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Habitat selection and time of breeding in the Great Blue Heron, (Ardea herodias) Butler, Robert William 1991

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HABITAT S E L E C T I O N AND TIME OF BREEDING IN THE GREAT BLUE HERON, {ARDEA HERODIAS) By ROBERT W. BUTLER B . S c , Simon F r a s e r U n i v e r s i t y , M . S c , Simon F r a s e r U n i v e r s i t y ,  1976 1980  A T H E S I S SUBMITTED IN P A R T I A L F U L F I L L M E N T OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF S C I E N C E (Department o f Zoology) We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e required standard  THE U N I V E R S I T Y OF B R I T I S H COLUMBIA © R o b e r t W. B u t l e r 1991  In presenting this thesis in partial fulfilment  of the requirements for an advanced  degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department  or  by his or  her  representatives.  It  is understood that  copying or  publication of this thesis for financial gain shall not be allowed without my written permission.  Department The University of British Columbia Vancouver, Canada Date  DE-6 (2/88)  ABSTRACT T h i s t h e s i s examines the causes and consequences o f h a b i t a t s e l e c t i o n and timing  of  breeding  of  the  Great  h y p o t h e s i s was t h a t the d u r a t i o n strongly season;  influenced habitat  the  shifts;  Blue  (Ardea herodias).  Heron  o f low t i d e s  location and the  of  and seasonal  colony-sites;  general  abundance o f  timing  use o f space by f o r a g i n g  My  of  prey  the  breeding  herons o f  different  age- and s e x - c l a s s e s . I  s t u d i e d Great  Blue Herons along the  b r e e d i n g seasons and f o u r w i n t e r s . of  85  to  visits  100  pairs  were made  British  to  about  Columbia.  availability, colonies, feeding  on Sidney  I  At  habitat recorded  areas  40  use the  and  Breeding herons were s t u d i e d at  Island  near  other  Sidney,  I  the  town o f  colonies  around  studied  the  reproductive  reproductive  and searched f o r  P a c i f i c c o a s t o f Canada f o r  nests of  the  in  a predator,  River  of j u v e n i l e  hypothesized that  away from p r e d a t o r s .  6 km o f greater low, eagle  and p o s t - b r e e d i n g  the  adult  Georgia,  behaviour,  food  At  other  their  main  Bald E a g l e .  In  the  d i s p e r s i o n pattern  herons  in  the  Fraser  delta. I  or  shifts  of  located  n o n - b r e e d i n g s e a s o n , I i n v e s t i g a t e d the f o r a g i n g b e h a v i o u r , and h a b i t a t  periodic  detail.  herons,  a colony  and  Strait  foraging  success  success o f  Sidney,  five  their  main  heron c o l o n y - s i t e s were l o c a t e d Twenty-nine  feeding  where e a g l e s nested  contrary  t o the  site.  near  food s u p p l i e s  o f 33 c o l o n y - s i t e s were l o c a t e d The  number  of  heron  pairs  i n high abundance than where e a g l e  h y p o t h e s i s t h a t breeding  was  within slightly  abundance was  herons a v o i d areas with  active  nests. I h y p o t h e s i z e d t h a t herons began breeding i n s p r i n g s h o r t l y a f t e r  females  a c q u i r e d enough food energy to make e g g s , or so c h i c k s were i n n e s t s when food was most p l e n t i f u l  to  their  parents.  E g g - l a y i n g began about  i i  9 days a f t e r  a  f e m a l e ' s d a i l y food i n t a k e c r o s s e d an energy t h r e s h o l d o f 1715 k J / d a y , whereas the peak a v a i l a b i l i t y  o f food energy to a d u l t s o c c u r r e d about 35 days before  the peak food demands o f t h e i r  chicks.  Food i n t a k e  rates  by a d u l t s i n c r e a s e d  g r a d u a l l y i n March and A p r i l with the i n c r e a s i n g d u r a t i o n o f low t i d e s and the i n s h o r e movement o f f i s h e s .  A d u l t food i n t a k e r a t e s reached a peak i n May when  sea perch were most abundant, Most j u v e n i l e to  and a d u l t  and d i m i n i s h e d through June and J u l y . female  herons foraged on beaches from February  October and i n marshlands and g r a s s l a n d s from November to  males r e t u r n e d until  the s t a r t I  when  tested  they  can  conspecifics maintain  to  territories  Some  i n August and remained  there  o f the next breeding season i n March. the  hypothesis that  no  longer  reduced  their  along r i v e r b a n k s  January.  energy  catch  foraging  herons l e a v e enough  intake  balance.  In  food  foraging or  rates  below  October  and  when a  habitats  in  autumn  interference  threshold  November  from  required  adults  moved  to to  marshlands and j u v e n i l e s moved to g r a s s l a n d s when they c o u l d no l o n g e r maintain d a i l y energy balance on beaches as a r e s u l t o f d e c l i n i n g d u r a t i o n o f low t i d e s and food i n t a k e r a t e s . habitat  I n t e r f e r e n c e c o m p e t i t i o n was too i n f r e q u e n t to e x p l a i n  s h i f t s by a d u l t o r j u v e n i l e  herons i n autumn.  i i i  AT B L U E HERO N CArdea herodias)  iv  TABLE OF CONTENTS  Abstract Frontispiece L i s t o f Tables L i s t Figures Acknowledgements  ii iv viii x xii  CHAPTER ONE. INTRODUCTION The Theory o f Habitat S e l e c t i o n The Timing o f Breeding The Study Species Aims o f t h e Thesis  1 1 3 4 5  CHAPTER TWO. STUDY SPECIES, STUDY AREA AND METHODS STUDY SPECIES Distribution Food Breeding Biology Non-breeding Season Age-class D e s c r i p t i o n s STUDY AREA Sidney Island Fraser River D e l t a GENERAL METHODS D i s p e r s i o n i n the Breeding Season D i s p e r s i o n i n the Non-breeding Season Foraging Oceanic Tides Time o f Breeding Reproductive Success Sex o f Adult Herons  6 6 6 6 8 8 10 10 10 13 13 14 16 17 17 17 18  . ....  CHAPTER THREE. THE EFFECT OF PREDATORS AND FOOD AVAILABILITY ON COLONY-SITE SPACING IN GREAT BLUE HERONS . Predictions METHODS Prey A v a i l a b i l i t y Distance between C o l o n y - s i t e s and Feeding S i t e s Distance between Heron C o l o n y - s i t e s and Eagle Nests Eagle Nest Abundance RESULTS Distance between C o l o n y - s i t e s and Feeding S i t e s Predation DISCUSSION V a l i d i t y o f Assumptions . . . Food and Predators as Determinants o f C o l o n y - s i t e Location ... F l i g h t Distance and Reproductive Success SUMMARY  v  20 21 21 21 22 22 22 23 23 23 30 30 31 32 33  Table o f Contents  (continued)  CHAPTER FOUR. SEASONAL PATTERNS OF HABITAT USE BY FORAGING GREAT BLUE HERONS Predictions I n t r a s p e c i f i c f o r a g i n g competition Foraging times and f o r a g i n g success STUDY AREA AND METHODS H a b i t a t use o f Age- and Sex-classes Breeding season d i s p e r s i o n Non-breeding season d i s p e r s i o n Habitat Selection I n t r a s p e c i f i c f o r a g i n g competition Interference Territoriality R e l a t i v e a v a i l a b i l i t y o f f o r a g i n g time Beaches Maintenance energy . Ingestion r a t e s Foraging time Grasslands Ingestion r a t e s H a b i t a t S h i f t s , D i s p e r s a l and M o r t a l i t y RESULTS Spacing o f Age- and Sex-classes Breeding season Non-breeding season Habitat Selection I n t r a s p e c i f i c f o r a g i n g competition Interference Territoriality Foraging success Beaches Foraging time Grasslands H a b i t a t S h i f t s , D i s p e r s a l and M o r t a l i t y DISCUSSION . V a l i d i t y o f Assumptions Beach-foraging herons G r a s s l a n d - f o r a g i n g herons Year-round Foraging D i s p e r s i o n o f Age- and Sex-classes Habitat Selection Foraging e f f i c i e n c y , h a b i t a t s e l e c t i o n and d i s p e r s a l Territoriality SUMMARY CHAPTER FIVE. TIME OF BREEDING IN GREAT BLUE HERONS STUDY AREA AND METHODS R e l a t i v e A v a i l a b i l i t y o f Energy t o Adult Herons Number o f f i s h i n the lagoon Energy estimates i n prey Nesting stages vi  34 35 35 35 36 36 36 36 37 37 37 37 38 38 38 38 39 39 39 40 40 40 40 42 44 44 44 48 52 52 52 52 54 57 57 57 57 58 59 59 61 62  ...  .  63 64 64 65 65 66  Table o f Contents (continued) Duration o f low t i d e s Estimated Energy Consumption by A d u l t s A d u l t consumption time o f Breeding Energy t h r e s h o l d f o r l a y i n g Energy consumption by e g g - l a y i n g females Food demands o f heron c h i c k s RESULTS R e l a t i v e A v a i l a b i l i t y o f Food Energy i n the Lagoon Seasonal Energy Consumption by A d u l t s , Chicks and E g g - l a y i n g Females Adults Chicks Egg-laying females DISCUSSION V a l i d i t y o f Assumptions Time o f Breeding Future D i r e c t i o n s SUMMARY  •  • . •  66 66 66 67 67 68 68 68 68 71 71 71 71 78 78 79 80 81  CHAPTER SIX. GENERAL DISCUSSION Choice o f C o l o n y - s i t e Colony Formation Habitat Selection Time o f Breeding I n t e r - r e l a t e d P a t t e r n s o f Ecology and Behaviour Adult versus j u v e n i l e t a c t i c s  82 82 83 84 84 85 86  LITERATURE CITED  88  APPENDICES Appendix I  99  vi i  Number o f s u c c e s s f u l and f a i l e d heron c o l o n i e s where eagles nested i n high (nests <6 km apart) and low (nests >6 km apart) abundance i n 1987~-89 Percentage o f s u c c e s s f u l (>1 f l e d g l i n g ) n e s t i n g p a i r s and mean brood s i z e s o f c o l o n i e s t h a t produced 1 f l e d g l i n g s near (<3 km) and f a r (>3 km) from an occupied Bald Eagle nest i n 1989 Percentage o f f l i g h t s to and from the Sidney lagoon d u r i n g low t i d e s (<1.7 m) and high t i d e s when the nests held eggs, small c h i c k s and l a r g e c h i c k s Number o f a d u l t and j u v e n i l e s herons counted i n g r a s s l a n d s and marshlands o f the F r a s e r R i v e r d e l t a i n J u l y 1987 March 1988 and J u l y to September 1988 Indices o f aggregation o f Great Blue Herons f e e d i n g i n marshlands and on i n t e r t i d a l beaches o f the F r a s e r R i v e r d e l t a d u r i n g the non-breeding season. The s p a t i a l p a t t e r n ranges from clumped (R approaches zero) to r e g u l a r , (R approaches 2.15). When the p a t t e r n i s random, R=l (Krebs 1989) Number o f each prey s p e c i e s caught by a d u l t and j u v e n i l e Great Blue Herons i n August and September 1987, 1988 & 1990 (N=33 days) D a i l y M e t a b o l i z a b l e Energy (ME) i n t a k e i n kJ l o s t to i n t e r f e r e n c e by c o n s p e c i f i c s i n autumn Numbers and d e n s i t i e s o f Great Blue Herons i n t h r e e h a b i t a t s along the r i v e r b a n k s o f the F r a s e r R i v e r d e l t a Average l e n g t h s o f f i s h caught i n beach s e i n e s between A p r i l - J u l y 1987-88 and t h e i r estimated weights from length-weight r e g r e s s i o n equations Estimates o f M e t a b o l i z a b l e Energy ( c a l c u l a t e d from weights o f prey i n Table 5-1) a v a i l a b l e to herons from the major prey s p e c i e s each summer month i n the lagoon at Sidney viii  Table 5-3. Table 5-4.  Table 5-5.  Table 5-6.  E x t r a p o l a t e d minimum populations o f f i s h i n the p a r t i a l enclosure i n Sidney lagoon Number o f low (<1.7 m) t i d e minutes per average day i n which herons could forage i n the lagoon e e l g r a s s beds during the breeding season. Estimated Metabolized Energy (ME) i n t a k e (kJ) per average day by an a d u l t heron average over f o u r p e r i o d s o f the breeding season on Sidney I s l a n d i n 1987-88. N i s the number o f herons watched. Numbers o f each o f the main prey s p e c i e s eaten by Great Blue Herons during the 1987-88 breeding season on Sidney I s l a n d .  ix  72  73  74  77  LIST OF FIGURES  F i g u r e 2-1.  F i g u r e 2-2.  F i g u r e 2-3.  F i g u r e 3-1.  F i g u r e 3-2.  F i g u r e 3-3.  F i g u r e 4-1.  F i g u r e 4-2.  F i g u r e 4-3.  L o c a t i o n o f t h e b r e e d i n g c o l o n y - s i t e and h a b i t a t s u s e d b y herons on Sidney I s l a n d Location o f habitats and three breeding colony-sites (diamond symbols) used b y herons i n t h e F r a s e r R i v e r d e l t a Census routes followed t o l o c a t e herons d u r i n g the breeding season i n the F r a s e r R i v e r d e l t a  Numbers o f b r e e d i n g p a i r s o f h e r o n s and t h e a r e a o f t h e i r f o r a g i n g s i t e s i n 1988 (A) and p e r c e n t a g e o f o c c u p i e d n e s t s t h a t f l e d g l i n g (B), i n ten heron c o l o n i e s versus m a j o r f e e d i n g s i t e i n 1988. Large dot in a l l other dots represent 1 c o l o n y - s i t e  P e r c e n t a g e o f h e r o n s u s i n g m a r s h l a n d s and b e a c h e s i n t h e F r a s e r R i v e r d e l t a between August a n d A p r i l 1986-87. Numbers a b o v e m o n t h s i n d i c a t e t h e number o f h e r o n s c o u n t e d i n t h a t month Numbers o f G r e a t B l u e H e r o n s c o u n t e d o n b e a c h e s i n t h e F r a s e r R i v e r d e l t a i n r e l a t i o n t o t h e a v e r a g e number o f f o r a g i n g h o u r s a t l o w t i d e (<2.3 m) f r o m A u g u s t 1986 t o A p r i l 1987 E s t i m a t e d p e r c e n t o f a v a i l a b l e l o w - t i d e (<1.7 m) f o r a g i n g t i m e r e q u i r e d by a d u l t ( s t i p p l e d ) and j u v e n i l e ( o p e n ) h e r o n s t o m e e t t h e i r e s t i m a t e d e n e r g y n e e d s ( 1 5 6 0 k J ) i n e a c h 24h b e t w e e n 1 S e p t e m b e r and 3 0 N o v e m b e r 1 9 8 7 - 1 9 8 8  x  12  non-  L o c a t i o n o f Great Blue Heron c o l o n y - s i t e s ( c l o s e d c i r c l e ) , k e l p b e d s , e e l g r a s s b e d s a n d m a r s h e s >10 h a ( a r r o w ) , i n r e l a t i o n t o Bald Eagle n e s t i n g abundance. Dashed l i n e s e n c l o s e a r e a s w h e r e e a g l e n e s t s a v e r a g e d <6 km a p a r t ( h i g h a b u n d a n c e ) and d o t t e d l i n e s e n c l o s e a r e a s w h e r e e a g l e n e s t s a v e r a g e d f e w e r t h a n one e v e r y 6 km ( l o w a b u n d a n c e )  Mean b r o o d s i z e h e l d one o r more distance, to the A = 2 colonies;  11  15  24  25  26  43  45  53  Figure 4-4.  Percentages of juvenile (A) and adult (B) herons counted on grasslands (stippled bars) and beaches (open bars) during the non-breeding season. Numbers above bars are number of herons counted 55  Figure 4-5.  Numbers of adult and juvenile Great Blue Herons counted each month on road surveys of the Fraser River delta 56  Figure 5-1.  Index of food a v a i l a b i l i t y during the breeding season of the Great Blue Heron. Percent a v a i l a b i l i t y is estimated from the product of numbers of minutes low tides (<1.7 m) expose the foraging habitat and the size of the f i s h population converted into units of energy. Vertical l i n e s are standard errors 75  Figure 5-2.  Numbers of days that available energy exceeded an estimated energy threshold for egg laying and the estimated dates when clutches were l a i d in 1987 and 1988 76  xi  ACKNOWLEDGEMENTS Several people a s s i s t e d me during t h i s study. Jamie Smith encouraged the development o f ideas and p a t i e n t l y e d i t e d d r a f t s o f t h i s t h e s i s . A r t M a r t e l l secured funding and along with Charley Krebs, Don McPhail and Dolph S c h l u t e r , served as a committee member. The Canadian W i l d l i f e S e r v i c e , P a c i f i c and Yukon Region and National W i l d l i f e Research Centre, Ottawa, funded the study. B r i t i s h Columbia M i n i s t r y o f Parks, e s p e c i a l l y B i l l Shaw and R i c Simmons, k i n d l y permitted and provided help i n e s t a b l i s h i n g a f i e l d camp i n Sidney S p i t Marine Park. Fred Stephenson and Tony Mau a t the I n s t i t u t e o f Ocean Sciences analyzed t i d a l data. Ian Moul, Mary T a i t t and e s p e c i a l l y T e r r y S u l l i v a n t i r e l e s s l y a s s i s t e d me i n the f i e l d . Don Blood, Andre B r e a u l t , S c o t t Forbes, Ian Moul and Phil Whitehead shared t h e i r data on heron c o l o n i e s . Darin Bennet provided data on food consumption o f c a p t i v e herons. G.E.John Smith provided s t a t i s t i c a l advice and Susan Garnham p a t i e n t l y typed many d r a f t s o f t h i s t h e s i s . Kees and Rebecca Vermeer provided warm h o s p i t a l i t y i n Sidney. I e s p e c i a l l y thank Sharon, H o l l y and Myrica B u t l e r f o r t h e i r support and a s s i s t a n c e through the e n t i r e study.  xi i  CHAPTER ONE. INTRODUCTION  T h i s study arose from my continuing i n t e r e s t i n how animals adapt to c o a s t a l  environments. I am p a r t i c u l a r l y i n t e r e s t e d i n animals that r e l y on  the i n t e r t i d a l zone f o r f o r a g i n g .  The rhythm of oceanic t i d e s pervades the  l i v e s of many b i r d s using seashores by determining when they w i l l feed, nest and  migrate.  (Ardea  The d i s c o v e r y of d i o x i n contamination  herodias)  of Great Blue Herons  and t h e i r prey i n B r i t i s h Columbia ( E l l i o t t  et  al.  1989)  provided a s p e c i f i c stimulus to begin t h i s study. The  Theory of Habitat Selection  One of the most important d e c i s i o n s an animal makes i n i t s l i f e t i m e i s where i t s e t t l e s . This d e c i s i o n may i n v o l v e a complex i n t e r p l a y between age, s o c i a l s t a t u s , sex, competitive a b i l i t y , f o r a g i n g s k i l l , presence of  predators  and the s u i t a b i l i t y of the h a b i t a t ( C a t t e r a l l et a l . 1989).  It i s well  e s t a b l i s h e d t h a t where animals s e t t l e a f f e c t s t h e i r s u r v i v a l and  reproductive  Whitham 1980, A l a t a l o et al. 1985).  Individuals  success  (e.g Krebs 1971,  s e t t l i n g i n good h a b i t a t s are thereby  rewarded by producing more o f f s p r i n g in  subsequent generations. Habitat s e l e c t i o n theory has a r i s e n i n the past two decades to e x p l a i n how i n d i v i d u a l s might s e t t l e s i t e s .  The ' i d e a l f r e e d i s t r i b u t i o n ' assumes  that i n d i v i d u a l s that are e q u a l l y s k i l l e d and have complete information about the h a b i t a t should move f r e e l y to wherever f i t n e s s rewards are thus should experience 1970,  F r e t w e l l 1972).  g r e a t e s t , and  the same average r a t e of r e t u r n ( F r e t w e l l and Lucas T h e i r ' i d e a l d e s p o t i c ' model p o s i t s that the best  competitors l i m i t settlement of i n f e r i o r competitors to smaller s i t e s in good  1  h a b i t a t o r t o s i t e s i n poor h a b i t a t . This r e s u l t s i n unequal rewards between individuals. Brown (1969) proposed a s i m i l a r explanation f o r d i f f e r e n t d e n s i t i e s o f b i r d s i n breeding h a b i t a t s . According t o h i s model, good h a b i t a t s are f i l l e d to a c r i t i c a l d e n s i t y a f t e r which s u c c e s s i v e l y poorer h a b i t a t s are occupied at lower d e n s i t i e s .  Non-breeding ' f l o a t e r s ' that roam between h a b i t a t s or  l i v e s e c r e t i v e l y i n t e r r i t o r i e s (e.g. Smith 1978), e v e n t u a l l y appear when populations are very dense. The i d e a l f r e e model e x p l a i n s the d i s p e r s i o n o f some b i r d s , i n s e c t s , f i s h , amphibians and r e p t i l e s well (e.g. Have et a7. 1984, Morse 1985, Pimm et al.  1985, and see review by Parker and Sutherland 1986). The i d e a l d e s p o t i c  and  breeding t e r r i t o r y models a l s o e x p l a i n d i f f e r e n t population d e n s i t i e s i n  h a b i t a t s that vary i n q u a l i t y (e.g. Krebs 1971, F r e t w e l l 1972, A l a t a l o et 1985,  Richner  non-territorial  al.  1986), but only the l a t t e r model e x p l a i n s the presence o f f l o a t e r s (e.g. Krebs 1971).  A l l three models p o s i t that  i n d i v i d u a l s s h i f t between h a b i t a t s as the d e n s i t y o f competitors  changes.  Another view i s that some animals switch between f o r a g i n g h a b i t a t s when food a v a i l a b i l i t y d e c l i n e s below some t h r e s h o l d l e v e l (Stephens and Krebs 1986). T h i s view p o s i t s that the length o f time a f o r a g i n g animal r e s i d e s i n a h a b i t a t depends on i t s r a t e o f prey capture (see review by Krebs et a7. 1984, Dugan e t al. 1988).  The marginal  value theorem (Charnov i976) p o s i t s that  animals move between food patches when t h e i r instantaneous r a t e o f food intake reaches  t h e average expected  gain i n the h a b i t a t .  The optimal  assessment  p o l i c y p o s i t s that animals switch h a b i t a t s when t h e i r net reward o f r e s i d i n g i n a f o r a g i n g h a b i t a t returns t o zero (McNamara 1982). changes  i n habitat  use by animals without  interactions. 2  T h i s theory  invoking  explains  density-dependent  Gathering availability  evidence  that  habitat  shifts  by  has been hampered by the d i f f i c u l t y  birds  are  related  of following  mobile  from summer t o w i n t e r h a b i t a t s and by measuring the a v a i l a b i l i t y For  example,  adult  and  Sullivan  immature  (1989, 1990) Yellow-eyed  to  food  animals  of t h e i r  food.  showed t h a t the d i s p e r s i o n and s u r v i v a l Juncos  b r e e d i n g season was p o s i t i v e l y r e l a t e d  (Junco  phaeonotus)  to t h e i r  ability  soon  after  of the  to a c q u i r e f o o d .  The Timing o f Breeding by B i r d s Most b i r d s breed d u r i n g a season when food i s r e l a t i v e l y 1950,  Lack 1950).  begin to their is  However, the mechanisms t h a t  breed are  less clear.  plentiful  (Moreau  determine p r e c i s e l y when b i r d s  Lack (1954,  1966)  proposed t h a t  birds  b r e e d i n g seasons so t h a t n e s t l i n g s are present when food f o r the  most abundant.  However,  average date were n o t , most p r o d u c t i v e An laying  female  studies  showed t h a t  as p r e d i c t e d by L a c k ' s (1954)  (Perrins  alternate  later  1965,  explanation  determines  when  Cave 1968, is  that  she w i l l  amount  lay  her  of  parents  clutches l a i d  on  the  h y p o t h e s i s , always  the  P e r r i n s and Birkhead  the  food  eggs  1983).  available  (Perrins  to  1965,  Perrins'  h y p o t h e s i s p r e d i c t s t h a t females should begin to l a y as soon as  foraging  sites  a l l o w them to make eggs as well  food a d d i t i o n  s t u d i e s with female b i r d s e a r l y  Perrins'  (see reviews by Daan et  view  al.  Evidence t h a t n e s t l i n g s hatch a f t e r mostly weak because o f m e t h o d o l o g i c a l of  food.  food  Most  studies  availability  (see  their  al.  Dijkstra parents'  et  studies of  Although t h i s  5 s o n g b i r d s and 2 r a p t o r s  3  Many  season support al.  1990).  the  to  seems  is  availability  food abundance as an  r e a s o n a b l e , the c o n c l u s i o n s o f these s t u d i e s are e q u i v o c a l . pieced together  their  food supply peaks  problems i n e s t i m a t i n g  1988).  the  1970).  themselves.  i n the b r e e d i n g  have used measures o f Daan et  as m a i n t a i n  1988,  time  index  of  intuitively  Daan et al.  (1988)  conclude t h a t  four  species  had n e s t l i n g s  at  a time when t h e i r  food s u p p l i e s were i n c r e a s i n g  in  abundance, and t h r e e w h i l e food abundance was waning. A  second m e t h o d o l o g i c a l  demands for  o f the n e s t l i n g s  the  food-gathering  o c c u r s d u r i n g the it  problem  in  relating  is establishing parent.  nestling  In  stage  if  food  and when a c r i t i c a l  some s p e c i e s ,  (see  review  o c c u r s a f t e r the young l e a v e the nest  availability  the  peak  to  time  the  occurs  demand f o r  by M a r t i n 1987)  while  in  (Weathers and S u l l i v a n 1989,  food  others  Sullivan  1989).  The Study Species The i d e a l a  year-round  sex-classes  for  studying habitat that  is  reproductive  life.  The  this  marshlands  distinguished  Great  ideal.  and from  in  adults  be  nest  in  et  al.  c o l o n i e s where  (Simpson  the  documented,  coast  1984,  grasslands.  age-  whose  easy to  of  is and  prey  catch  British  Juveniles  Butler  (Palmer  1962,  can be sexed from a d i s t a n c e 1990).  Most  their 1989).  small f i s h on beaches near c o l o n y - s i t e s and a d u l t  on  recognizable  and i s  by plumage d i f f e r e n c e s  (Butler  lengths  Heron  neighbouring  culmen  populations  can  has  breeding  and  Columbia  The s p e c i e s i s sedentary and c o n s p i c u o u s on beaches,  and most a d u l t s  documented  visible,  performance  Blue  Kushlan 1984),  Columbia  highly  s e l e c t i o n and time o f  and food consumption can be measured,  approximates in  for  resident  whose  availability mark  animal  consumption can be  4  Great  Blue  are  easily  Hancock and  by e s t i m a t e s  Herons  in  British  be  readily  reproductive  success  can  Herons  British  Columbia  in  (Krebs 1974, estimated  Simpson 1984) (Simpson  of  1984).  catch  where prey  Aims of the Thesis In t h i s thesis I describe seasonal changes in habitat age(i)  and sex-classes and test herons  locate  the  3 sets of  colony-sites near  (Chapter 3 ) ,  (ii)  interference  from foraging  food  begin  to  supplies or  foraging herons move between habitats conspecifics or  i n s u f f i c i e n t foraging time during t i d a l herons  alternative  lay  eggs after  are  use by  different  hypotheses that:  away  from  predators  in autumn because of  forced to  move because of  cycles (Chapter 4 ) and, ( i i i )  a threshold  of  available  energy  female for  production has been passed or match the period of peak food demands of chicks  (Chapter 5 ) .  In  Chapter 6 , I  examine the year-round  egg their  relationships  between habitat use, dispersal and foraging e f f i c i e n c y of age- and sex-classes.  5  CHAPTER TWO.  STUDY SPECIES, STUDY AREA AND METHODS  STUDY SPECIES Distribution ardeid  in  continent  from  Islands from  North  -  The Great  America the  Blue Heron  (Hancock  Arctic  Circle  is  and Kushlan in  Alaska  most o f  its  northern  range  Food  - Great  Blue Herons i n h a b i t  pass w i t h i n  terminology)  fish  or  and f i s h  caught  striking  "Walk Slowly"  are  between  the  immobilized  range  crustaceans,  (Kushlan  1978).  Breeding b i o l o g y  Laporte Werschkul  1979, et  (Calambokidi  finely  a,b,  Large  pers. obs.).  on the  Galapagos  coasts of  British  and l a k e s where they  and wait"  et  al.  al.  English  until  prey  (1976a)  i s encountered.  I  Most  mandibles,  prey  spines  with dangerous  with the b i l l  until  the  are  spines  Great Blue Herons a l s o eat mammals, reptiles,  birds  and  carrion  Blue Heron has been  (Vermeer  1969)', Nova S c o t i a  Quinney 1983), Quebec (DesGranges and  1979), 1978,  1985), Montana (Parker  Oregon Blus  (Henny et  al.  and  Bethers  1980),  (Powell  1971,  Washington  1980), C a l i f o r n i a ( P r a t t 1970,  1985), and F l o r i d a  6  Kushlan's  for  open upper and lower  amphibians,  P r a t t and Winkler  B l a c k and C o l l u p y  in  - The breeding b i o l o g y o f the Great  DesGranges  Brandman 1976,  on the  the  d e s c r i b e d by Kushlan (1976a).  serrated  gastropods,  1977,  across  s t a n d i n g i n water where they wait  (Kushlan 1976a)  Quinney and Smith 1979,  et al.  and  rivers  documented a c r o s s North America i n c l u d i n g A l b e r t a (McAloney 1973,  breeds  (Brandman 1976,  sometimes taken ashore where they are speared  insects,  widespread  The Great Blue Heron migrates  except  ("Stand  by s h a k i n g .  r e l a x o r break (Forbes 1982,  It  Mexico  seashores,  saw the 28 o t h e r f e e d i n g t e c h n i q u e s  are  and most  resides year-round.  Great Blue Herons mostly hunt by  to  rarely  to  in winter  wade i n search o f f i s h d u r i n g day and n i g h t 1983).  largest  1984).  (American O r n i t h o l o g i s t s ' Union 1983).  Columbia and A l a s k a where i t  prey  the  1983,  1972 Powell  and Powell 1986).  In B r i t i s h Columbia, breeding Great Blue Herons have been  s t u d i e d near Pender Harbour on t h e S e c h e l t P e n i n s u l a ( K e l s a l l and  Simpson  1980, Simpson 1984, Simpson et a7. 1987), and on the U n i v e r s i t y o f  British  Columbia Endowment Lands (now c a l l e d P a c i f i c S p i r i t Park) (Urhahn 1972, Krebs 1974). compared  by  1968, Paine  The breeding b i o l o g y o f several c o a s t a l c o l o n i e s were  Forbes et a7. (1985a), and I have reviewed these and other  c o l o n i e s i n more d e t a i l elsewhere ( B u t l e r 1989). I d e f i n e a c o l o n y - s i t e as the l o c a t i o n where n e s t i n g herons gather, and a colony as t h e group o f herons t h a t gather t h e r e (Kushlan 1986). B r i e f l y , male Great Blue Herons on the c o a s t o f B r i t i s h Columbia r e t u r n to t h e i r c o l o n y - s i t e s near dusk i n l a t e February o r e a r l y March.  They  settle  on p r e v i o u s y e a r s ' nests o r i n t r e e limbs which they defend a g a i n s t one another. When females a r r i v e about a week l a t e r , the males v i g o r o u s l y d i s p l a y for their attention.  Over the next few weeks females begin t o a r r i v e ever  e a r l i e r i n t h e afternoon u n t i l by l a t e March o r e a r l y A p r i l they remain near the c o l o n y - s i t e a l l day. Meyerriecks (1960) and Mock (1976) have d e s c r i b e d the d i s p l a y s o f the Great Blue Heron i n c o l o n y - s i t e s elsewhere i n North America. Great Blue Herons mate monogamously i n each breeding attempt but a p p a r e n t l y choose new mates each y e a r (Simpson 1984). In North America, Great Blue Herons nest on a v a r i e t y o f s u b s t r a t e s i n c l u d i n g t h e ground, i n bushes, on man-made s t r u c t u r e s and i n t r e e s (Palmer 1962).  In B r i t i s h Columbia they nest i n c o n i f e r o u s and deciduous t r e e s  ( B u t l e r 1989). Herons, i n B r i t i s h Columbia l a y eggs mostly i n A p r i l  ( B u t l e r 1989).  Incubation d u t i e s are shared by t h e mated p a i r ; males incubate mostly during the day and females a t n i g h t (Paine 1972, Brandman 1976, Kaufmann and Cawley 7  1986).  Incubation  lasts  about  asynchronously (Mock 1985). (Vermeer  1969)  27 days  (Vermeer  1969)  and the  eggs hatch  Clutch sizes average from f i v e eggs in Alberta  to fewer than three in Florida (Powell  coastal B r i t i s h Columbia the average clutch size i s 4.1  and Powell  1986).  eggs (Butler  In  1989).  Nestlings are fed on f i s h caught near the c o l o n y - s i t e during the day (Simpson 1984,  pers. o b s . ) .  S i b l i n g aggression results in the reduction in the size  of some broods (Mock 1985). per successful nesting  On average,  attempt  about 2.5 young reach independence  in B r i t i s h  Columbia (Forbes  et  a7.  1985a,  Butler 1989). Non-breeding Season - Less is known about the Great Blue Heron outside its  breeding  season.  post-breeding  herons  Band recovery data in  northern  indicate  populations  Columbia and Washington (Henny 1972, Byrd 1978). band recovery data show about three-quarters first  birthday (Henny 1972,  efficient  Bayer 1981).  a southward movement  excluding  coastal  of  British  Mortality estimates based on  of the juveniles die before  their  Juvenile Great Blue Herons are less  foragers than adults after the close of the breeding season (Quinney  and Smith 1980).  Some post-breeding adults hold exclusive feeding  territories  (Bayer 1978). Age-class Descriptions - Age-classes of the Great Blue Heron have been described by Bent (1963), Palmer (1962), and Hancock and Kushlan (1984), and by M i l l stein et a7. (1970) for the Gray Heron (A. cinerea).  The classes used  in t h i s thesis are defined below.  Chick:  A heron between  hatching  and fledging  is c a l l e d  a chick.  This  period l a s t s about two months, usually May and June; Fledgling: Chicks are c a l l e d fledglings from when they f i r s t leave the nest in late  June or early J u l y , until about two weeks l a t e r when no trace 8  o f the  i s most apparent  on  F l e d g l i n g s become j u v e n i l e s once they l o s e the n e o s s o p t i l e down  in  the Juvenile:  n e o s s o p t i l e down r e m a i n s .  crown and on the  mid-July which  and remain  begins  slate-grey occipital Yearling:  T h i s 'down'  face;  in  this  i n March.  class  until  next  They can be r e c o g n i z e d  breeding  i n the  season  field  by a  crown, the absence o f body plumes, and very s h o r t or no plumes;  J u v e n i l e s become y e a r l i n g s about 11 months a f t e r h a t c h i n g with commencement o f  the  adult  breeding  season i n March.  i n t h i s a g e - c l a s s f o r twelve months. by  a  small  (c.3  cm)  presence o f o c c i p i t a l wing.  However,  white  crown  Adults  have  a  white  i n t o l o n g plumes. chest  remain  Y e a r l i n g s can be d i s t i n g u i s h e d patch,  grey  forehead  and  the the  crown,  juvenile's;  white  or  white  flecked  with  grey  s t r i p e s f u s i n g at the p o s t e r i o r end  Long plumes cascade over the back and s p l a y out  and b e l l y  most n o t i c e a b l y  The gray body plumage has no brown e d g i n g . the b i l l  the  they l a c k the long a d u l t body plumes, and some wing  f o r e h e a d , and b l a c k post o r b i t a l  from the  They  plumes and by a c h e s t n u t - c o l o u r e d bend o f  c o v e r t s are edged with brown l i k e Adult:  the  approaches an o r a n g e - y e l l o w  9  in  the  breeding  season.  In the breeding  season,  hue ( c o l o r  18,  Smithe  1975).  STUDY AREA I studied Great Blue Herons mostly in the Gulf Islands and in the Fraser River delta in the southwest corner of B r i t i s h Columbia, Canada. climate  ensures a mild winter and cool summer, despite  the  The maritime high  latitude  (48-49°N).  Infrequent v i s i t s were made to about 40 colonies around the S t r a i t  of Georgia  and two areas were studied in d e t a i l .  Sidney Island - The breeding component of my study focused on a colony of 85 to  100 nesting pairs  on Sidney Island,  about  4 km east  Sidney and 23 km ENE of V i c t o r i a , B r i t i s h Columbia. a  dozen large Canadian Gulf  (San Juan)  Islands  of the  town of  Sidney Island is one of  that l i e  between Vancouver  Island and the mainland of B r i t i s h Columbia. Sidney Island i s mostly covered in second growth Douglas F i r (Pseudotsuga menziesii)  and mature Arbutus  (Alnus rubra) trees. in which most  female  exposed by f a l l i n g virginica  Littoral  is  The herons nested  in  Red Alder  d r i f t has created a lagoon ( F i g . 2-1)  herons fed  tides  and Distichlis  menziesii.  each day.  a saltmarsh  spicata.  the  first  part  community dominated  of  (100  the  by  ha)  lagoon  Salicornia  Herons used t h i s marsh as a loafing  site.  Adjacent to the saltmarsh i s a 60 ha mudflat that supports sea lettuce (1/7va lactuca)  and sparse growth  of eelgrass  (Zostera  marina).  Some herons  fed  there before and after the tides exposed the most heavily used zone, which was dominated  by a dense growth of eelgrass (40  Fraser River Delta in  the Fraser River delta  ( F i g . 2-2).  ha).  I studied herons during autumn and winter mostly immediately  south of Vancouver, B r i t i s h Columbia  the Fraser is the largest estuary on the P a c i f i c coast of  A general description i s given by Butler and Campbell (1987).  10  Canada.  Figure 2-1.  Location of the breeding c o l o n y - s i t e and habitats used by herons on Sidney Island.  11  F i g u r e 2-2.  Location of habitats and t h r e e breeding c o l o n y - s i t e s symbols) used by herons i n the F r a s e r R i v e r d e l t a .  12  (diamond  Briefly,  most  agricultural,  of  the  680 km  delta  2  residential  above  and i n d u s t r i a l  and s l o u g h s d r a i n s much o f the d e l t a . summer  and  fields  either  lie  Outside  the  dikes,  between  the  channels o f the  of  Point  over  Roberts  4 km wide  a 600  east  at  to  their  use.  fallow  or  grow  1000 m wide  f l o u r i s h i n g beds o f e e l g r a s s  (Z.  (Fig.  japonica  been  diked  for  c r o p s are grown i n f i e l d s  band o f  width  has  cold-hardy  Boundary Bay l i e  greatest  tide  An e x t e n s i v e network o f d i t c h e s  Vegetable  Fraser River  into  high  in  winter.  b r a c k i s h marshland  2-2).  extends  From the western  extensive  (Fig.  crops  2-2).  in  shore  sand and mud beaches These beaches  and Z . marina)  (Fig.  2-2).  3 April  1986  support  GENERAL METHODS Great August  Blue Herons were s t u d i e d y e a r - r o u n d  1990.  I  studied  herons  for  five  15 August 1986,  21 March to 24 September 1987,  17 March  August  to  15  Herons were watched most o f t e n  at  1989, all  1 April times  from l a t e summer t o  to  1987  r e p o r t e d by Forbes et and by A . B r e a u l t  Forbes et requests I relative  a7.  al.  (pers.  seasons,  -  31 August  1990)  i n the  examined  31  (3 A p r i l  to  Nesting  on  Sidney  15 March  c o l o n y - s i t e s of  (1985b) were v i s i t e d by v o l u n t e e r s comm.)  in  1988-89.  1988,  Island.  Fraser River delta  but  1986-90).  Great  Blue  and me i n  C o l o n y - s i t e s unknown  (1985b) were found by c o n t a c t i n g n a t u r a l i s t s  in local  and  21 February to 18 August  s p r i n g (15 August to  D i s p e r s i o n i n the Breeding Season Herons  breeding  o f the y e a r  early  between  and through  to  public  newspapers. how herons chose a  importance  of  the Bald Eagle (Haliaeetus  site  for  their  c o l o n y by comparing  food s u p p l i e s and the presence o f p r e d a t o r s , leucocephalus).  13  the  notably  Dispersion in the outside  the  observers  breeding  Non-breeding Season - The d i s p e r s i o n p a t t e r n of herons season was determined  independently  mapped a l l  herons  above  the  (Fig.  2-3).  The maximum number o f herons o v e r a l l  the  route  airplane The  was c o n s i d e r e d as  f l e w at an  flight  1986 path  followed  the  1987 tide  beach i n the o p p o s i t e d i r e c t i o n . over the e n t i r e beach. From 17 J u l y 1987 I  drove  dikes  at  observers the  a 89.4  herons  the  line  i n one  Each heron  during  the  number  daytime  and 1 September  to 31 March 1988  tide  1987  direction  (3 m)  between  and 30 A p r i l and the  1988.  top o f  the  both o b s e r v e r s to r e c o r d herons  and from 5 J u l y to  once a week t h a t  was  mouth  assigned  and i t s  censuses were used to  following  of  mid-winter  T h i s allowed  Bay and the  Age-class Descriptions) road  delta  i n each about 4 km segment  of  c r o s s e d the  the  Fraser  an  age  using  non-breeding  habitat  season.  chapters.  14  (Fig.  plumage  2-3).  characters  the Two  methods  (see  Data from these  use and seasonal  Detailed  behind  1988  on o p p o s i t e s i d e s of  l o c a t i o n was noted on a map.  estimate  16 September  farmlands  River  searched f o r herons i n the d i t c h e s and f i e l d s  road.  River  The e n t i r e survey took about 70 m i n u t e s .  km route  Boundary  Fraser  km/h.  lowest  index  entire  aircraft  a l t i t u d e o f about 90 m and an a i r s p e e d o f about 140  and 31 March  best  the  185  The  the  the  of  a Cessna  present.  c o i n c i d e d with  1 September The f l i g h t  and marshlands  seen from  flown  of  beaches  Every two weeks 2  i n two ways.  are  mortality  of  provided  in  F i g u r e 2-3.  Census routes followed to l o c a t e herons during the non-breeding season i n the F r a s e r R i v e r d e l t a .  15  Foraging -  Feeding herons were watched on Sidney Island in spring and  summer and in the Fraser River delta in summer and autumn through a 15-60x and a 20x spotting telescope.  I opportunistically chose a feeding heron and an  assistant recorded the time of the observations, the geographical l o c a t i o n , the tide height (Department of Fisheries and Oceans 1986, 1987, 1988, 1989) and the age of the heron (juvenile or a d u l t ) .  Over the next ten minutes I recorded the  time of each s t r i k e  second,  to the  gunnel, s c u l p i n , etc.)  nearest  the  type  of  f i s h caught  and i t s estimated total length as a proportion of the  culmen length of the heron (<l/4, 1/4-1/2, 1/2-3/4, 3/4-1, >1). Blue  Heron  (S.E.=5.1, Observer  in  British  n=29,  bias  Columbia has an average  Simpson 1984)  using this  analysis (see Chapter Prey populations  (e.g.  so these  method was  culmen  increments  tested  A female Great  length  of  represent  123.9 mm  about  and accounted for  31 mm.  during  the  5). in the  lagoon on Sidney Island were sampled using a  2.4 x 18 m beach seine with a 6 mm mesh in A p r i l to July 1987-88 during the breeding season of the Great Blue Herons.  Fish caught in seine hauls were  i d e n t i f i e d using Hart's (1973) d e s c r i p t i o n s .  The entire catch was emptied into  buckets and  counted l a t e r .  spring balances,  Samples of the catch were weighed with Pesola  measured (total length) on a f i s h r u l e r and then released.  These data were  used to estimate the seasonal occurrence of major f i s h species  and to produce  length-weight  regression equations.  Detailed methods appear  in Chapter 5. In winter, herons in the Fraser River delta feed on voles, Microtus daily  townsendii,  and seasonal  Alaksen National of M.  townsendii  caught in grasslands (Taitt and Krebs 1983). use of  grasslands,  I  counted  the  number  of  especially To assess herons on  W i l d l i f e Area f i e l d s on Westham Island, where large numbers occur (Beacham 1980,  16  Taitt  et  al.  1981,  Taitt  and Krebs  1983), from 13 November 1986 to 23 March 1987, 12 February  and from 23 October 1987  1988.  Oceanic tides  -  Tides  in the  Strait  of  Georgia expose and cover  i n t e r t i d a l foraging s i t e s of herons twice about every 25 h. each day i s and low  followed about 8 h l a t e r by the lowest t i d e .  tide complete the 25 h c y c l e .  gradually amplitude  to  An intermediate  in December.  about 4.5 m. The number of minutes  eelgrass beds (<1.7  The highest tide high  Within a year the lowest tides change  from midday in June to midnight is  the  The maximum t i d a l  that low tides  uncovered  m) each day at Sidney and the Fraser was derived from a  computer model of predicted tides (A. Mau, pers. comm.). Time of Breeding - Great Blue Herons throw hatched eggshells from their nests and the chicks begin c a l l i n g soon after hatching (Brandman 1976; obs.).  Once the f i r s t  s h e l l s in the colony.  pers.  chick was heard, I made nearly d a i l y v i s i t s to gather Two observers were used to reduce the total search time  in the colony to about ten minutes.  I used the date when the f i r s t  found below a nest as the date of hatching.  shell was  On days when I was unable  to  v i s i t the colony, I gathered eggshells the following day and assigned them a hatching date based on the  amount of faeces splattered  on them and on the  freshness of the inner membranes. Reproductive Success - In 1986 I counted the number of in nests at c o l o n y - s i t e s : University  of  Washington),  British  fledglings raised  on Sidney Island, near the town of Crofton, in the Columbia  and beside the  Endowment  Nicomekl  Lands,  on  Point  Roberts  River near Crescent Beach ( F i g .  (in 2-1).  Volunteers gathered data on the number of nests used by herons and the number of chicks they produced at 23 other colony-sites on the B r i t i s h Columbia coast in  1987.  A. Breault  In  1988-89, these and other colony-sites were v i s i t e d by me and  (pers. comm.) to  record nesting data.  17  I April  numbered  all  nest  1987-90 b e f o r e  plates  to  within  sight  1985).  all  the  trees or  trees  at  Sidney with a f e l t  eggs were l a i d .  with n e s t s .  earshot  of  I later  Herons  their  nests  To reduce t h i s d i s t u r b a n c e  are  in  polyethylene  p l a s t i c b l i n d was b u i l t  beginning  1988  and 1989.  at the b e a c h .  and had no  numbered  sensitive  season ( e g .  observed  on the ground at the s o u t h e a s t c o r n e r  on the s e t t l e m e n t  obscured n e s t s . sides of taken  the  about  considered  or  when  a 100 m covered walkway  vantage  once each week u n t i l have  used a l l  Adult Herons -  points  along  a s s i g n the age  the  nests  the  Fraser  herons found i n  determined u s i n g the  'gradicule  culmen l e n g t h s  of a t e l e s c o p e .  chicks  that  The d i s t a n c e  beak  accuracy  including  the  active  during  directly  feedings  at  angle  of  the  These counts were  became f l e d g l i n g s .  Herons  eggs or  had  chicks,  or  were  hatched  them. 1988  to  and major  30 A p r i l  roost  1989  and f e e d i n g  I also o p p o r t u n i s t i c a l l y  (Butler  a gradicule  et  al.  sites  a s s i g n e d the  the d e l t a over the same p e r i o d . method'  I drove  1990).  to to age  The sex was  This  technique  s c a l e mounted i n the  eye-piece  between the heron and the t e l e s c o p e i s measured  and the  The  held  River  against  using a tape-measure, lengths.  became  From 15 October  and sex to h e r o n s .  and sex o f o t h e r  estimates  nests  or n e s t i n g success of h e r o n s .  and a count was made u s i n g b i n o c u l a r s .  e g g s h e l l s o r f e c e s on the ground below Sex of  of  For many n e s t s , two o b s e r v e r s used vantage p o i n t s on o p p o s i t e  nest  to  they  to  A black  T h i s b l i n d was used t o observe herons at t h e i r  nests,  al.  i n 1987  the eggs h a t c h e d .  Access was gained v i a  noticeable effect  easily  humans  Vos et  About one month a f t e r h a t c h i n g the s u r v i v i n g c h i c k s were counted in  early  aluminum to  I r e s t r i c t e d most c o l o n y v i s i t s  50 m o f the edge o f the c o l o n y u n t i l  colony in  very  the  w i t h i n about  the  attached  often  early  pen i n March and  g r a d i c u l e - s c a l e measure i s c o n v e r t e d i n t o of  culmen  this  method  relative  18  to  depends the  on  several  sight-line  actual factors  through  the  telescope and the distance  between  heron and observer.  Nevertheless,  method allowed me to estimate the sex of about three-quarters  the  of the herons  within 65 m with 95% confidence. Records for the 25% of birds of unknown sex were discarded.  19  CHAPTER THREE.  THE EFFECT OF PREDATORS AND FORAGING HABITAT ON COLONY-SITE SPACING IN GREAT BLUE HERONS  In  the  next  two chapters,  Herons select habitats.  I  examine factors  affecting  how Great Blue  In this chapter I focus on the relationship between  the d i s t r i b u t i o n of foraging habitat and presence of predators on the spacing of heron c o l o n y - s i t e s . Lack (1954, 1968)  postulated that food a v a i l a b i l i t y  was more  important  than predators in determining the location of most breeding colonies of b i r d s . Several studies have shown that colonies are located near food supplies Perrins and Birkhead 1983, Kushlan 1976b, Gibbs et al. Lack (1954, 1968).  However,  as predicted by  predators are often d i f f i c u l t to locate or t h e i r  populations  reduced  understood.  Predators cause c o l o n y - s i t e abandonments in herons (Dusi and Dusi  1968,  Simpson 1984)  so  1987)  (see  their  impact  on  colony-site  spacing  is  poorly  and might affect the spacing of t h e i r c o l o n y - s i t e s .  The Great Blue Heron in B r i t i s h Columbia i s suitable for exploring the relationship between food a v a i l a b i l i t y , d i s t r i b u t i o n of site  spacing because:  (i)  colony-sites can be located by following herons  returning from feeding areas, the Bald Eagle  (Haliaeetus  predators and colony-  (ii)  the major predator in B r i t i s h Columbia is  leucocephalus)  (Vermeer et al.  1989) which nests in conspicuous places, and ( i i i )  1988, Norman et  al.  most female herons feed on  i n t e r t i d a l beaches near the colony-sites during the breeding season (Chapter 4) and  only females provision chicks under 3 weeks of age.  tested  were that  alternately  (ii)  herons located  colony-sites:  away from predators.  20  (i)  near  The hypotheses I food supplies  or,  Predictions Food s u p p l i e s (i)  More herons should breed near h a b i t a t s near h a b i t a t s  (ii)  Fledged  with low prey  brood-size  should  with high prey a v a i l a b i l i t y  than  availability; decline,  and  should i n c r e a s e , with d i s t a n c e between  the  the  number  of  deserted  food supply and the  nests  colony-  site; (iii)  The number o f b r e e d i n g herons should be p o s i t i v e l y area o f f o r a g i n g  c o r r e l a t e d with  the  habitat;  Predators (iv)  Fewer herons should breed i n areas f r e q u e n t l y infrequently  (v)  used by e a g l e s than  areas  used by e a g l e s ;  Fledged b r o o d - s i z e s and the number o f s u c c e s s f u l n e s t i n g p a i r s should be greater  in  colonies  outside  eagle  territories  than  inside  eagle  territories.  METHODS Great Georgia,  Blue  Herons  British  were  Columbia  fledged b r o o d - s i z e s , estimate l o c a t e c o l o n y - s i t e s are  studied (Fig. the  at  colony-sites  3-1).  General  around  methods  number o f n e s t s used at  g i v e n i n Chapter  the  Strait  used t o :  of  estimate  a c o l o n y - s i t e , and  2.  Prey A v a i l a b i l i t y I (mostly prey  assumed t h a t shallow beaches with e e l g r a s s (Zostera Nereocystis  availability  assumption  is  luetkeana whereas  discussed  and Laminaria  other  later.  habitats All  spp.)  21  beds,  kelp  beds and marshes had high  had low  eelgrass  marina)  beds,  prey  availability.  kelp  beds  This  and marshes  >10  ha  in  area  in  the  Strait  p u b l i s h e d maps (Hutchinson  or  1982,  Georgia  were  Hutchinson et  located  al.  from  searches  and  1989).  Distance between colonies and feeding s i t e s Most (Chapter major  breeding 4).  Herons  feeding  distance  herons  feed  departing  location  when  at  one  from  22  their  major  site  near  their  c o l o n y - s i t e s were  nests  held  eggs  and  colony-site  followed small  to  their  chicks.  The  between the c o l o n y - s i t e and c e n t r e o f the f e e d i n g s i t e was measured  on 1:50,000 s c a l e maps.  Distance between heron colonies and eagle nests I  assumed t h a t  predation Eagle  pressure  nests  (Vermeer  et  territories 1987). be  the  were al.  extend  an  by  ground  Bald  (6  and by p u b l i c  outwards  for  Eagle  to  was  T h i s assumption cases)  request  up  nests  and (74  air  is  index  discussed  searches  cases).  3 km around  a good  of  later.  (113  cases)  Bald Eagle  feeding  their  nests  (Stalmaster  heron c o l o n y - s i t e s >3 km from an e a g l e nest were assumed to  eagle's  p r e d a t o r s , whereas ' n e a r ' assumed t o be  of  heron c o l o n i e s .  found  1989)  Therefore,  outside  at  proximity  territory  and  were  considered  to  be  'far'  from  c o l o n y - s i t e were l o c a t e d <3 km from an e a g l e nest and  w i t h i n an e a g l e ' s  territory.  Eagle-nest abundance I assumed t h a t e a g l e s o c c u r r e d i n high abundance where 3 or more n e s t i n g territories  were  contiguous  (i.e.  nests  o c c u r r e d i n low abundance where t h e i r (i.e.  n e s t s where >6 km a p a r t on  were  <6 km a p a r t  nesting t e r r i t o r i e s  average).  22  on  average)  were not  and  contiguous  RESULTS Distance between c o l o n y - s i t e s and feeding s i t e s Colony-sites were near feeding grounds; the average distance was 2.3 km (S.D.=1.3, N=22). pairs)  Of 33 colony s i t e s used in 1987-89, a l l but four small  colonies were near  marshes ( F i g . 3-1). However,  all  The  shallow beaches with kelp beds, eelgrass beds or feeding s i t e s of these four colonies were unknown.  of them were within 2 km of the  herons foraged.  (5-40  Three apparently  suitable  Strait  feeding areas had no colonies  nearby during t h i s study but have been used in the The number of breeding pairs correlated  of Georgia where lone  past (Forbes et al.  1985b).  p o s i t i v e l y and s i g n i f i c a n t l y with the  area of the feeding s i t e s used by each  colony ( F i g . 3-2).  Brood size and  nesting success were not s i g n i f i c a n t l y  correlated with the f l i g h t  between the c o l o n y - s i t e and feeding area  ( F i g . 3-3),  distance  although brood sizes were  weakly negatively correlated with f l i g h t distance as predicted. These feeding  findings  areas  with  support abundant  the  prediction  prey,  and  the  that  colonies  number  of  p o s i t i v e l y correlated with the area of foraging habitat. fail  are  breeding  sited pairs  near is  However, my results  to support the prediction that reproductive success i s  related to the  f l i g h t distance between colony-sites and feeding areas.  Predation There was no support for the hypothesis that c o l o n y - s i t e s were located far from eagle nests ( F i g . 3-1).  Four-hundred and ninety-seven herons nested in  23  Figure 3-1.  L o c a t i o n of Great Blue Heron c o l o n y - s i t e s (closed circles), k e l p b e d s , e e l g r a s s beds and marshes >10 ha (arrow) i n r e l a t i o n to Bald E a g l e n e s t i n g abundance. Dashed l i n e s e n c l o s e areas where e a g l e n e s t s averaged l e s s than 6 km apart (high abundance) and d o t t e d l i n e s e n c l o s e areas where eagle nests averaged fewer than one every 6 km (low abundance).  24  1200' 1000  y m 19.9 + 0.2x r =0.796 2  JS  800'  <  UJ  600-  <  400H 200-  50  100  150  200  250  300  NUMBER O F B R E E D E R S  Figure 3-2.  Numbers of breeding pairs of herons and the area of t h e i r foraging s i t e s in 1988.  25  UJ N CO  o o o  2-  cc  00  2  4  6  DISTANCE (km)  100  co  B  co 80, UJ'  oO  60  »  40,  204  "I  I  "  2 . 4  'n 6  DISTANCE, (km)  Figure 3-3.  Mean brood size (A) and percentage of occupied nests that held one or more f l e d g l i n g (B), in ten heron colony-sites versus distance to the major feeding s i t e in 1988. Large dot in A = 2 colonys i t e s ; a l l other dots represent 1 c o l o n y - s i t e . 26  19 c o l o n i e s i n areas where e a g l e s o c c u r r e d i n high abundance, nested  in  13 c o l o n i e s  proportion  of  in  colony-site  low  abundance e a g l e  abandonments  areas.  was a c t u a l l y  abundance e a g l e areas than i n low abundance areas Victoria,  a  away. These e a g l e s r e g u l a r l y  and 1990.  The  c o l o n y abandoned i t s eggs i n 1990.  colony  all  5  years  of  this  d u r i n g the egg stage i n 1989 killed  importantly,  slightly  (Table  herons  3-1).  lower  in  by e a g l e s  At one s i t e  in  i s l a n d s where  1989  abandoned a f t e r one attempt and Eagles a l s o attacked 23 c o l o n y - s i t e s i n  herons i n  At l e a s t 1990.  3 adult In  1989,  site  third  time.  In  1990,  twice  herons were many herons  few had been seen i n 1987-88 so I  the herons d i d not attempt to nest a  1989  E a g l e s a t t a c k e d the Sidney  The c o l o n y abandoned the  and 5 i n  in  nested  a t t a c k e d the herons i n 1988,  and once i n 1990.  i n the c o l o n y  were seen on nearby  study.  the high-  p a i r o f e a g l e s nested i n the c o l o n y - s i t e and a second p a i r  l e s s than 1 km  in  More  and 515  presume  the Sidney herons  r e n e s t e d about 6 km away on Vancouver  Island.  7 o f 21 o t h e r c o l o n y - s i t e s i n 1988 and i n 9 of  1989.  Although e a g l e s sometimes cause abandonments o f e n t i r e c o l o n y - s i t e s , t h e r e is  no  evidence  reproductive  that  the  3-2).  was n e a r l y  of  nesting  eagles  affected  the  average  success i n heron c o l o n i e s t h a t r a i s e d f l e d g l i n g - a g e d c h i c k s .  mean brood s i z e was n e a r l y (Table  proximity  identical  More i m p o r t a n t l y ,  identical  The  i n c o l o n i e s near and f a r from eagle nests  the mean percentage o f s u c c e s s f u l n e s t i n g p a i r s  i n c o l o n i e s near and f a r from n e s t i n g e a g l e s (Table  27  3-2).  Table 3-1.  Number o f s u c c e s s f u l and f a i l e d heron c o l o n y - s i t e s nested i n high (nests <6 km a p a r t ) and low (nests abundance i n 1987-89.  Eagle-nest  abundance  Success  Fail  High  23  5  Low  17  6  28  where eagles >6 km a p a r t )  Table 3-2.  Percentage o f s u c c e s s f u l (>1 f l e d g l i n g ) n e s t i n g p a i r s and mean brood s i z e s o f c o l o n y - s i t e s t h a t produced f l e d g l i n g s near (<3 km) and f a r (>3 km) from an o c c u p i e d Bald Eagle nest i n 1989.  Near  Percent  success  Brood s i z e  Mean  SD  81.1  11.9  2.6  0.4  29  Far Mean  SD  N  12  85.1  13.7  7  12  2.6  0.4  7  N  DISCUSSION V a l i d i t y of assumptions This' analysis was based on two key assumptions. prey " a v a i l a b i l i t y  was greater  First,  I assumed that  on beaches with kelp beds, eelgrass beds  marshes than on beaches without these habitats.  This assumption is supported  by several d i s t r i b u t i o n studies of prey ,species eaten by herons in the of Georgia (see Hughes 1985, Gordon and Levings 1984, Hay et a / . I  feel  less  confident  with  the  assumption  that  Bald  Strait  1989).  Eagle  nesting  abundance was a good measure of predation pressure in heron c o l o n i e s . my analysis of heron nesting  First,  success might have been more sensitive i f  used more than 2 categories of eagle nest abundance. lotor),  Red-tailed Hawks (Buteo jamaicensis),  and  I had  Second, raccoons {Procyon  and Turkey Vultures  (Cathartes  aura) also occasionally prey upon heron eggs and chicks in B r i t i s h Columbia (Simpson et  al.  1987,  measure t h e i r impact.  A. Breault,  pers.  comm., pers.  obs.)  but  I  did not  Most importantly, the proximity of nesting eagles might  be a poor measure of predator pressure.  For example, the 1990 abandonment at  Sidney occurred after an immature-plumaged eagle k i l l e d an adult heron in the colony although a pair of eagles nested less than 1 km away.  Thus, the number  of attacks on herons by eagles might be unrelated to the proximity of occupied eyries.  On 9-10 A p r i l 1987 when female herons  in most c o l o n y - s i t e s , over one-third  (293  from  Islands  aircraft  in  the  Southern  Gulf  were laying or incubating eggs  out of 797) were  of the eagles counted  immature-plumaged  birds.  Moreover, the abundance of eagles in t h i s part of B r i t i s h Columbia might be so great  that herons can not avoid them.  There was s l i g h t l y more than one  eagle per km of coastline on average (797 eagles along 750 km of shoreline) the southern Gulf Islands in 1987 (data in Vermeer et al.  30  1989).  in  Food and Predators as Determinants o f Colony L o c a t i o n Several  s t u d i e s have shown t h a t wading b i r d s  Osborne 1978)  i n c l u d i n g herons  Simpson et al. first is  1987)  ( F a s o l a and B a r b i e r i  nest near p r o d u c t i v e f o r a g i n g  to my knowledge  unrelated  (Kushlan  that  1977,  C u s t e r and  Gibbs et  areas  shows t h a t the d i s t r i b u t i o n  to the d e n s i t y o f breeding  1976b,  but  al.  1987,  my study i s  the  o f heron c o l o n y - s i t e s  eagles.  Many heron c o l o n y - s i t e s were l o c a t e d near eagle n e s t s presumably because both  s p e c i e s have  wide  assortment  similar  o f prey  habitat  (mostly  needs.  Eagles i n  vertebrates)  near  (Hughes  1985,  habitats  foraging Hay  al.  (Fig.  1989,  3-1)  al.  resulted  colony-sites  1989).  with  Chapter 5 ) .  (1987) suggested t h a t the  from d i s p e r s e r s s e t t l i n g  where  they  would  their  model,  new  According  to  foraging  ranges  Eventually,  an  Heron c o l o n y - s i t e s are  abundance  These  of  get  s p a c i n g of  outside  full  colonies  neighbouring  assumes t h a t  sites  (Gibbs et al.  should  be a f u n c t i o n  than small o n e s .  small  fish  productive  the  access would  colonies  heron c o l o n y - s i t e s  foraging  to  settle  once  the  1987). of  herons d e p l e t e If  at  the  best  agree with Gibbs et  sites  of  were  the full.  coast.  food s u p p l i e s near t h e i r  size  since large  colonies  require  colony-  deplete  more  study even  their  the  spacing  food s u p p l i e s  Herons i n my study foraged mostly near the  31  food  herons.  (1987) t h a t food s u p p l y determines  o f c o l o n y - s i t e s but I doubt t h a t herons i n general measureable e x t e n t .  other  resources.  edges  c o l o n y - s i t e s were evenly spaced i n t h e i r  al.  of  in  herons d e p l e t e l o c a l food s u p p l i e s then spacing  colony  However,  range  unexploited  though the number o f breeding p a i r s ranged from 4 to 252  to a  of  biologically  c o l o n y - s i t e s would become e v e n l y - s p a c e d along the  T h i s model  I  a  a l s o a t t r a c t the prey s p e c i e s taken by e a g l e s .  Gibbs et Maine  et  habitats  Columbia eat  caught along s e a c o a s t s of which  herons are o f minor consequence (Vermeer et al. located  British  colony-  site  (Chapter 4)  and had i n s u f f i c i e n t time d u r i n g low t i d e s to d e p l e t e  food  s u p p l i e s markedly  (Chapter  their  5).  F l i g h t Distance and Reproductive Success A d u l t herons must i n c r e a s e t h e i r p r o v i s i o n i n g r a t e s to r a i s e l a r g e broods to f l e d g i n g age ( S u l l i v a n 1988). time a n d / o r reduce t h e i r  Thus, a d u l t s should i n c r e a s e t h e i r  own energy requirements  by d e c r e a s i n g t r a v e l  between the c o l o n y - s i t e and f o r a g i n g area (Orians and Pearson 1979, Turner 1982). as  Therefore,  i t makes i n t u i t i v e  foraging  Bryant and  sense f o r herons to nest as c l o s e  p o s s i b l e to food s u p p l i e s . All  c o l o n y - s i t e s were l o c a t e d too c l o s e to  food s u p p l i e s to  differences  in t h e i r  average f l e d g e d brood s i z e s .  statistical  tests  detect  small  near  to  food  their  feeding  detectable.  sites  It  is  before  unclear  t o f e e d young.  how f a r  differences  Herons c a r r y l a r g e  here  in  if  better  any  the power of  the  is  low because of  ones are  herons would  have  reproductive  success  amounts o f food each t r i p  Marion (1989) r e p o r t e d  reveal  from f e e d i n g g r o u n d s .  herons should a v o i d poorer s i t e s  supplies.  However,  a significant difference  sample s i z e s o f c o l o n y - s i t e s d i s t a n t Moreover,  individual  to  available nest  and make few  reproductive success.  Blue Herons  a c o l o n y - s i t e on the  success  than  distant-feeders  distant-feeding travelled  supplemented t h e i r (Clupea  harengus)  British  diet  (pers.  showed t h a t  Columbia c o a s t  birds,  but  comm.).  he  study.  32  trips  Great  had h i g h e r did  Moreover,  not  d i s t a n c e s to feeding  reproductive know  how  local-feeding  by making r e g u l a r t r i p s to tanks h o l d i n g l i v e  i n Simpson's (1984)  be  Gray Herons f e e d i n g up to flight  Simpson (1984)  from  should  38 km from c o l o n y - s i t e s i n France but he d i d not r e l a t e  near  time  far  herons herring  I  c o n c l u d e t h a t average n e s t i n g success i n c o l o n i e s i s u n r e l a t e d  to  the  d i s t a n c e to the f e e d i n g s i t e over the d i s t a n c e s I measured.  SUMMARY  1)  Great Blue Heron c o l o n y - s i t e s within  2)  The  6 km o f major f o r a g i n g  number  of  abandonments o f  breeding  location  evidence of  of  the S t r a i t o f G e o r g i a were l o c a t e d  areas.  pairs,  reproductive  success,  heron c o l o n y - s i t e s were s i m i l a r  eagle-nesting areas. the  (n=22) i n  number  low and high  c o l o n y - s i t e s were  o f predators,  not  supported  i n c l u d i n g Bald E a g l e s ,  but is  of  density  P r e d i c t i o n s from the h y p o t h e s i s t h a t p r e d a t o r s  heron  effects  in  or  more  affect direct  required  to  t e s t the h y p o t h e s i s f u r t h e r . 3)  The s p a c i n g o f heron c o l o n y - s i t e s i n B r i t i s h Columbia i s best e x p l a i n e d by the  distribution  of  shallow  beaches with  kelp  marshes where most herons foraged i n summer.  33  beds,  eelgrass  beds  and  CHAPTER FOUR.  SEASONAL PATTERNS OF HABITAT USE BY FORAGING GREAT BLUE HERONS  In the p r e v i o u s c h a p t e r I showed t h a t the s p a c i n g o f heron c o l o n y - s i t e s was e x p l a i n e d by the d i s t r i b u t i o n the  habitat  use  distribution One  to  view  increases,  of  the  1970,  foraging  herons  and  In  relate  habitat  best  selection  competitors  posits  that  establish  this  1972,  Sutherland  I  describe  shifts  in  this  availability.  as  the  exclusive  and Parker  density  feeding  i n d i v i d u a l s who then move to marginal  Fretwell  chapter,  seasonal  i n t r a s p e c i f i c c o m p e t i t i o n and food  dominate weaker Lucas  by  o f food s u p p l i e s .  foragers  territories  or  (Fretwell  and  habitats  1985,  of  Chapter  1).  Another  view p o s i t s t h a t an i n d i v i d u a l ' s f o r a g i n g s k i l l mostly determines where i t forage,  i n d e p e n d e n t l y o f the d e n s i t y o f c o m p e t i t o r s  Chapter  (Stephens and Krebs  of  the  hampered  foraging  by  in  and food a v a i l a b i l i t y  tracking  This is p a r t i c u l a r l y  review by Burger 1988) C u s t a r d and d i t  competition  difficulties  through the y e a r .  Durell  mobile  important  animals  between  since foraging  and c o m p e t i t i v e s k i l l s ( P a r t r i d g e 1987a,b)  hypotheses have  (Chapter  (3)  5),  others  juveniles  (2)  some defend f e e d i n g t e r r i t o r i e s alone  or  (see Goss-  in  are l e s s p r o f i c i e n t  groups at  to s t u d i e s  they feed y e a r - r o u n d i n open  can be observed and food i n g e s t i o n r a t e s  feed  skills  improve with age i n many a n i m a l s .  f o r a g i n g h a b i t a t s e l e c t i o n because: (1) where i n t e r a c t i o n s  habitats  and Green 1985,  Great Blue Herons i n B r i t i s h Columbia l e n d themselves w e l l  while  1986,  1).  Tests been  will  can be  (Brandman 1976,  (Krebs  f o r a g i n g than  1974, adults  Kushlan  of  habitats estimated  Bayer  1978)  1978),  and  (Quinney and Smith  1980). The aim o f t h i s c h a p t e r i s to r e l a t e the d i s p e r s i o n o f age- and s e x - c l a s s e s of  herons  across  availability  of  habitats foraging  to  the  time.  presence o f First,  34  I  intraspecific  describe  the  competitors  year-round  use  and of  habitats  by heron age- and s e x - c l a s s e s .  f o r a g i n g herons move between h a b i t a t s  Next,  I compare the hypotheses t h a t (i)  interference  from f o r a g i n g c o n s p e c i f i c s ( S u t h e r l a n d and Parker 1985), o r ( i i )  insufficient  foraging  time  foraging s k i l l  (Sullivan  1990)  of juvenile  i n autumn because o f :  d u r i n g low t i d e .  and a d u l t  Finally,  I  herons a f f e c t s t h e i r  discuss  how  the  survival.  Predictions Intraspecific foraging competition (i)  J u v e n i l e herons should d e p a r t from  adults  maintenance (ii)  reduces  ingestion  rates  below  when the  interference daily  energy  requirement;  Feeding t e r r i t o r i e s larger  energy  from f o r a g i n g h a b i t a t s  and h e a v i e r  should be defended by a d u l t (Simpson 1984)  than a d u l t  males because they  are  females and j u v e n i l e s ,  and  thus b e t t e r a b l e to exclude o t h e r age- and s e x - c l a s s e s .  Foraging time and foraging success (i)  Herons s h o u l d l e a v e f o r a g i n g h a b i t a t s when t h e r e tide  cycle  to  catch  enough  maintenance requirement (ii)  Juveniles  are  less  foraging habitats  food  there  to  i s too l i t t l e  meet  their  time per  daily  energy  therefore  vacate  (McNamara 1982);  proficient  foragers  b e f o r e a d u l t s i n autumn.  35  and  should  STUDY AREA AND METHODS Great Blue Herons were s t u d i e d mostly on Sidney I s l a n d d u r i n g the breeding season  and  d e s c r i b e the there ii)  how  in  the  Fraser  study areas I:  i)  River  delta  and general  estimated  tide  during  the  non-breeding  methods i n Chapter 2, heights,  diets  season.  and a l s o d e s c r i b e  and  foraging  determined use o f f o r a g i n g h a b i t a t s by breeding a d u l t s , i i i )  i n m a r s h l a n d s , on beaches, i n g r a s s l a n d s , and along r i v e r s ,  rates,  counted herons  and i v )  assigned  age and sex to h e r o n s . I e s t i m a t e the energy i n heron prey and the d a i l y maintenance needs o f herons i n Chapter  I  energy  5.  Habitat use of Age- and Sex-classes Breeding  season  foraging f l i g h t s  dispersion  -  The  directions  4 May) large  outgoing  recorded f l i g h t s  visited  about  once every  When n e s t s  during f i v e  two  the  Aprilheld  low t i d e s  and  i s l a n d were drawn  weeks  to  search  for  A r r i v a l and d e p a r t u r e d i r e c t i o n s at these s i t e s confirmed t h a t  the herons came from Sidney  Island.  Non-breeding season dispersion c a r and b o a t .  efforts  were uneven.  but  remains  it  I  c h i c k s (10-31 May).  B e a r i n g s o f major f l i g h t d i r e c t i o n s o f f  on a map and d e s t i n a t i o n s  an a i r p l a n e ,  and  I observed b i r d s on 4 days when most n e s t s h e l d eggs (12  c h i c k s (16 June-10 J u l y ) ,  feeding herons.  incoming  and 17 high t i d e s on 16 days at 3 times  and 4 days when n e s t s h e l d small  one high t i d e .  all  by breeding a d u l t s from the Sidney c o l o n y - s i t e were recorded  from dawn to dusk d u r i n g 19 low t i d e s i n the s e a s o n .  of  open  I censused herons i n p o t e n t i a l  These covered a l l  heron h a b i t a t s  I c o n s i d e r these h a b i t a t s that  some herons  habitats.  36  might  habitats  although census  were most important  have  been  from  overlooked  to herons in  other  Herons feed on beaches, i n marshlands and g r a s s l a n d s and along r i v e r banks i n the F r a s e r R i v e r d e l t a . o f the  Herons were p l o t t e d on maps d u r i n g a e r i a l  F r a s e r R i v e r d e l t a beaches and marshlands ( F i g .  1986 and February 1987.  2-3)  between  censuses September  N e a r e s t - n e i g h b o u r d i s t a n c e s were l a t e r measured from  these maps.  A 300 m b u f f e r  edge-effect  bias  (Krebs  zone was drawn around the census areas to  1989).  An  index  of  aggregation  and  reduce  tests  of  s i g n i f i c a n c e f o r i t s d e v i a t i o n from randomness were d e r i v e d f o r marsh and beach habitats  (pp.  126-129 i n Krebs 1989).  Habitat Selection I n t r a s p e c i f i c f o r a g i n g competition Interference  -  indicator  intraspecific  the on  of  I  used  interference,  f r e q u e n c y and d u r a t i o n beaches i n the  1987  interference  competition of chases,  for  territorial  d i s p l a y s and f i g h t s  Fraser River d e l t a  was q u a n t i f i e d  behaviour,  food and f o r a g i n g  and 1988 breeding season at  3 November 1987-89 in the this  including  of  the  I  foraging  reduction  in  Territoriality  herons on the  c o a s t was about cases). (i)  (Bayer  defended by 32  1978)  and about  I assumed t h a t o t h e r f e e d i n g t e r r i t o r i e s  single  cases),  200 m long  territory  (ii)  herons  repeatedly  used a s i t e  a heron I d i s t u r b e d r e t u r n e d  used by o t h e r herons (3 c a s e s ) , or ( i i i ) unoccupied  habitat  extended  on  one  or  cases).  37  Oregon  e x i s t e d i n my study area by n e i g h b o u r i n g same s i t e  of  interactions.  300 m i n my study area  avoided to the  herons  Metabolized  from  The s m a l l e s t  noted  The e f f e c t  Energy (ME,see Chapter 5) to a f o r a g i n g heron r e s u l t i n g -  an  Sidney and from 1 August-  (see Chapter 2 ) .  by e s t i m a t i n g  sites.  as  herons  (6 if: (4  and avoided areas  a t o t a l o f 200 m or more o f continuous both  sides  of  a  solitary  heron  (25  Habitats (Fig.  2-3)  and the l o c a t i o n s o f herons along the banks o f the F r a s e r R i v e r  were mapped from a boat on 10 January 1990.  than 200 m l o n g were e l i m i n a t e d  from the  Habitat  segments l e s s  a n a l y s i s because they were  to be l a r g e enough to h o l d a heron t e r r i t o r y  unlikely  (see Bayer 1978).  Relative a v a i l a b i l i t y of foraging time Relative available  time  availability required  by  maintenance  energy  season.  assumed t h a t  I  needs  of  f o r a g i n g time  adult  and j u v e n i l e  on beaches adult  i s the  and  and  in  proportion of  herons  to  meet  grasslands in  juveniles  required  the  the  the  total  their  daily  non-breeding  same  amount  time  to  of  maintenance energy because they have s i m i l a r body masses. Beaches -  I  estimated  the  relative  availability  of  f o r a g i n g on beaches i n September-November u s i n g the F  i  foraging formula:  + <vyj  =  herons  xioo  M where Fj. i s the percentage o f the low t i d e f o r a g i n g p e r i o d r e q u i r e d by a heron to meet  its  d a i l y maintenance energy i n each 24 h p e r i o d , M i s the  energy (kJ)  r e q u i r e d by herons f o r d a i l y energy maintenance,  respective  ingestion  rates  during  day  and  night,  and  l"  amount o f  1^ and I d  and  T  n  n  are the are  the  r e s p e c t i v e number o f minutes o f low t i d e s a v a i l a b l e f o r f o r a g i n g d u r i n g day and night.  Each o f these terms i n the e q u a t i o n i s now e x p l a i n e d i n  Maintenance  energy  maintenance  (=1560 kJ)  adult  and j u v e n i l e  (M)  -  I  estimated  the  u s i n g methods o u t l i n e d  herons had s i m i l a r  amount  of  energy  i n Chapter 5.  I  detail. needed  for  assumed t h a t  energy maintenance needs because they  have s i m i l a r body masses. Ingestion rates (I) minute  of  foraging  -  I n g e s t i o n r a t e i s the amount o f energy (kJ) by  an  adult  and  juvenile 38  heron.  It  was  consumed per estimated  by  multiplying  foraging  rates  of adult  average amount o f energy i n t h e i r  and j u v e n i l e  respective diets  herons  (see below)  by the  (see Methods i n Chapter 5 ) .  ' F o r a g i n g r a t e s o f a d u l t and j u v e n i l e herons on beaches on the F r a s e r R i v e r 3  -  d e l t a were e s t i m a t e d  on 33 days between 1 August and 3 November 1987-89.  The  mean c a p t u r e r a t e d u r i n g high t i d e s at Sidney i n May-June was 3.6 times  faster  during  night  (x=9.4  f i s h caught d u r i n g the day and n i g h t  are the  the  day  mins./fish, same s i z e ,  (x=2.6  S.E.=2.1,  mins/fish,  N=9).  If  S.E.=0.3,  N=81)  than  at  then herons i n g e s t e d about 3.6 times more energy p e r minute  while  f o r a g i n g d u r i n g the day as at n i g h t from August t o November i n t h e F r a s e r R i v e r delta.  T h i s assumption i s d i s c u s s e d l a t e r .  I assumed t h a t herons caught the  same s p e c i e s o f f i s h d u r i n g n i g h t and day on the F r a s e r R i v e r d e l t a because sea perch and s c u l p i n s were caught i n beach s e i n e s d u r i n g day and n i g h t i n e e l g r a s s beds i n the Yaquina e s t u a r y ,  Foraging time (T)  Oregon (Bayer  - The number o f minutes t h a t low (<1.7 m) t i d e s uncovered the  eelgrass  beds each 24h from  estimate  o f the t o t a l .amount o f a v a i l a b l e  Grasslands  -  1985b).  1 September  I e s t i m a t e d the r e l a t i v e  F  f o r a g i n g time on beaches.  availability  f e e d i n g on Townsend's v o l e s (Microtus January u s i n g t h e f o r m u l a :  t o 30 November- 1987 was used as an  townsendii)  t =IiWl  o f f o r a g i n g time o f herons i n g r a s s l a n d s i n November -  x 100  M where  F^. i s the percentage  energy  maintenance  maintenance rate day.  need,  M is  the  estimated  by a heron t o meet amount  of  energy  its  daily  needed  for  (=1560 kJ) u s i n g methods o u t l i n e d i n Chapter 5, I i s the i n g e s t i o n  (kJ) o f herons e a t i n g How I determined  Ingestion  o f the day r e q u i r e d  rates  (I)  -  voles,  and T i s the number o f d a y l i g h t  ingestion rates  i s now e x p l a i n e d  Ingestion  (1^)  rates  hours per  in d e t a i l .  d u r i n g the day were estimated by  m u l t i p l y i n g the number o f v o l e s caught per minute o f g r a s s l a n d f o r a g i n g by the 39  amount o f energy  (kJ)  c o n t a i n e d i n an average v o l e .  I watched 54 herons  v o l e s i n g r a s s l a n d s between 18 November 1985 and 17 January 1987. the  average  amount o f m e t a b o l i z a b l e  by c o n v e r t i n g units  (kJ).  the  mean weight  (56g,SD  V o l e s were caught  the  i n the  of  herons  k J / g dry wt) digested  78%  61  Fraser River d e l t a .  the  energy  in  each  be  dry wt  x 23.5  assumed t h a t :  et  (67%)  and  1984), and al.  1989).  i n a v o l e eaten by a heron digestive  assumed t h a t the d e c l i n e i n numbers o f herons along an 89.4  km census  339kJ.  (56g x 0.33  I  (Castro  energy  in g r a s s l a n d s  Wijnandts'  vole  voles  x 0.78  efficiency=)  to  into  same amount of water  T h e r e f o r e , the average m e t a b o l i z e d energy a v a i l a b l e was e s t i m a t e d  v o l e s caught  as the Common v o l e (M. arvalis, of  estimated  to a heron e a t i n g  on 2 December 1986  average Townsend's v o l e c o n t a i n e d the  energy (23.5 (2)  = 12)  in l i v e - t r a p s  used by herons on Westham I s l a n d (1)  energy a v a i l a b l e  I  hunt  k J / g dry wt  These assumptions are d i s c u s s e d  later.  Habitat S h i f t s , Dispersal and Mortality I route  (Fig.  River  delta.  2-3)  approximated  This  their  assumption  is  death  and d i s p e r s a l  discussed  later.  r a t e from the  Four road  censuses were  conducted each month between August and February 1987-88, except (3 c e n s u s e s ) , November  (5  censuses) and August (6 c e n s u s e s ) .  herons found by me or o t h e r s fat  i n the  s t o r e s and d e p l e t e d p e c t o r a l  Fraser River d e l t a  Fraser  i n September  Emaciated dead  t h a t had no abdominal  muscle mass were assumed to have s t a r v e d .  RESULTS Spacing of Age- and Sex-classes Breeding season - During low t i d e s , most a d u l t lagoon (Table  4-1).  herons foraged i n nearby Sidney  Herons f l y i n g o f f the i s l a n d fed i n e e l g r a s s and kelp beds  on n e i g h b o u r i n g i s l a n d s w i t h i n 10 km o f the c o l o n y .  40  Table 4 - 1 .  Percentage o f f l i g h t s t o and from the Sidney lagoon versus other s i t e s o f f the i s l a n d d u r i n g low t i d e s (<1.7 m) and high t i d e s when the n e s t s h e l d e g g s , small c h i c k s and l a r g e c h i c k s .  Low t i d e s Stage  Percent  High N~  tides  Percent  N  Egg  94.1  271  27.2  213  Small c h i c k  90.5  541  51.3  298  Large c h i c k  62.6  460  10.3  78  41  During high t i d e s ,  herons with eggs and l a r g e  the lagoon and f l e w mostly o f f estuaries However,  the  i s l a n d (Table  4-1)  an average o f 14 km (SD=8, range=6-27, n=13) d u r i n g the small  chick stage,  were t o and from the lagoon (Table Only females 31 May 1988.  about h a l f  (2 c a s e s ) were f i r s t  identified  i s l a n d in l a t e afternoon  the  cases).  the f l i g h t s  themselves and t h e i r  and e a r l y  tide  and the  1 June.  fed  c h i c k s on f i s h caught i n the lagoon d u r i n g the day when  chicks,  both parents  lagoon d u r i n g low t i d e and o f f Y e a r l i n g herons r a r e l y  1989  evening b e f o r e  the  i s l a n d in  the  the f o l l o w i n g morning when t i d e s were mostly h i g h .  nests held large  1987,  high  c h i c k s most females  t i d e s were low and males fed themselves mostly o f f  season.  at  colony-site.  Seven male herons and no females were  c o n c l u d e t h a t when n e s t s h e l d eggs and small  until  from the  seen i n the lagoon on 1 June in  (11  afternoon  to beaches and  (18 c a s e s ) were r e c o r d e d i n the lagoon between 9 A p r i l  Males  off  avoided  4-1).  company o f females  I  chicks in nests  the  fed the c h i c k s f i s h caught  i s l a n d d u r i n g high  When  in  the  tides.  v i s i t e d Sidney I s l a n d d u r i n g the  breeding  One was seen i n the lagoon f o r about 3 weeks i n l a t e J u n e - J u l y in  1988  and 1989.  but d i d  A yearling  not f i n d  a mate (I.  among a d u l t s on f o r t n i g h t l y  o c c u p i e d a nest  i n the c o l o n y i n J u n e - J u l y  M o u l , p e r s . comm.).  I found 1-8  v i s i t s made i n M a y - J u l y 1988 t o 4 e s t u a r i e s  6 beaches up t o 27 km away from Sidney I s l a n d and on beaches and marshlands i n the  yearlings  Fraser River  and  in  delta.  Non-breeding season - A d u l t female and j u v e n i l e herons i n the F r a s e r R i v e r delta mainly (Fig.  shifted (56%) 4-1).  from f e e d i n g mainly  (92%)  i n marshlands i n J a n u a r y .  on the beaches i n August to T h i s t r e n d was r e v e r s e d  The number o f herons counted on the beaches at  August 1986 to A p r i l  1987 was s i g n i f i c a n t l y  42  in  feeding spring  low t i d e  ( t = 7 . 6 , p<0.001) and  from  positively  100  MARSHLANDS  j  no data  BEACHES 20 t» 10 CU  eg i»  10  to N  N  ca co  © ©  CO  T-  N  D  ^ •* w  J  to N  to  10  M  MONTH  Figure 4-1.  Percentage o f herons u s i n g marshlands and beaches i n the F r a s e r R i v e r d e l t a between August and A p r i l 1986-87. Numbers above months i n d i c a t e the number o f herons counted i n t h a t month.  43  c o r r e l a t e d with the number o f hours o f a v a i l a b l e tide  (Fig.  4-2).  low  The marshlands were seldom covered by more than 30 cm o f  water f o r l o n g e r than 2h each day. used g r a s s l a n d s r a t h e r  S i g n i f i c a n t l y more j u v e n i l e s  than marshlands (Table  4-2).  clumped on the beaches and i n marshlands between (Table  f o r a g i n g itime d u r i n g  than  adults  Herons were mostly  September and February  4-3). In  October,  summary, herons used beach h a b i t a t s  mainly  from March to  and i n c r e a s i n g l y used marshlands and g r a s s l a n d s from November  to  January a f t e r which they began to r e t u r n t o beaches.  Habitat Selection I n t r a s p e c i f i c f o r a g i n g competition Interference the d a y .  - Most breeding herons fed p e a c e f u l l y  I saw one i n t e r a c t i o n  non-schooling f i s h .  i n over 2700 mins. o f watching  example,  d i d not r e s u l t  on 28-29 June 1988  I recorded 41 i n t e r a c t i o n s  D i s p l a c e d b i r d s f l e w a few meters  perch per m i n . 100.4  aggregata).  flocked  However,  i n herons being chased from the l a g o o n .  herons t h a t pursued perch f o r 23 m i n s .  about 30s l a t e r .  herons catch  I n t e r f e r e n c e i n c r e a s e d when herons b r i e f l y  pursue s c h o o l s o f S h i n e r Sea Perch (Cymatogaster interferences  i n Sidney lagoon d u r i n g  (0.04  between  kJ o f M e t a b o l i z e a b l e  x 100.4  o f an i n t e r f e r e n c e interferences/min  For  about  50  interferences/heron/min.).  away and began f e e d i n g on sea perch  Each sea perch c o n t a i n s an  again  Energy (ME)  (Table  5-2,  k J / s e a perch=) 20.1 was (0.5 x 20.1  kJ/min.  Chapter 5 ) .  The estimated  Therefore, (0.2  energetic  min. to s t a r t f o r a g i n g per i n t e r f e r e n c e  kJ/min.=) 0.4  sea  estimated  the e s t i m a t e d ME i n t a k e r a t e f o r herons f e e d i n g on sea perch was fish/min.  these  The median c a p t u r e r a t e i n 27 o f these groups was 0.2  (range=0.04-l.7).  to  x  cost 0.04  k J / m i n / h e r o n o r 2% o f the t o t a l ME  44  500  o  £  30  o  o CC LL) CQ  ID 100  100 NUMBER  F i g u r e 4-2.  200  300  OF FORAGING, HOURS  Numbers o f Great Blue Herons counted on beaches i n the F r a s e r R i v e r d e l t a in r e l a t i o n to the average number o f f o r a g i n g hours at low t i d e (<2.3 m) from August 1986 to A p r i l 1987.  45  Table 4-2.  Number o f a d u l t and j u v e n i l e herons counted i n g r a s s l a n d s and marshlands o f the F r a s e r R i v e r d e l t a between J u l y 1987-March 1988 and J u l y - S e p t e m b e r 1988.  Adult  Juvenile  Grasslands  698  302  Marshlands  158  16  46  Table 4-3.  I n d i c e s o f a g g r e g a t i o n o f Great Blue Herons f e e d i n g i n marshlands and on i n t e r t i d a l beaches o f the F r a s e r R i v e r d e l t a d u r i n g the n o n - b r e e d i n g s e a s o n . The s p a t i a l p a t t e r n ranges f r o m clumped (R approaches zero) to r e g u l a r , (R approaches 2 . 1 5 ) . When the p a t t e r n i s random, R=l (Krebs 1989).  Index o f A g g r e g a t i o n Year  Date  Marshlands  (R)  n  Beach  n  1986  30 29 12 27 10  Sept Oct Nov Nov Dec  0.53** 0.96 0.98 1.25** 0.80**  69 73 105 28 43  0.59** 0.87** 0.83** 0.98 1.20*  224 164 96 54 22  1987  6 21 6 20  Jan Jan Feb Feb  0.81** 0.95 0.97 0.63**  45 46 27 34  0.73** 0.57** 0.74** 0.61**  32 42 47 84  * = the s p a t i a l p a t t e r n i s s i g n i f i c a n t l y (p<0.05) and * * = h i g h l y s i g n i f i c a n t l y (p<0.01) d i f f e r e n t from random.  47  while  f e e d i n g on sea p e r c h .  average  ME i n t a k e  rate of  Therefore  these  interference  herons although  had l i t t l e  it  effect  on the  c o u l d have a f f e c t e d  some  non-average b i r d s who got c o n t i n u a l l y d i s p l a c e d . Adult  and  juvenile  significantly different fewer sea p e r c h . rates  of  adult  therefore,  The  and j u v e n i l e  Fraser  herons  by  delta  were  because j u v e n i l e s  about  1% o f  their  4-6).  densities  individual  of  ate  herons  and  I conclude t h a t  herons every  month o f  sexed along r i v e r b a n k s  significantly different  identified.  in  river-edge  t h a t c o u l d be viewed both day and n i g h t  the n o n - b r e e d i n g season were f e m a l e s , c o u l d not be c o n f i d e n t l y  (n=38) on the F r a s e r  occurred  the  year.  and a l l  out o f 28 a d u l t s whose sex c o u l d be determined  prediction that i t  rate  f o r a g i n g d i s t a n c e s but does  along r i v e r b a n k s  territorial  Nine t e r r i t o r i e s  herons c o u l d be c o n f i d e n t l y  (Fisher  average  i n autumn (Table 4 - 5 ) .  - Adults held t e r r i t o r i e s  were o c c u p i e d by s o l i t a r y  riverbanks  River  s h i f t s among herons f o r a g i n g on b e a c h e s .  highest  marshes (Table  the  i n A u g u s t / September (Table 4-4)  by c o n s p e c i f i c s m a i n t a i n s  Territoriality  on  I n t e r f e r e n c e from f o r a g i n g c o n s p e c i f i c s reduced the ME i n t a k e  not e x p l a i n h a b i t a t  Nineteen  diets  i n t e r f e r e n c e was unimportant  interference  River.  heron  Three  out  were a d u l t  of  5  males.  i n marshlands d u r i n g  one was a male and the None were t e r r i t o r i a l .  sex o f 8 o t h e r s These v a l u e s  are  from an expected even sex r a t i o i n marshlands and along  Exact T e s t X =11.8, 2  p<0!003).  These r e s u l t s  i s u s u a l l y a d u l t male herons who defend f e e d i n g  48  support  the  territories.  Table 4-4.  Number o f each prey s p e c i e s caught by a d u l t and j u v e n i l e Great Blue Herons i n August and September 1987, 1988 and 1990 (N=33 days).  Adult Number  Prey s p e c i e s  Perch Sculpins Gunnels Others  82 39 4 5  2  %  Number  63.0 30.0 3.1 3.9  27 40 3 7  130  Total  X =15.7,  Juvenile  df=3,  77  p=0.002  49  % 35.1 52.0 3.9 9.0  Table 4-5.  D a i l y M e t a b o l i z a b l e Energy (ME) i n t a k e i n kJ l o s t i n t e r f e r e n c e by c o n s p e c i f i c s i n autumn.  Adult  to  Juvenile  MeanS.E.  Mean  S.E.  E s t i m a t e d ME i n g e s t e d Mean n o . f i s h c a u g h t / m i n . Mean M E / f i s h Mean ME/min.  0.46 48.6 22.4  0.24 42.6 10.2  0.03 10.5  Cost o f i n t e r f e r e n c e No. i n t e r f e r e n c e s / m i n . ME l o s t t o i n t e r f e r e n c e / m i n .  0.01 0.22  a  0.06 14.0  0.01 0.10  c a l c u l a t e d by w e i g h t i n g the average m e t a b o l i z e a b l e energy per s p e c i e s (Chapter 5) by the p r o p o r t i o n o f the d i e t i n T a b l e 4 - 4 .  50  Table 4-6.  Numbers and d e n s i t i e s o f t e r r i t o r i a l Great Blue Herons i n h a b i t a t s along the r i v e r b a n k s on the F r a s e r R i v e r d e l t a .  Length of habitat(km)  Number  Density  (herons/km)  F o r e s t edge  17.1  7  0.41  Marsh w i t h o r without f o r e s t  19.2  25  1.30  Industry/agriculture with o r w i t h o u t marsh  62.1  6  0.10  51  three  Foraging success Beaches - The mean daytime f o r a g i n g r a t e s delta declined significantly 3 November.  o f a d u l t s on the F r a s e r R i v e r  ( t = 4 . 9 , r = 0 . 2 4 , p<0.001) between 2  In A u g u s t , a d u l t s needed an average o f 1.9  mins (SE=0.5)  c a p t u r e a f i s h v e r s u s 3.5 mins (SE=0.3) i n September and 5.0 i n October and e a r l y November. not s i g n i f i c a n t l y 3.3  mins.  October,  (t=2.57, r = 0 . 1 2 , p=0.13).  and 6.0  mins (SE=0.9)  mins  to  (SE=0.4)  also declined,  but  J u v e n i l e s needed an average  2  (SE=0.4) to c a p t u r e a f i s h i n A u g u s t , 4.7  September, took 74%,  Juvenile foraging rates  1 August and  mins (SE=0.5)  i n October and e a r l y November.  of  in Juveniles  34% and 20% more time to c a t c h a f i s h i n A u g u s t , September and respectively,  than  adults.  Foraging time The average j u v e n i l e  runs out o f time (F >100%) t o meet i t s t  energy needs on beaches on 5 days i n September, days i n November ( F i g .  4-3).  In c o n t r a s t ,  daily  7 days i n October and 12  the average a d u l t meets i t s  daily  energy need d u r i n g low t i d e on beaches on a l l  but 2 days i n September, 4  days i n October and 7 days i n November ( F i g .  4-3).  Grasslands - An average heron r e q u i r e d about 160 m i n s . to c a t c h a v o l e v o l e s caught i n 1938 m i n s . )  between  18 November 1986 and 17 January  An average v o l e c o n t a i n e d an e s t i m a t e d 339 kJ o f m e t a b o l i z a b l e heron i n g e s t e d about 127 kJ per hour (339 f e e d i n g on v o l e s caught i n g r a s s l a n d s . day i n November and 8h o f d a y l i g h t of t h i s  study ( 4 9 ° N ) .  and January (127  k J / h x 9h)  kJ/h x 8h).  so a  while  There were about 9h o f d a y l i g h t  i n December and January at the  each  latitude  A heron p r e y i n g o n l y on v o l e s i n g e s t e d about  per day i n November (127  1987.  energy,  kJ per v o l e / 1 6 0 m i n s . )  (12  1144  kJ  and 1016 kJ per average day i n December  Therefore,  52  a heron o b t a i n e d an average o f 73%  Figure  4-3.  Estimated p e r c e n t of a v a i l a b l e l o w - t i d e (<1.7 m) f o r a g i n g time r e q u i r e d by a d u l t ( s t i p p l e d ) and j u v e n i l e (open) herons to meet t h e i r estimated energy needs over 24 h (1560 kJ) each day i n September to November 1987-1988.  o f i t s d a i l y need i n November (1144 kJ/1560 and January (1016 kJ/1560  kJ x 100%) and 65% i n December  kJ x 100%).  H a b i t a t S h i f t s , D i s p e r s a l and M o r t a l i t y About o n e - t h i r d o f a l l j u v e n i l e s were seen i n g r a s s l a n d s i n SeptemberOctober and over t h r e e - q u a r t e r s on  beaches u n t i l  November.  marshlands r a t h e r  i n November ( F i g . 4 - 4 ) .  Between November and February a d u l t  than g r a s s l a n d s t o a g r e a t e r  (X =32.1, p<0.001, T a b l e 4 - 2 ) . 2  Most a d u l t s  degree than  stayed  females used  juveniles  P r o p o r t i o n a t e l y more j u v e n i l e s  than  adults  f o r a g e d d u r i n g the day on 17 r o a d - s i d e censuses between November 1986 and February 1987.  Ninety-nine  out o f 139 (71.2%) j u v e n i l e  compared t o 218 out o r 551 (39.6%) a d u l t s  herons were f o r a g i n g  (p<0.001).  J u v e n i l e s d i s a p p e a r e d from the F r a s e r R i v e r d e l t a (Fig.  4-5)  and more s t a r v e d t o death than a d u l t s .  (60.9%) j u v e n i l e  at a f a s t e r  Twenty-eight  rate  out o f 46  herons s t a r v e d t o death compared t o 3 out o f 16 (18.8%)  a d u l t s found between August and February (p<0.009). These f i n d i n g s support the p r e d i c t i o n s t h a t : herons d e p a r t tide  from beaches when t h e r e  t o meet t h e i r  before adults  i s too l i t t l e  d a i l y energy need a n d , ( i i )  i n autumn.  54  (i)  juvenile  and a d u l t  f o r a g i n g time d u r i n g low  juveniles  v a c a t e beaches  2B  100  37 li:::;:;:;:;:;  B  30  141  40  ••.-•••'•••'I 121  Z UJ  O  CC  128  50  UJ 0.  98  S  Figure 4-4.  i  1 0  t  y  N D J MONTH  23  i  S O N D J MONTH  F  F  Percentages o f j u v e n i l e (A) and a d u l t (B) herons counted on g r a s s l a n d s ( s t i p p l e d bars) and beaches (open b a r s ) d u r i n g the non-breeding season. Numbers above bars are number o f herons counted.  55  A S  O  N  D  J  F  MONTH  F i g u r e 4-5.  Numbers o f a d u l t and j u v e n i l e Great Blue Herons counted each month on road surveys o f the F r a s e r R i v e r d e l t a .  DISCUSSION Validity of assumptions T h i s a n a l y s i s i s based on s e v e r a l  key assumptions about herons  f o r a g i n g on beaches and i n g r a s s l a n d s .  Beach-foraging herons - I assumed t h a t : ( i ) intake  r a t e at a s i t e  (see Swennen et  al.  herons can not change t h e i r 1989),  (ii)  f o r a g i n g time  by low t i d e s and f o r a g i n g s k i l l  were the o n l y v a r i a b l e s t h a t  affected daily  and ( i i i )  ingestion rates,  f i s h d u r i n g the day as at n i g h t  herons caught t h r e e times as many  from August to November.The food  my study took p l a c e when winds were g e n e r a l l y l i g h t .  night.  and i t  intake 1979),  but  I have l e s s c o n f i d e n c e  t h a t t h r e e times as many f i s h are caught d u r i n g the day as at  My e s t i m a t e  summer.  dictated  significantly  r a t e s o f herons are a l s o a f f e c t e d by high winds (Bovino and B u r t t  i n my e s t i m a t e  food  i s based on a small n i g h t - t i m e  sample (N=9)  taken  in  However, most f o r a g i n g i s done d u r i n g daytime at t h i s time o f y e a r i s r e a s o n a b l e t o expect t h a t herons c a t c h fewer  d u r i n g the d a y .  It  f i s h at n i g h t  i s u n l i k e l y t h a t herons caught l a r g e r  than  f i s h at n i g h t  than  d u r i n g the day because l a r g e f i s h move i n t o deeper water d u r i n g low t i d e s when most herons f e e d . All  o f t h e s e assumptions c o n t r i b u t e a d d i t i o n a l  o f the amount o f time herons need to balance t h e i r However, these e r r o r s do not g r e a t l y would r e q u i r e 6-165% (median 83%) their  d a i l y energy need.  e e l g r a s s on a f u r t h e r  affect  e r r o r t o my e s t i m a t e s  d a i l y energy b u d g e t s .  my c o n c l u s i o n s because j u v e n i l e s  more time on 10 days i n November to meet  T i d e s were too high to a l l o w herons to f o r a g e  in  two d a y s .  Grassland-foraging herons - I assumed t h a t : Townsend's v o l e s eaten by herons in t h i s  study weighed an average o f 56g, c o n t a i n e d the same amount o f water  and energy per gram as the Common v o l e i n W i j n a n d t s '  57  (1984) s t u d y , 78%  energy a s s i m i l a t i o n e f f i c i e n c y by h e r o n s , and herons caught o f one every  v o l e s at  a rate  160 m i n s .  Other s t u d i e s have found s i m i l a r body weights  f o r Townsend's v o l e s  in  the F r a s e r R i v e r d e l t a between November and January (Beacham 1980). Moreover, the water and energy c o n t e n t rodents  (Cummins and Wuycheck 1971,  is relatively  constant  Zwarts and Blomert 1990)  d i g e s t i v e e f f i c i e n c y o f herons i s not s i g n i f i c a n t l y d i f f e r e n t other b i r d s  ( C a s t r o et  associated error.  a/.  1989).  I have l e s s c o n f i d e n c e i n my e s t i m a t e s  v o l e s were c a p t u r e d by h e r o n s . about a h a l f - h o u r on average walked out o f s i g h t . time i n t e r v a l voles,  i n c l u d i n g the Great Blue Heron.  the  Other f a c t o r s ,  o f the r a t e  that  for  the  such as the a c t i v i t y  the v u l n e r a b i l i t y  o f v o l e s to  For example, T a i t t  predators,  and Krebs (1983)  i n f i e l d s d u r i n g heavy  between November 1980 and February 1981  compared to 160 mins.  same f i e l d when f l o o d i n g was not w i d e s p r e a d .  Thus,  the  t o herons o f v o l e s i n g r a s s l a n d s i s p r o b a b l y more v a r i a b l e  availability  of  c o v e r , and e s p e c i a l l y the degree o f w i n t e r  herons c a t c h i n g v o l e s every 20 mins.  availability  of  has a small  Herons c o u l d be watched c o n t i n u a l l y  between prey c a p t u r e s .  i n my study o f the  from t h a t  No heron c o u l d be watched long enough to determine  flooding of grasslands, affect  precipitation  and the  (X=36 m i n s , SD=22, N=54), b e f o r e they flew or  the amount o f v e g e t a t i o n  reported  Each o f these e s t i m a t e s  between  than  o f f i s h on beaches.  Year-round Foraging Dispersion of Age- and Sex-classes T h i s i s the f i r s t  study to my knowledge t h a t d e s c r i b e s the  h a b i t a t use o f Great Blue Heron age- and s e x - c l a s s e s . and small c h i c k s and food a v a i l a b i l i t y  year-round  When n e s t s h e l d eggs  i n the lagoon was high (Chapter  females f e d c l o s e t o the c o l o n y w h i l e males tended n e s t s d u r i n g the  58  5),  day.  Other s t u d i e s have shown t h a t most herons feed near t h e i r Brandman 1976, sex or  Pratt  1980,  Dowd and F l a k e 1985,  o f those herons was not known.  colony-sites  Simpson et  al.  1987)  When t i d e s were high i n l a t e  but  the next morning.  left  they  returned  Other s t u d i e s o f c o a s t a l herons have shown t h a t most  p a i r s exchange d u t i e s d u r i n g high t i d e 1990).  up t o 27 km away u n t i l  (Paine 1972,  Brandman 1976,  Moul  Both p a r e n t s fed i n the lagoon l e s s o f t e n when the c h i c k s were  weeks o l d than i n e a r l i e r  stages (Table 4 - 1 ) .  This s h i f t  s i t e s c o i n c i d e d w i t h a d e c l i n e i n food a v a i l a b i l i t y (Chapter  the  afternoon  e v e n i n g , females i n my study exchanged d u t i e s w i t h males who then  the c o l o n y t o feed alone on t e r r i t o r i e s  (e.g.  i n use o f  3-4  foraging  i n the lagoon  5).  Habitat Selection Foraging e f f i c i e n c y , h a b i t a t s h i f t s and d i s p e r s a l others that dispersal Durell  i n d i c a t e t h a t f o r a g i n g e f f i c i e n c y , d a i l y use o f t i m e ,  are c l o s e l y i n t e r r e l a t e d  1984,  - My study j o i n s  in birds ( e . g .  Weathers and S u l l i v a n 1989,  showed t h a t j u v e n i l e  r e q u i r e d more f o r a g i n g time to meet t h e i r d e p a r t e d from beach h a b i t a t s  and  G o s s - C u s t a r d and d i t  S u l l i v a n 1989,  herons were l e s s e f f i c i e n t  several  S u l l i v a n 1990).  foragers  (Table  4-4),  energy needs ( F i g . 4 - 3 ) ,  sooner i n autumn than a d u l t s  I  and  (Fig. 4-4).  I  a l s o i n d i c a t e d t h a t p r o p o r t i o n a t e l y more j u v e n i l e s used g r a s s l a n d s than marshlands compared to a d u l t s  (Table 4 - 2 ) .  Lastly,  I showed t h a t  d i s a p p e a r e d from the study area at a h i g h e r r a t e than a d u l t s  juveniles  (Fig.  F o r a g i n g t h e o r y p o s i t s t h a t animals s w i t c h f e e d i n g s i t e s when food i n t a k e d e c l i n e s to the average g a i n i n the h a b i t a t T h i s t h e o r y p r e d i c t s t h a t herons s h o u l d a s s e s s t h e i r  their  (Charnov 1976).  f o r a g i n g success on  both beaches and i n g r a s s l a n d s by sampling each h a b i t a t .  59  4-5).  Most a d u l t s  (94.5%, n=l10) and j u v e n i l e s d u r i n g high t i d e s  (85%,  i n autumn r a t h e r  the p r e d i c t i o n o f h a b i t a t  n=35) r o o s t e d i n marshes and  fields  than f o r a g i n g i n g r a s s l a n d s , c o n t r a r y  sampling.  Therefore,  p r e d i c t when herons departed beach h a b i t a t s  in  I used a hunger t h r e s h o l d  o f F i s h e r i e s and Oceans 1987,  populations  (Gordon and Levings 1984)  c o u l d meet t h e i r  1988)  tides  and d i m i n i s h i n g prey  on beaches through autumn.  Adults  d a i l y energy needs i n about 55-85% o f the a v a i l a b l e  d u r i n g most low t i d e s  i n September, October and November.  r e q u i r e d about 73-100% o f the a v a i l a b l e p e r i o d and had i n s u f f i c i e n t herons s t o r e body f a t  time  Juveniles  time on most days over the same  time on many days ( F i g .  4-3).  A d u l t and  ( u n p u b l . data) presumably on days when t i d e s  juvenile remain  low f o r many hours t o i n s u r e a g a i n s t p r e d i c t a b l e p e r i o d s o f s h o r t a g e . o p t i o n ceased f o r j u v e n i l e s c o u l d h a r d l y meet t h e i r juveniles  d a i l y energy needs on most days ( F i g . 4-4)  4-3).  where they  Most  acquired  d a i l y energy need each day i n November and 65% o f  d a i l y energy need each day i n December and J a n u a r y . their  at  night.  These f i n d i n g s suggest t h a t herons l e a v e f o r a g i n g h a b i t a t s  when t h e i r  energy p o t e n t i a l  1982).  efficiency  d e c l i n e s below a c r i t i c a l differences  among j u v e n i l e s  I d e s c r i b e d can g r e a t l y  S u l l i v a n 1990).  level  affect  (McNamara  at the l e v e l  their  As w i n t e r approached, j u v e n i l e  survival  of (Gill  food  foraging et  al.  1975,  herons spent l e s s time on  the beaches and more time i n g r a s s l a n d s ( F i g . 4-4) mammals.  their  Presumably they met  energy needs by f o r a g i n g i n marshlands d u r i n g low t i d e s  Individual  This  on beaches a f t e r about m i d - O c t o b e r when they  f l e w t o g r a s s l a n d s a f t e r October ( F i g .  about 73% o f t h e i r  to  autumn.  Herons are f a c e d with a d e c l i n i n g d u r a t i o n o f low day time (Department  to  where they hunted small  I h y p o t h e s i z e t h a t the low f o r a g i n g success i n g r a s s l a n d s l e f t  60  especially juveniles telephone wires It  emaciated and v u l n e r a b l e to c o l l i s i o n s with v e h i c l e s ,  and f e n c e s .  i s u n c l e a r why more j u v e n i l e s do not f o r a g e with a d u l t s  marshlands d u r i n g the d a y . f o r a g i n g times  i n the day (Draulans  f o r high v a r i a n c e 1981).  J u v e n i l e s might l a c k the  intake  rates  and Vessem 1985)  A d u l t s might  knowledge o f the or t r a d e  i n g r a s s l a n d s (Caraco et  J u v e n i l e s f o r a g e i n marshlands at n i g h t  (pers.  a7.  (Bovino and B u r t t  1979)  off  1980,  best  low mean Caraco  o b s . R i c h n e r 1986).  l e a v e marshlands on b l u s t e r y days when t h e i r  d e c l i n e s below some t h r e s h o l d  in  foraging  or t o  success  exploit  temporary bonanzas c r e a t e d when r a i n s f l o o d v o l e s from t h e i r  underground  burrows i n g r a s s l a n d s , o r b o t h .  Territoriality Several territories  s t u d i e s have shown t h a t a d u l t  and t h a t i n t e r a c t i o n s  R i c h n e r 1986,  1972)  (Bayer  feeding  1978,  Mine i s the f i r s t  Cook  study to show  habitat  e x p l a i n well  s e l e c t i o n models ( F r e t w e l l  the s p a c i n g o f t e r r i t o r i a l  and Lucas  herons.  The  d e n s i t i e s o c c u r r e d i n marshes along r i v e r b a n k s where prey d e n s i t i e s probably g r e a t e s t . this  and o t h e r  intake  Fretwell rates  habitats,  1970, greatest are  Moreover, j u v e n i l e s were excluded from t e r r i t o r i e s  studies  p r e d i c t e d from the 1970,  1978,  herons were mainly and perhaps o n l y m a l e s .  Density-dependent Fretwell  are i n f r e q u e n t  Draulans and Hannon 1988).  that t e r r i t o r i a l  herons defend  (e.g.  ideal  1972).  Bayer 1978,  R i c h n e r 1986).  d e s p o t i c d i s t r i b u t i o n model  These r e s u l t s  (Fretwell  Future s t u d i e s are needed to examine i f  are g r e a t e r  i n the high d e n s i t y h a b i t a t s  as p r e d i c t e d by the  ideal  than  despotic d i s t r i b u t i o n .  61  in are  and Lucas the  average  i n low d e n s i t y  SUMMARY 1)  When n e s t s h e l d eggs and small c h i c k s ,  b r e e d i n g female herons  at  Sidney m o s t l y f e d w i t h i n 2 km o f the c o l o n y - s i t e d u r i n g the day when t i d e s were low and, b r e e d i n g males mostly fed along beaches from 627 km from the c o l o n y - s i t e d u r i n g l a t e a f t e r n o o n morning when t i d e s were h i g h .  until  the  next  Breeding males and females fed w i t h i n  2 km o f the c o l o n y - s i t e when c h i c k s were l a r g e . 2)  The s h i f t  in habitat  use by a d u l t  females and j u v e n i l e s  from beaches  t o marshlands and g r a s s l a n d s i n autumn was best e x p l a i n e d by a s h o r t a g e o f f o r a g i n g time d u r i n g low t i d e s c o u p l e d with s h r i n k i n g prey populations. 3)  A d u l t males spent the n o n - b r e e d i n g season i n t e r r i t o r i e s riverbanks  and t h e i r  distribution  s p a c i n g was p r e d i c t e d w e l l  (Fretwell  and Lucas 1970).  62  by the  along  ideal  despotic  CHAPTER FIVE. TIME OF BREEDING IN GREAT BLUE HERONS In the p r e v i o u s c h a p t e r s , foraging habitats. availability  Here,  and time o f  I examined how herons s e l e c t e d breeding and  I f o c u s on the r e l a t i o n s h i p  s e l e c t i o n f a v o u r s a d u l t s whose  n e s t l i n g s are p r e s e n t when food i s most a v a i l a b l e hypothesis predicts that early  often see  the  Perrins prevent  Several  s t u d i e s have supported  (see P e r r i n s and Birkhead 1983), but e a r l y  most p r o d u c t i v e  Noordwijk  (1965,  1970)  most females  (e.g.  Cave 1968,  but  proposed t h a t food shortages d u r i n g  from breeding e a r l i e r  it  food s u p p l i e s had peaked.  i s not c l e a r whether  Tests of Lack's  availability, (Daan et  a/.  methodological has 1988).  problems.  are but  egg-laying  (1965,  Moreover,  (Drent and Daan 1980,  al.  1970)  by M a r t i n 1987,  1).  63  is  hypotheses have been than  food  studies  1970)  or adults  (Lack 1954), although many s t u d i e s have found t h a t breeding reviews  et  1988).  these hypotheses assume t h a t food i s 1965,  Daan  i n n e s t s when food  Food abundance, r a t h e r  (Perrins  the  s t u d i e s suggest  been used as an index o f food s u p p l y i n most  are not s h o r t o f food (see Chapter  Several  most young are  (1954) and P e r r i n s '  s h o r t s u p p l y t o the e g g - l a y i n g female nestlings  clutches  so t h a t young would be i n  i n c r e a s i n g o r d e c r e a s i n g i n abundance (see Daan et  hampered by  than  Lack's  Davies and Lundberg 1985,  t h a t females breed when food becomes p l e n t i f u l 1988)  This  1983).  nest a f t e r the p a r e n t s '  al.  to the p a r e n t s .  and l a t e n e s t e r s s h o u l d f a r e l e s s well  those n e s t i n g on the average d a t e . hypothesis  food  breeding.  Lack (1954) p o s t u l a t e d t h a t n a t u r a l  (1954)  between  Linden and M o l l e r  in with birds 1989,  A suitable  s p e c i e s i n which to compare t i m i n g o f b r e e d i n g i s one i n  which  food a v a i l a b i l i t y  Great  Blue Heron i s a s u i t a b l e  whose  p o p u l a t i o n s can be sampled with b e a c h - s e i n e n e t s ,  of  and prey consumption can be measured d i r e c t l y . s p e c i e s because:  consumption o f f i s h can be e s t i m a t e d The aim o f t h i s c h a p t e r  Next,  1970)  plentiful  and, (Lack  (ii)  First,  small  and ( i i )  fish  its  rate  Bayer 1985).  I estimate  the  o f food  relative  herons when they have e g g s , small  then examine i f  I compare the hypotheses  threshold of available 1965,  I  eats  i s t o r e l a t e the seasonal a v a i l a b i l i t y  o f prey energy to a d u l t  c h i c k s and l a r g e c h i c k s .  it  (Simpson 1984,  energy t o the time o f b r e e d i n g o f h e r o n s . availability  (i)  The  that:  (i)  energy f o r egg  critical  food s h o r t a g e s o c c u r .  females begin t o l a y eggs a f t e r a  p r o d u c t i o n has been passed  c h i c k s are i n the  nest when food f o r a d u l t s  (Perrins i s most  1954).  STUDY AREA AND METHODS Great Blue Herons were s t u d i e d on Sidney I s l a n d methods used t o :  estimate  when eggs were l a i d , Chapter  foraging rates,  and e s t i m a t e  (Figure  2-1).  sample prey p o p u l a t i o n s ,  reproductive  success,  General determine  are o u t l i n e d  in  2.  R e l a t i v e A v a i l a b i l i t y o f Energy t o Adult Herons Relative  availability  o f food energy i s the biomass o f f i s h p r e s e n t  the  lagoon expressed i n u n i t s o f energy  low  (<1.7  estimated  (kJ),  multiplied  by the d u r a t i o n  m) t i d e s d u r i n g each stage o f the b r e e d i n g s e a s o n . from the  formula: ME =((F )(W )-C)T S  S  64  S  S  It  was  in of  where ME i s the amount of available energy in nesting stage S; F $  estimated number of the f i s h in the partial  enclosure in  s  enclosure  for metabolizeable energy  content of  f i s h equal to 4.76 kJ/g dry weight,  of minutes  of low tide foraging in nesting stage S.  the equation is now  is the  nesting stage S,  w" is the average weight in grams of f i s h caught in the partial in nesting stage S; C is a constant m u l t i p l i e r  $  and T  i s the average number  $  Each of these terms i n  explained in d e t a i l .  Number of f i s h in the lagoon (F) - A polyethylene fence lined with galvanized  chicken-wire was erected around poles enclosing three sides of a  9x9 m portion  of eelgrass {Zostera marina) bed on an ebbing tide in Sidney  lagoon when nests stages, below)  held eggs, small chicks and large chicks (see Nesting  in 1988.  A 1.5 x 18 m (6 mm stretched mesh) beach seine was  then quickly and repeatedly contents were quickly emptied caught.  hauled toward the opening.  into an empty bucket until most f i s h had  These data (Appendix I)  162-166 in Krebs 1989)  On each haul, the  to estimate  been  were used in L e s l i e and Ricker models (pp. the number of f i s h in the  partial  enclosure. The Shiner Sea Perch (Cymatogaster aggregata) and Tube-snout (Aulorhynchus flavidus)  avoided the partial  enclosure so I estimated  numbers by assuming that the proportion of a l l hauls outside the partial partial  their  species caught in 19 seine-  enclosure each month was the same as in the  enclosure prior to i t s i n s t a l l a t i o n .  This assumption i s discussed  later.  Energy estimates in prey (W,C) - A l l f i s h caught in the p a r t i a l  enclosure  were  equations  measured and samples were weighed to derive length-weight 65  f o r each stage  species.  (see  1989)  (i)  (ii)  the  and,  (iii)  all  each  Nesting stages (S)  chick  These  -  I estimated  about  i n c l u d e the p e r i o d  each n e s t i n g stage from the median date The  so t h a t most n e s t s h e l d small chicks  (Vermeer  1969,  Brandman 1976).  from  Incubation Therefore,  the  b a c k - d a t e d 28 days from the median h a t c h i n g date 15 A p r i l  to  13 May.  Egg-laying was e s t i m a t e d  to  to  occur  April.  Duration of  low t i d e s (T)  Sidney l a g o o n .  each day  kJ (Cummins  hatched e g g s h e l l was found below each n e s t .  f o u r weeks  al.  later.  8 J u n e , and large chicks from 9 June t o 1 J u l y .  incubation stage was  from 1-14  o f f i s h c o n t a i n e d 21.3  assumptions are d i s c u s s e d  stage was d i v i d e d i n h a l f  requires  at  f i s h c o n t a i n e d 71% water (Holmes and Donaldson  gram d r y - w e i g h t  t h a t the f i r s t  14 May t o  I  a s s i m i l a t i o n e f f i c i e n c y o f herons was 77% ( C a s t r o et  and Wuycheck 1971).  (14 May)  f i s h caught d u r i n g each n e s t i n g  below) was c o n v e r t e d i n t o u n i t s o f energy as f o l l o w s .  assumed t h a t : 1969),  The mean weight o f a l l  -  F o r a g i n g o c c u r r e d w h i l e t i d e s were low (<1.7  The number o f minutes o f low t i d e a v a i l a b l e  o f the breeding season was generated  housed at  the T i d a l  O f f i c e o f the  to  m)  herons  by computer u s i n g t i d a l  data  I n s t i t u t e o f Ocean S c i e n c e s .  Estimated Energy Consumption by Adults Adult consumption - The l e n g t h o f each f i s h caught by herons was as  a p r o p o r t i o n o f the h e r o n ' s culmen l e n g t h  significant length  o f the  (Chapter  2).  b i a s i n t h i s method i s the o b s e r v e r ' s a b i l i t y prey  a range o f s i z e s  (Bayer  1985).  o f the f o u r major  The most to e s t i m a t e  Observer p r e c i s i o n was t e s t e d prey s p e c i e s h e l d between  66  estimated  the  the  by showing mandibles  o f a dead heron to  o b s e r v e r s u s i n g t e l e s c o p e s 65-100 m away.  Significant  d i f f e r e n c e s d i d not  o c c u r between o b s e r v e r s (X =1.86, p=0.8, d . f . = 4 ) .  correctly  all  identified  Both  2  f i s h and both underestimated the l e n g t h s o f some  f i s h s i z e s by one s i z e - c l a s s .  Therefore,  I adjusted a l l  fish  length  e s t i m a t e s by one s i z e - c l a s s t o reduce o b s e r v e r b i a s and then c o n v e r t e d each f i s h i n t o energy  u n i t s f o l l o w i n g the methods o u t l i n e d above.  Time o f Breeding Energy t h r e s h o l d f o r l a y i n g  - C i c o n i i f o r m e s r e q u i r e about 30% more energy  above maintenance c o s t s to f u e l  their  activities  (Kushlan 1977).  The  e s t i m a t e d maintenance c o s t from K e n d e i g h ' s (1970) e q u a t i o n s f o r a caged 2100 g  heron i s 1200 kJ and her a c t i v i t y  360 kJ (30% o f 1200 kJ)  for a total  c o s t s an a d d i t i o n a l  estimated  d a i l y energy need o f 1560 k J .  Twenty-seven heron eggs c o l l e c t e d i n B . C . near the b e g i n n i n g o f l a y i n g p e r i o d c o n t a i n e d a mean o f 4.77 and p r o t e i n  (P. Whitehead,  g of fat  unpubl. d a t a ) .  and 3.03  the  of carbohydrate  Each gram o f f a t  c o n t a i n s about  39 kJ o f energy w h i l e a gram o f c a r b o h y d r a t e and p r o t e i n h o l d s about 18 k J . Therefore,  t h e r e are about 240 kJ [ ( 4 . 7 7 g x 39 k J / g ) + ( 3 . 0 3  average heron e g g .  x 18 k J ) ]  The e f f i c i e n c y o f c o n v e r t i n g m e t a b o l i z e d energy  eggs i s e s t i m a t e d to be about 70% (King 1973) about 312 kJ per egg (240kJ+(0.30x240 k J ) ) .  i n an  into  so an average female musters S i n c e herons l a y eggs i n two  day i n t e r v a l s ,  a female r e q u i r e s about h a l f the 312 kJ per day (Murton and  Westwood 1977)  o r 155 k J .  days p r i o r to egg f o r m a t i o n  She a l s o r e q u i r e s energy f o r o v o g e n e s i s f o r (Murton and Westwood 1977).  heron must exceed a t h r e s h o l d o f about 1715 kJ (1560 +155  k J / d f o r each egg)  f o r at l e a s t  Therefore,  kJ/d for  a female  maintenance  10 days (Murton and Westwood  complete her 4 egg c l u t c h where one egg i s l a i d every two d a y s .  67  a few  1977)  to  Energy consumption by e g g - l a y i n g females - A v a i l a b l e energy t o females before  and w h i l e they l a i d eggs was estimated by m u l t i p l y i n g the mean d a i l y  energy  i n g e s t i o n r a t e d u r i n g the e g g - l a y i n g stage (1-14 A p r i l ) by the  number o f minutes t h a t low (<1.7 m) t i d e s uncovered the e e l g r a s s bed each day from 1 February t o 14 May. Food  demands  of  heron c h i c k s - S i x c h i c k s were reared from the egg i n  c a p t i v i t y and f e d f i s h  ad libitum  (D. Bennet, p e r s . comm.).  They were moved  i n t o outdoor a v i a r i e s at about 3 weeks o f age. I used three-day running averages o f the weight o f f i s h eaten t o estimate the age when the  maximum  food demand  1982)  o c c u r r e d . Growth curves o f c a p t i v e and w i l d (Quinney  c h i c k s do not d i f f e r s i g n i f i c a n t l y (D. Bennet, p e r s . comm.).  RESULTS Relative Availability  of Food  Herons caught tube-snouts (Gasterosteus  acuieatus),  (Signathus griseolineatus), (Pholis  energy  ornata  and  Energy i n t h e Lagoon (Aulorhynchus fiavidus),  sea perch sculpins  P. laeta).  sticklebacks  (Cymatogaster aggregata), (Leptocottus  armatus)  pipefish  and gunnels  The average l e n g t h , weight and estimated  c o n t a i n e d by these f i s h through the breeding season i s shown i n  Tables 5-1 and 5-2.  68  Table  5-1.  Average l e n g t h s o f f i s h caught i n beach s e i n e s A p r i l - J u l y 198788 and t h e i r e s t i m a t e d weights from l e n g t h - w e i g h t r e g r e s s i o n equations.  Pholis ornata April May June July Gasterosteus April May June July Leptocottus April May June July  Total  length  (mm)  Mean  SD  N  87.0 80.8 99.0 94.3  28.1 18 19.7 141 17.2 92 16.5 73  Weight Mean wt(g)  (g) SE  1.8 1.3 2.6 2.3  0.2 0.1 0.1 0.1  aculeatus 60.8 67.2 70.4 73.8  6.1 5.5 4.4 4.8  27 98 42 12  1.8 2.6 3.0 3.5  0.8 0.4 0.7 1.2  64.1 66.4 90.9 95.8  15.2 24.5 38.5 22.9  21 47 66 58  2.9 3.1 6.4 7.6  3.2 1.7 1.5 1.5  24.9 21.1 12.0  6.7 2.4 3.7  3.9 4.4 4.8 4.8  0.7 0.2 0.4 0.4  35.3  7.5(  armatus  Cymatogaster aggregata May June July Sygnathus griseolineatus April May June July  110.0 104.1 52.1  34.2 15 12.1 119 5.3 50  138.0 156.7 173.1 173.1  58.0 48.2 39.2 39.2  7 118 31  Aulorhynchus April  158.8  47.3  5  31  flavidus  69  Table  5-2.  Estimates o f M e t a b o l i z a b l e Energy ( c a l c u l a t e d from weights from T a b l e 5-1) a v a i l a b l e t o herons from the major prey s p e c i e s each summer month i n the lagoon at S i d n e y .  M e t a b o l i z e a b l e energy Mean  Pholis ornata April May June July Gasterosteus April May ' June July Leptocottus April May June July  SE  8.6 6.2 12.4 11.0  0.2 0.1 0.1 0.1,  8.6 12.4 14.3 16.7  1.0 0.5 0.9 1.5  13.8 14.8 30.5 36.2  4.1 2.2 1.9 1.9  118.5 100.4 57.1  8.6 3.1 4.7  18.6 20.9 22.9 22.9  0.9 0.2 0.5 0.5  168.0  9.7  aculeatus  armatus  Cymatogaster May June July Synathus April May June July  (kJ)  aggregata  griseolineatus  a  Aulorhynchus April  flavidus  a - no p i p e f i s h were caught i n s e i n e s i n J u l y so I assumed those caught by herons were the same s i z e as i n J u n e .  70  Numbers o f prey i n d i v i d u a l s were g r e a t e s t chicks in  n e s t s (Table  5-3).  the l o n g e s t p e r i o d when (Table  5-4).  greatest  However,  by f a r when herons had small  Low t i d e s exposed the e e l g r a s s h a b i t a t  for  l a r g e c h i c k s (9 June-1 J u l y ) were i n n e s t s the r e l a t i v e  availability  when small c h i c k s were p r e s e n t  (Figure  o f food energy was 5-1).  Seasonal Energy Consumption by A d u l t s , Chicks and Egg-laying Females Adults when  - A d u l t herons i n g e s t e d n e a r l y f o u r times more food energy per day small c h i c k s were i n t h e i r  laying  n e s t s (Table  5-5)  than when they were  (p<0.001), or i n c u b a t i n g eggs (p<0.001), and n e a r l y t h r e e  times as  much as when r a i s i n g l a r g e c h i c k s (p<0.05, K r u s k a l l - W a l l i s ANOVA and multiple  comparison t e s t ;  p.  200,  Zar 1984).  The surge i n  consumption r e s u l t e d from the l a r g e number o f sea perch (Table  Chicks  energy  i n the  diet  5-6). - The g r e a t e s t  about 37 days of age. Therefore,  c h i c k s o c c u r r e d at  The median h a t c h i n g date at Sidney was 14 May.  the e s t i m a t e d  June when l a r g e relatively  food demands by 6 c a p t i v e - r e a r e d  peak food demands f e l l  37 days l a t e r  on about  20  c h i c k s were i n n e s t s , and when i n g e s t i o n r a t e s were  low.  Egg-laying females  -  Egg l a y i n g began when the f e m a l e s '  energy  i n g e s t i o n exceeded t h e i r  i n 1987  and 9 days i n 1988  daily  energy t h r e s h o l d f o r e g g - l a y i n g f o r 7 days  (Figure  below the e g g - l a y i n g t h r e s h o l d f o r March compared t o o n l y 1-2  estimated  5-2).  Moreover,  available  10 c o n t i n u o u s days i n l a t e  days i n e a r l y May ( F i g u r e  71  5-2).  energy  fell  February-early  Table 5-3.  E x t r a p o l a t e d minimum p o p u l a t i o n s e n c l o s u r e i n Sidney l a g o o n .  Extrapolated population  Period  Incubation  of f i s h  i n the  partial  S.E.  360  48  chicks  3297  6  Large c h i c k s  403  5  Small  a - See Appendix  I.  72  T a b l e 5-4.  Number o f low (<1.7 m) t i d e minutes per average day i n which herons c o u l d f o r a g e i n the lagoon e e l g r a s s beds d u r i n g the breeding season.  Number o f minutes per average day  Breeding stage  Egg l a y i n g  151  Incubation  176  Small c h i c k  208  Large c h i c k  216  73  Table 5-5.  Estimated M e t a b o l i z e d Energy (ME) intake (kJ) per average day by an a d u l t heron averaged over f o u r p e r i o d s o f the b r e e d i n g season on Sidney I s l a n d i n 1987-88. N i s the number o f herons watched.  Breeding p e r i o d Egg-laying 4S  ME i n t a k e  rate  (kJ/min)  Mins. a v a i l a b l e to f o r a g e per day Estimated intake/day  7.7  Small c h i c k  SE  N  x  SE  N  x  3.7  27  6.8  1.1  155  20.5  151 1163  Incubation  176 555  1197  SE  3.7  208 194  4264  Large c h i c k N  x  48  7.4,  SE  0.7  216 764  1598  151  N  109  1.0 X LU  a •  1  SMALL CHICK  LARGE CHICK  0.5  5 <  < > < Figure 5-1.  i n  INCUBATION  Index o f food a v a i l a b i l i t y d u r i n g the b r e e d i n g season o f the Great Blue Heron. Percent a v a i l a b i l i t y i s e s t i m a t e d from the product o f numbers o f minutes when low t i d e s (<1.7 m) exposed the f o r a g i n g h a b i t a t and the s i z e o f the f i s h p o p u l a t i o n converted into u n i t s of energy. V e r t i c a l l i n e s are standard errors.  75  1987  Figure 5-2.  Number of clutches  Numbers o f days t h a t a v a i l a b l e energy exceeded an estimated energy t h r e s h o l d f o r egg l a y i n g and the estimated dates when c l u t c h e s were l a i d i n 1987 and 1988.  76  Table 5-6.  Numbers o f each o f the main prey s p e c i e s eaten by Great Blue Herons d u r i n g the 1987-88 breeding season on Sidney I s l a n d .  Courtship Species  Gunnel  N  %  20  41.7  Egg  Small  chick  Large c h i c k  %  N  %  188  40.9  56  34.6  175  65.3  439  46. ,8  N  %  Total  N  N  °/0  Stickleback  0  0  42  9.1  10  6.2  8  3.0  60  6. .4  Sculpin  2  4.2  78  17.0  9  5.5  53  19.8  142  15. ,1  Sea Perch  0  0  20  4.3  68  42.0  11  4.1  99  10. ,6  Pipefish  0  0  4  0.9  9  5.5  1  0.4  14  1..5  0  1  0.4  13  1..4  19  7.0  171  18, .2  Tubesnout  11  22.9  1  0.2  0  Unknown  15  31.2  127  27.6  10  48  460  162  77  6.2  268  938  In  summary:  (i)  energy demands o f the c h i c k s o c c u r r e d s e v e r a l  a f t e r the peak i n a v a i l a b i l i t y their I  to t h e i r  eggs soon a f t e r the e s t i m a t e d  parents,  and ( i i )  female  1965,  herons  laid  e g g - l a y i n g t h r e s h o l d had been exceeded.  c o n c l u d e t h a t my f i n d i n g s best support the energy t h r e s h o l d  (Perrins  weeks  hypothesis  1970).  DISCUSSION Validity In  o f Assumptions this  analysis  I assumed t h a t :  (i)  the  relative  abundance o f  the p a r t i a l e n c l o s u r e r e p r e s e n t e d the f i s h p o p u l a t i o n elsewhere i n the (ii)  fish  lagoon,  the p r o p o r t i o n o f t u b e - s n o u t s and sea perch caught i n s e i n e hauls  lagoon  closely  lagoon,  (iii)  reflected each f i s h  their  abundance r e l a t i v e  contained  (iv)  the  herons  maintain  they  hatched,  and  (v)  female  themselves and an e x t r a  other  71% water and 21.3  o f which 77% was a s s i m i l a t e d by h e r o n s , day  to  species  kJ per  of  required  and consumption.  not a l t e r my  c o n c l u s i o n s because the  was more  eight-fold  large  than  c h i c k s were  populations  in  present  British  abundance (Weibe 1968, errors  greater  1560  kJ  from e s t i m a t i n g  (Table  Columbia  1964,  Dunn  sample o f  small  5-3). have  c h i c k s were  Other found  water and energy content  1975,  Kushlan  12 prey  fishes  1977,  studies  similar  K e l s a l l and Simpson 1980,  and maintenance energy requirements  However,  r e l a t i v e abundance o f  when  Castro  et  i n my study c o n t a i n e d  78  in  the  weight  a/.  day  to  e r r o r to the  these e r r o r s prey  do  populations  in  nests  than  when  of  intertidal  fish  Hughes 1985). of f i s h ,  per egg.  seasonal  o f herons are p r o b a b l y  1978,  the  I found e g g s h e l l s below n e s t s on  155 kJ each day t o produce an  energy a v a i l a b i l i t y  in  gram dry  Departures from a l l o f these assumptions c o n t r i b u t e a d d i t i o n a l estimates  in  patterns  The  in  additional  assimilation  rates  a l s o low (see Kahl  1989).  71-73% water  For. example,  a  and 3 seaperch  c o n t a i n e d about 3 kJ l e s s energy than my e s t i m a t e the  (118  kJ)  using values  from  literature. I  have l e s s c o n f i d e n c e with my e s t i m a t e s o f energy consumption e a r l y and  l a t e i n the n e s t i n g s e a s o n . published values energy  (1715  My energy t h r e s h o l d e s t i m a t e  kJ)  was g r e a t e r  than my f i e l d  f o r e g g - l a y i n g using  estimate  (=1163 kJ)  of  i n g e s t i o n by l a y i n g females but the e r r o r around my e s t i m a t e was g r e a t  (Table  5-5).  University  Moreover, e x i s t e n c e energy o f herons h e l d i n outdoor cages at of  British  Columbia  maintenance  energy  estimate  (D.  unpubl.  data).  Bennet,  is  about  derived Thus,  270 kJ  from  my  lower  than  Kendeigh's  estimates  of  the  (1970)  energy  the  1200 kJ equations  required  for  maintenance and the t h r e s h o l d f o r egg l a y i n g are p r o b a b l y s l i g h t l y h i g h e r than what  female  threshold  is  availability  herons  require.  required  Also,  to make small  a  large  change  change t o  exceeds the t h r e s h o l d i n F i g u r e  the  in  the  position  number o f  of  days when  the food  5-2.  Time of Breeding My r e s u l t s (i)  are  best e x p l a i n e d by P e r r i n s '  1970)  hypothesis  energy a v a i l a b i l i t y determines when females w i l l l a y eggs and, ( i i )  are i n the nest a f t e r food a v a i l a b i l i t y most  (1965,  females  crossed being  in  laid  (Figure  Some o f body  Thus,  days this  condition  after  the  variability (see  Drent  the c h i c k s to match t h e i r  chicks  p a r e n t s has peaked.  However,  estimated  had been  probably and  Daan  threshold arose from 1980,  the Great Blue Heron f a c e s an i m p o s s i b l e  once females c r o s s the t h r e s h o l d f o r  remains f o r supply.  5-2).  different  B i r k h e a d 1983). problem:  eggs s e v e r a l  to t h e i r  egg l a y i n g ,  food demands with the  peaks.  79  females  Perrins  and  optimization  insufficient  time  peak i n the  food  C h i c k s are fed o n l y by the female when food i s most a v a i l a b l e  both p a r e n t s a f t e r food a v a i l a b i l i t y  that:  and by  It  is  influences  unlikely  that  (see r e f s .  photoperiod  in O r e l l  or  air  and Ojanen 1983)  temperature  are  on when herons breed  l a y i n g d a t e s are not s y n c h r o n i z e d between nearby c o l o n y - s i t e s . i n t h r e e y e a r s o f t h i s s t u d y , 3 c o l o n i e s i n the F r a s e r R i v e r to the n o r t h )  and a c o l o n y i n V i c t o r i a (25  2-3 weeks e a r l i e r west)  hatched  than S i d n e y .  eggs  in  the  km t o the  Moreover, a c o l o n y  same week  as  significant  For  example,  delta  (30-40 km  southeast)  hatched eggs  near C r o f t o n (35  Sidney.  I  because  hypothesize  km to  the  that  the  asynchrony i n l a y i n g dates between c o l o n i e s r e s u l t s from d i f f e r e n c e s i n i n s h o r e movements o f l o c a l  f i s h populations.  The d a i l y d u r a t i o n o f low t i d e s determines how much food herons a c q u i r e on  beaches throughout  the  year  (Chapter  4,  Figure  5-2).  In  Chapter  4,  I  proposed t h a t female herons might s t o r e energy on w i n t e r days when t i d e s were very low t o  s a f e g u a r d a g a i n s t p r e d i c t a b l e p e r i o d s o f food s c a r c i t y .  Here,  I  propose t h a t as p e r i o d s o f food s c a r c i t y become l e s s f r e q u e n t i n s p r i n g , female herons devote l e s s f o r a g i n g time t o m a i n t a i n i n g t h e i r time t o the b r e e d i n g  energy b a l a n c e , and more  effort.  Future D i r e c t i o n s Future  work  e g g - l a y i n g date  might  examine  the  strength  and food a v a i l a b i l i t y  w i l d and c a p t i v e herons o f f e r  to  of  the  the f e m a l e .  relationship  between  Food m a n i p u l a t i o n s  of  the best p r o s p e c t s .  The bonanza i n food energy a v a i l a b l e when sea perch a r e p l e n t i f u l  i n May  might a l l o w females t o r e c o v e r body c o n d i t i o n from e g g - l a y i n g and c h i c k - r e a r i n g and  begin  their  feather  moult.  Sea  September i n e e l g r a s s b e d s .  80  perch  are  abundant  between  May  and  SUMMARY 1)  The g r e a t e s t  amount o f energy  available  i n May when young c h i c k s were i n the 2)  The g r e a t e s t  energy  demands o f  The t i m i n g  of breeding  i n Great  e x p l a i n e d by the a v a i l a b i l i t y 1965,  1970)  rather  occurred  nest.  chicks occurred  month a f t e r energy was most a v a i l a b l e 3)  and consumed by a d u l t s  to t h e i r  Blue Herons i n  in  late  June  about  parents. B r i t i s h Columbia i s  o f energy t o the e g g - l a y i n g female  than t o the demands o f the c h i c k s (Lack  81  one  1954).  best  (Perrins  CHAPTER SIX. GENERAL DISCUSSION The aims o f t h i s t h e s i s were to c o n s i d e r the f a c t o r s t h a t determine herons  locate  foraging food  their  habitats  breeding  through the y e a r ,  availability,  integrate  colony-sites,  habitat  how  age-  and  and to examine the  selection  and time o f  where  sex-classes  relationship  breeding.  I  use  between  discuss  and  my main c o n c l u s i o n s now.  Choice o f C o l o n y - s i t e Three main c o n c l u s i o n s emerged from t h i s s t u d y .  F i r s t , colony spacing i s  best e x p l a i n e d by the d i s t r i b u t i o n o f f e e d i n g s i t e s and not by the presence o f a key p r e d a t o r near  rich  the  B a l d Eagle  intertidal  (Chapter 3 ) .  feeding  areas.  Twenty-nine  The  average  grounds o f a sample o f 22 c o l o n i e s was 2.4 km  o f 33 c o l o n i e s were  distance  (Chapter 3 ) .  to  the  feeding  In c o n t r a s t ,  the  number o f b r e e d i n g p a i r s o f h e r o n s , the number o f s u c c e s s f u l n e s t s and the s i z e o f broods d i d not vary s i g n i f i c a n t l y with e a g l e s nest d e n s i t i e s .  Herons might  be unable t o a v o i d e a g l e s , my a n a l y s i s might be too c o a r s e o r the power o f test  might have been t o weak, Several  studies  have  to d e t e c t  shown t h a t  the  p o s i t i v e l y r e l a t e d t o the area o f t h e i r 1977,  Kushlan  continuous (Fasola  1978,  habitat  Chapter 3 ) . is  and B a r b i e r i  assumption t h a t capacity.  In my s t u d y ,  three  I  habitats  have  reproductive  breeding  success  to  the  disagree  not  vary  (e.g.  number with  i n general  f o r a g i n g areas  did  b r e e d i n g wading  shown t h a t  are  birds  Werschkul et  colony of  breeding  Gibbs et near  spacing  al.  their  Forbes et al. with  al. in  (1987) carrying the  (1985a) found number  of  p a i r s i n a c o l o n y , and I found no evidence t h a t food was l i m i t i n g  at  82  significantly  is  pairs  used by b r e e d i n g herons i n  p a s t were not used d u r i n g t h i s study (Chapter 3 ) . that  difference.  feeding habitat  related  However,  heron breeding  number o f  Others  positively 1977).  a significant  the  the  the c o l o n y l e v e l  among breeding herons (Chapter  5).  At the  individual  level,  brood r e d u c t i o n a d j u s t s the s i z e o f broods to the food p r o v i s i o n i n g a b i l i t y o f the  parents. I  hypothesize  b r e e d i n g male choice of  that  the  number  herons through  colony s i t e s  is  the  of  suitable  habitat  (see Marion  c o n s t r a i n e d by how f a r  assume t h a t male t e r r i t o r y h o l d e r s exclude a l l habitat  when t e r r i t o r i e s  p o p u l a t i o n s are at t h e i r  1989)  they w i l l  s h r i n k t o a minimum s i z e  area o f f o r a g i n g h a b i t a t  territories  spaces  and t h a t fly  to  (Fretwell  their  nest.  o t h e r males from s e t t l i n g  T h i s e x p l a i n s the d i s t r i b u t i o n o f c o l o n i e s and s i z e and the  feeding  I  i n the  and Lucas  1970).  the r e l a t i o n s h i p between c o l o n y  without having t o  carrying capacity of t h e i r  assume t h a t  regional  habitat.  Colony Formation P r e d a t o r s have been suggested to p l a y an important o r maintenance o f n e s t i n g c o l o n i e s o f b i r d s (Lack 1968, not a l l  o t h e r s agree  is often predators  (see  review by Forbes 1989).  used as a measure o f the e f f e c t i v e n e s s (see  Wittenberger  and  reviews  by  Wittenberger  Hunt  1985).  1981,  For example,  out c o l o n i a l n e s t i n g as an a n t i - p r e d a t o r and c h i c k s u r v i v a l d i d not d i f f e r  of  role  i n the  Forbes 1989)  although  Egg and n e s t l i n g  survival  colonial  Perrins  and  nesting  against  Birkhead  Vessem and Draulans  adaptation  formation  (1986)  1983, ruled  i n Gray Herons because egg  s i g n i f i c a n t l y in c o l o n i e s of d i f f e r e n t  sizes  and a d u l t s d i d not mob p r e d a t o r s .  However, c o l o n i a l n e s t i n g might have a r i s e n  primarily  of adults  to favour  the  survival  c l o s e t o one another would i n c r e a s e t h e i r being  caught  by  eagles  (see  Pulliam  (Forbes 1989).  Herons t h a t  v i g i l a n c e and reduce the chances o f and  Caraco  1984).  This  p r e d i c t s t h a t heron n e s t s should be more clumped i n e a g l e areas than  83  nest  hypothesis elsewhere  in  their  range.  Thus,  Lack's  (1968)  herons from the p e r s p e c t i v e o f a d u l t  Habitat  needs  second main  preferred  foraging  be examined  conclusion  habitats  is  that  herons  that s h i f t  lagoons,  in  my  seasonally,  study  have  as d i c t a t e d  beds and e e l g r a s s beds are the  for  feeding  on beaches was s i g n i f i c a n t l y  low t i d e s  breeding  kelp  habitat  female  herons.  each month of the y e a r  I  showed  that  a  set  correlated  (Chapter  4).  with the Moreover,  estimated  energetic  needs  Blue Herons might  could  no  longer  have a f l e x i b l e  be met  social  on  Time o f  and  waters, feeding  of  herons  number o f hours juveniles  and l a t e r ,  g r a s s l a n d s when beaches  their  (Chapter  some o t h e r  elsewhere  of  in t h e i r  4).  birds range.  Breeding  My t h i r d  main c o n c l u s i o n i s t h a t the time o f  seasonal  availability  energetic  demands o f  estimated  energy t h r e s h o l d  females, about  number  system, l i k e  (see Stacey and Koenig 1990), and behave d i f f e r e n t l y  of  by t i d e s  preferred  the  p o s t - b r e e d i n g a d u l t s vacated beaches f o r marshlands and  Great  for  survival.  Because o f a s t r o n g seasonal movement o f small f i s h e s to i n s h o r e  shallow t i d a l  the  to  Selection  My  food.  hypothesis  whereas  one  month  the  their  (Chapter  5).  for  occurred  i n May (Table  the  surge  food  to  chicks.  greatest  after  The  of  Egg-laying  females  and  began about  not  energy  food  demands o f  energy  the  available  9 days  energy  to  growing  to  the  females  an  egg-laying  in  had  peaked  the  lagoon  s u r p l u s might be used t o r e c o v e r body c o n d i t i o n and prepare to moult the  flight  elsewhere  in  British  84  Columbia  in  after  chicks occurred  adults  feeding  the  This  Herons  when sea perch a r r i v e d  by  shallow waters.  feathers.  5-5)  egg-laying  i s e s t a b l i s h e d by  egg p r o d u c t i o n had been c r o s s e d by  food  in  breeding  (Simpson  1984)  and  in  California  (Brandman  1976)  have  chicks in  However, mine i s one o f the f i r s t rather  the  nest  when  fish  are  s t u d i e s i n which food energy  availability,  than food abundance, has been shown t o be r e l a t e d t o energy  e g g - l a y i n g a d u l t s and growing c h i c k s (see Daan et availability  is  more a c c u r a t e  than  indices of  al.  1988).  abundant.  demands of  My index o f food  food abundance used i n  other  t i m i n g o f b r e e d i n g s t u d i e s because i t combines e s t i m a t e s o f prey abundance with those o f f o r a g i n g time by h e r o n s . tide  foraging  periods  abundance o f a l l  in  early  This i s June  important  did  the major prey s p e c i e s .  not  affects  in  their  the  amount  timing of  of  time  coincide  with  I assumed t h a t a l l  beach c o n t i n u o u s l y through each low t i d e . variation  because the  Further  individuals  longest  the  low  peaks  in  herons fed on the  s t u d i e s might  spend f o r a g i n g  address how  during  low  tide  breeding.  I n t e r - r e l a t e d Patterns o f Ecology & Behaviour Empirical individual's  evidence fitness  is  from  many  related  to  scattered the  sources  combined  indicate  effects  habitat,  s p a c i n g p a t t e r n s , d i s p e r s a l and f o r a g i n g e f f i c i e n c y  1989).  The  important  distribution  factor  and  underlying  availability the  ecological  of  food  and  of  its  that  an  choice  of  ( C a t t e r a l l et  resources  behavioural  is  the  al. most  relationships  between a g e - c l a s s e s o f Y e l l o w - e y e d Juncos (Juncos phaeonotus) s h o r t l y a f t e r the b r e e d i n g season ( S u l l i v a n (Catterall  et  al.  1989).  1990) I  and S i l v e r e y e s (Zosterops now extend  the  examination  lateralis) of  in winter  ecological  and  b e h a v i o u r a l r e l a t i o n s h i p s throughout the y e a r u s i n g age- and s e x - c l a s s e s o f the Great Blue Heron as an example.  85  A d u l t versus j u v e n i l e t a c t i c s Juvenile  herons  are  poorly  suited  to  foraging  on beaches  in  autumn  w i n t e r because o f t h e i r low f o r a g i n g e f f i c i e n c y and spend more time than h u n t i n g v o l e s i n g r a s s l a n d s (Chapter 4 ) . food consumption, narrow mortality,  compared to  limits  on t h e i r  s p e c i a l i z e at  t o beaches the  f i s h r e t u r n i n g to inshore h a b i t a t s . foraging Green  specialization  1985).  feeding survival  Through  skills  use o f  pattern  habitats  i s one o f  and high  low  rates  of  are the  so t h a t  well  following  can  s p r i n g as y e a r l i n g s  to feed on  A g e - r e l a t e d s h i f t s i n use o f h a b i t a t s documented i n  summer and  they  c a t c h i n g small mammals i n g r a s s l a n d s  autumn  catch  the  other  animals  yearlings food  (Partridge  might  necessary  and  improve  for  and their  good  winter  and r e p r o d u c t i o n as 2 - y e a r - o l d s .  A d u l t females have high f o r a g i n g e f f i c i e n c y  (Chapter  4)  which makes them  w e l l s u i t e d t o f o r a g i n g on beaches d u r i n g the b r e e d i n g season and i n t o and i n g r a s s l a n d s and marshlands i n w i n t e r females  adults  adults.  J u v e n i l e herons might i n w i n t e r and r e t u r n  The r e s u l t a n t  and  (Chapter  4).  autumn,  As a r e s u l t ,  have high food consumption, use o f a wide range o f h a b i t a t s ,  rates of mortality  compared to  and low  juveniles.  Some a d u l t male herons foraged on t e r r i t o r i e s  through the y e a r (Chapter  During the b r e e d i n g s e a s o n , some males t r a v e l l e d up t o 27 km each n i g h t territories  and t h e i r  n e s t s (Chapter 4 ) .  f o r a g e d w i t h females near the c o l o n y . territories  to  spend the  adult  autumn  and w i n t e r  mostly s o l i t a r y f e e d e r s on t e r r i t o r i e s .  between  When n e s t s h e l d l a r g e c h i c k s , P o s t - b r e e d i n g males r e t u r n e d (Chapter  4).  Thus,  4).  males  to  their  males  were  The f o r a g i n g e f f i c i e n c y o f t e r r i t o r i a l  males i s unknown. My study concurs with those o f S u l l i v a n (1989) and C a t t e r a l l t h a t food a v a i l a b i l i t y  i s the most important  86  factor  shaping the  et a l .  (1989)  behaviour  of  age- and s e x - c l a s s e s . individual 1980, study,  fitness  Bildstein  unimportant  to  and s u r v i v a l  1983,  dominance  In many s p e c i e s , dominance i s an important  is adult  for  all  A r c e s e and Smith important females  to and  age1985,  adult  and sex c l a s s e s ( e . g . Catterall  males  juveniles  grasslands.  87  feature  on  et  al.  feeding  feeding  in  of  Monaghan  1989).  In my  territories  but  marshlands  and  LITERATURE CITED A l a t a l o , R . V . , A . Lundberg and S. U l f s t r a n d . 1985. H a b i t a t s e l e c t i o n i n the P i e d F l y c a t a c h e r Ficedula hypoleuca. IN: M . L . Cody, H a b i t a t s e l e c t i o n i n b i r d s . Academic P r e s s , N.Y. Pp. 59-83. American O r n i t h o l o g i s t s ' U n i o n . ed. A l l e n P r e s s , Kansas.  1983.  Checklist  of  North American b i r d s .  6th  A r c e s e , P. and J . N . M . S m i t h . 1985. Phenotypic c o r r e l a t e s and e c o l o g i c a l consequences o f dominance i n song sparrows. J . Anim. E c o l . 54:817-830. B a y e r , R.D. 1978. A s p e c t s o f an Oregon e s t u a r i n e IN: A . Sprunt IV, J . C . Ogden and S. W i n k l e r . Soc. Res. R p t . No. 7, N.Y. Pp:213-218. B a y e r , R.D. 1981. Regional O r n i t h o l . 52:210-213.  variation  o f Great  B a y e r , R.D. 1985. B i l l l e n g t h o f herons s i z e . C o l o n i a l Waterbirds 8:104-109.  Great Blue Heron p o p u l a t i o n . Wading b i r d s . N a t i o n . Audub.  Blue Heron l o n g e v i t y .  and e g r e t s  J.  as an e s t i m a t o r  B a y e r , R.D. 1985b. S h i n e r perch and P a c i f i c staghorn e s t u a r y , Oregon. Northwest S c i . 59:230-240.  of  s c u l p i n s i n the  Beacham, T . D . 1980. Breeding c h a r a c t e r i s t i c s o f Townsend's townsendii) d u r i n g p o p u l a t i o n f l u c t u a t i o n s . Can. J . Z o o l . B e n t , A.C% 1963. L i f e h i s t o r i e s o f North American marsh b i r d s . Mus. B u l l . 135 (Dover r e p r i n t ) . N.Y.  Field  prey  Yaquina  vole (Microtus 58:623-625. U.S.-  National  B i l d s t e i n , K . L . 1983. A g e - r e l a t e d d i f f e r e n c e s i n the f l o c k i n g and b e h a v i o u r o f White I b i s e s i n a South C a r o l i n a s a l t marsh. Waterbirds 6:45-53.  foraging Colonial  B l a c k , B . B . and M.W. C o l l u p y . 1983. Nocturnal a c t i v i t y o f Great Blue in a north F l o r i d a saltmarsh. J . F i e l d O r n i t h o l . 53:403-406.  Herons  B l u s , L . J , C . J Henny, and T . E . K a i s e r . 1980. P o l l u t i o n e c o l o g y o f breeding Great Blue Herons i n the Columbia B a s i n , Oregon and Washington. M u r r e l e t 61: 6 3 - 7 1 . B o v i n o , R.R. and E . H . B u r t t , J r . 1979. Weather-dependent Herons (Ardea herodias). Auk 96:628-629.  f o r a g i n g o f Great Blue  Brandman, M. 1976. A q u a n t i t a t i v e a n a l y s i s o f the annual c y c l e o f b e h a v i o r i n the Great Blue Heron (Ardea herodias). Unpubl. P h . D . D i s s e r t . , U n i v . C a l i f . Los Angeles. Brown, J . L . 1969. The b u f f e r N a t u r a l . 103:347-354.  e f f e c t and p r o d u c t i v i t y  88  in t i t  populations.  Amer.  Bryant, D.M. and A . K . T u r n e r . 1982. C e n t r a l - p l a c e f o r a g i n g by swallows ( H i r u n d i n i d a e ) : the q u e s t i o n o f l o a d s i z e . Anim. Behav. 30:845-856. B u r g e r , J . 1988. E f f e c t s of O r n i t h o l . C o n g r . , Ottawa.  age on f o r a g i n g Pp. 1127-1140.  birds.  Proc.  XIX  Internat.  B u t l e r , R.W. 1989. Breeding e c o l o g y and p o p u l a t i o n t r e n d s o f the Great Blue Heron (Ardea herodias fannini), i n the S t r a i t o f G e o r g i a , B r i t i s h C o l u m b i a . I N : K. Vermeer and R.W. B u t l e r ( e d s . ) . The s t a t u s and e c o l o g y o f marine and s h o r e l i n e b i r d s i n the S t r a i t o f G e o r g i a , B r i t i s h Columbia. Can. W i l d l . S e r v . S p e c . P u b l . , Ottawa. Pp: 112-117. B u t l e r , R.W. and R.W. C a m p b e l l . 1987. The b i r d s o f the F r a s e r R i v e r p o p u l a t i o n s , e c o l o g y and i n t e r n a t i o n a l s i g n i f i c a n c e . Can. W i l d l . O c c a s . Paper No. 65, Ottawa. B u t l e r , R.W., A . M . B r e a u l t and T . M . S u l l i v a n 1990. a t e l e s c o p e . J . F i e l d O r n i t h o l . 61:111-114.  Measuring animals  delta: Serv.,  through  B y r d , M.A. 1978. D i s p e r s a l and movements o f s i x North American C i c o n i i f o r m e s . IN: A . Sprunt IV, J . C . Ogden and S . W i n k l e r . Wading b i r d s . N a t i o n . Audub. S o c . R e s . Rpt. No. 7, N.Y. Pp:161-186. C a l a m b o k i d i , J . , S . M . S p e i c h , J . P e a r d , G . H . S t e i g e r , J . C . Cubbage, D.M. Fry and L . J . Lowenstine. 1985. B i o l o g y o f Puget Sound marine mammals and marine b i r d s : p o p u l a t i o n h e a l t h and evidence o f p o l l u t i o n e f f e c t s . N a t i o n . Oceanic and Atmosph. Admin. T e c h . Memorand. NOSOMA 18, R o c k v i T l e , M a r y l a n d . C a r a c o , T . S . , S . M a r t i n d a l e and T . S . * Whitham. 1980. An e m p i r i c a l demonstration o f r i s k - s e n s i t i v e f o r a g i n g p r e f e r e n c e s . Anim. Behav. 28:820-830. Caraco,  T . S . 1981.  R i s k - s e n s i t i v i t y and f o r a g i n g g r o u p s . E c o l o g y . 62:527-531.  C a s t r o , G . , N. Stoyan and J . P . Myers. 1989. A s s i m i l a t i o n e f f i c i e n c y i n b i r d s : a f u n c t i o n o f taxon o f food type? Comp. Biochem. P h y s i o . V o l . 92A:271- 278. C a t t e r a l l , C P . , J . Kikkawa, and C . Gray. 1989. I n t e r - r e l a t e d age-dependent p a t t e r n s o f e c o l o g y and behaviour i n a p o p u l a t i o n o f S i l v e r e y e s (Aves: Z o s t e r o p i d a e ) . J . Anim. E c o l . 58:557-570. Cave, A . J . 1968. The b r e e d i n g o f the k e s t r e l , FaIco tinnunculus L., r e c l a i m e d area O o s t e l i j k F l e v o l a n d . Neth. Z . Z o o l . 18:313-407. Charnov, E . L . 1976. Optimal f o r a g i n g : the marginal v a l u e theorem. 9:129-136. Cook, D . C . 1978. Grey Herons Ardea cinera Ythan E s t u a r y . B i r d Study 2 5 : 1 1 - 1 6 .  in  the  Popul. B i o l .  h o l d i n g f e e d i n g t e r r i t o r i e s on the  Cummins, K . C . and J . C . Wuycheck. 1971. C a l o r i c e q u i v a l e n t s f o r i n v e s t i g a t i o n s i n e c o l o g i c a l e n e r g e t i c s . M i t t . i n t . V e r . t h e o r . angew. L i m n o l . 18:1- 158.  89  C u s t e r , T.W. and R . G . Osborn, 1978. Feeding h a b i t a t use by c o l o n i a l l y - b r e e d i n g h e r o n s , e g r e t s , and i b i s e s i n North C a r o l i n a . Auk 95:733-743. Daan, S . , C. D i j k s t r a , R. Drent and T . M e i j e r . 1988. Food s u p p l y and the annual t i m i n g o f avian r e p r o d u c t i o n . A c t a XIX Congr. P r o c . I n t e r n a t . O r n i t h o l . , Pp: -392-407, U n i v . Ottawa P r e s s , Ottawa, Canada. D a v i e s , N . B . and A . Lundberg. 1985. The i n f l u e n c e o f food on the time budgets and t i m i n g o f breeding o f the Dunnock Prunella /nodularis. I b i s 127:100110. Department o f F i s h e r i e s & Oceans. 1986. Canadian t i d e and c u r r e n t tables. V o l . 5. Juan de Fuca S t r a i t and S t r a i t o f G e o r g i a . F i s h e r i e s & Oceans, Inform. P u b l . Branch, Ottawa. Department o f F i s h e r i e s & Oceans. 1987. Canadian t i d e and c u r r e n t tables. V o l . 5. Juan de Fuca S t r a i t and S t r a i t o f G e o r g i a . F i s h e r i e s & Oceans, Inform. P u b l . B r a n c h , Ottawa. Department o f F i s h e r i e s & Oceans. 1988. Canadian t i d e and c u r r e n t tables. V o l . 5. Juan de Fuca S t r a i t and S t r a i t o f G e o r g i a . F i s h e r i e s & Oceans, Inform. P u b l . B r a n c h , Ottawa. Department o f F i s h e r i e s & Oceans. 1989. Canadian t i d e and c u r r e n t tables. V o l . 5. Juan de Fuca S t r a i t and S t r a i t o f G e o r g i a . F i s h e r i e s & Oceans, Inform. P u b l . B r a n c h , Ottawa. DesGranges, J - L . and P. L a p o r t e . 1979. Second t o u r o f i n s p e c t i o n o f h e r o n r i e s , 1978. Can. W i l d l . Serv.. P r o g r . Note No. 105, Ottawa.  Quebec  :  DesGranges, J - L . , P. Laporte and G. C h a p d e l a i n e . 1979. F i r s t t o u r o f i n s p e c t i o n o f Quebec h e r o n r i e s , 1977. Can. W i l d l . S e r v . P r o g r . Note No. 93, Ottawa. D i j k s t r a , C , A . B u l t , A . B i j l s m a , S. Daan, T . M e i j e r and M. Z i j l s t r a . 1990. Brood m a n i p u l a t i o n s i n the k e s t r e l , Falco tinnunculus: e f f e c t s on f i t n e s s o f p a r e n t s and o f f s p r i n g . J . Anim. E c o l . 59:269-286. Dowd, E.M. and L . D . F l a k e . 1985. Foraging h a b i t a t and movements o f n e s t i n g Great Blue Herons i n a p r a i r i e r i v e r ecosystem, South Dakota. J . F i e l d O r n i t h o l . 56:379-387. D r a u l a n s , D. and J . van Vessem. 1985. A g e - r e l a t e d d i f f e r e n c e s time and space by r a d i o - t a g g e d Grey Herons (Ardea cinerea) Anim. E c o l . 54:771-780.  i n the use o f in winter. J .  D r a u l a n s , D. and J . Hannon. 1988. D i s t r i b u t i o n and f o r a g i n g behaviour o f Grey Herons Ardea cinerea i n adjacent t i d a l and n o n - t i d a l a r e a s . O r n i s Scand. 19: 297-304. D r e n t , R.H. and S. Daan. 1980. The prudent avian b r e e d i n g . Ardea 68:225-252.  90  parent:  energetic  adjustments  in  Dugan, P . , H. Hafner and V. Boy. 1988. H a b i t a t switches and f o r a g i n g success i n the L i t t l e Egret (Egretta grazetta). P r o c . I n t e r n a t . O r n i t h o l . Congr. 19:1868-1877. Dunn, E i H . 1975. 92:553-565.  Caloric  intake  of  nestling  Double-crested  Cormorants.  D u s i , J . L . and R . T . D u s i . 1968. E c o l o g i c a l f a c t o r s c o n t r i b u t i n g f a i l u r e i n a heron c o l o n y . Wilson B u l l . 80:458-460.  to  Auk  nesting  E l l i o t t , J . E . , R.W. Butler, R.J. Norstrom and P.E. Whitehead. 1989. Environmental contaminants and r e p r o d u c t i v e success o f Great Blue Herons (Ardea herodias) i n B r i t i s h Columbia, 1986-87. E n v i r o n . P o l l u t . 59:91- 114. E n g l i s h , S . 1978. D i s t r i b u t i o n and e c o l o g y o f Great Blue Heron c o l o n i e s on the W i l l a m e t t e R i v e r , Oregon. IN: A . Sprunt IV, J . C . Ogden and S . W i n k l e r . Wading b i r d s . N a t i o n a l Audubon S o c . Res. Rpt. No. 7. Pp:235-246. F a s o l a , M. and F. B a r b i e r i . 1977. Factors a f f e c t i n g h e r o n r i e s i n n o r t h e r n I t a l y . I b i s 120:537-540. F o r b e s , L . S . 1982.  Prey m a n i p u l a t i o n  the  distribution  i n the Great Blue Heron. M u r r e l e t  Forbes, L.S. 1989. Coloniality in r e c o n s i d e r e d . C o l o n i a l Waterbirds  herons: Lack's 12:24-29.  predation  of  63:89.  hypothesis  F o r b e s , L . S . , K. Simpson, J . P . K e l s a l l and D.R. F l o o k . 1985a. Reproductive s u c c e s s o f Great Blue Herons i n B r i t i s h Columbia. Can. J . Z o o l . 63:11101113. F o r b e s , L . S . , K. Simpson, J . P . K e l s a l l and D.R. F l o o k . 1985b. Great Blue Heron c o l o n i e s i n B r i t i s h Columbia. Unpubl. R p t . , Can. W i l d l . S e r v . , D e l t a , B . C . F r e t w e l l , S . D . and H . L . L u c a s , J r . 1970. On t e r r i t o r i a l b e h a v i o r and other f a c t o r s i n f l u e n c i n g h a b i t a t d i s t r i b u t i o n i n b i r d s . A c t a B i o t h e o r . 19:16-36. F r e t w e l l , S . D . 1972. P o p u l a t i o n s i n a seasonal environment. B i o l . 5. P r i n c e t o n U n i v . P r e s s , P r i n c e t o n , N . J .  Monogr.  Populat.  G i b b s , J . P . , S . Woodward, M.L. Hunter and A . E . H u t c h i n s o n . 1987. Determinants o f Great Blue Heron colony d i s t r i b u t i o n i n c o a s t a l Maine. Auk 104:38-47. Gill,  F . B . , L . L . W o l f , and F.R. Hainsworth. 1975. F o r a g i n g e f f i c i e n c i e s time-budgets i n n e c t a r f e e d i n g ' b i r d s . E c o l . 56:117-128.  and  Gordon, D.K. and C D . L e v i n g s . 1984. Seasonal changes o f inshore fish p o p u l a t i o n s on Sturgeon and Roberts banks, F r a s e r R i v e r e s t u a r y , B r i t i s h C o l u m b i a . Can. T e c h . Rep. F i s h . A q u a t . S c i . 1240. Goss-Custard, mortality IN: P.R. wildfowl  J . D . and S . E . A . Le V. d i t D u r e l l . 1984. Feeding e c o l o g y , w i n t e r and the p o p u l a t i o n dynamics o f O y s t e r c a t c h e r s on the Exe e s t u a r y . Evans, J . D . G o s s - C u s t a r d and W.G. H a l e . C o a s t a l waders and i n w i n t e r . Cambridge U n i v . P r e s s , Cambridge. Pp:190-208.  91  G o s s - C u s t a r d , J . D . and S . E . A . Le V. d i t D u r e l l . 1987a. A g e - r e l a t e d e f f e c t s i n O y s t e r c a t c h e r s , Haematopus ostralegus, f e e d i n g on m u s s e l s , Mytilus edulis. I: f o r a g i n g e f f i c i e n c y and i n t e r f e r e n c e . J . Anim. E c o l . 56:521- 536. G o s s - C u s t a r d , J . D . and S . E . A . Le V. d i t D u r e l l . 1987b. A g e - r e l a t e d e f f e c t s i n O y s t e r c a t c h e r s , Haematopus ostralegus, f e e d i n g on m u s s e l s , Mytilus edulis. II: a g g r e s s i o n . J . Anim. E c o l . 56:536-548. G o s s - C u s t a r d , J . D . and S . E . A . Le V. d i t D u r e l l . 1987c. A g e - r e l a t e d e f f e c t s i n O y s t e r c a t c h e r s , Haematopus ostralegus, f e e d i n g on m u s s e l s , Mytilus edulis. Ill: the e f f e c t o f i n t e r f e r e n c e on o v e r a l l i n t a k e r a t e . J . Anim. E c o l . 56:549-558. Hancock, J . York.  and J . K u s h l a n .  1984.  The herons  handbook.  H a r t , J . L . 1973. P a c i f i c f i s h e s o f Canada. F i s h . Ottawa. 740 pp. Hay,  Harper  and Row,  Res. B d . Canada, B u l l .  New  180,  D . E . M.C. H e a l e y , L . J . R i c h a r d s and J . B . M a r l i a v e . 1989. Distribtuion, abundance, and h a b i t a t o f prey f i s h e s i n the S t r a i t o f G e o r g i a . I N : K. Vermeer and R.W. B u t l e r ( e d s . ) The s t a t u s and e c o l o g y o f marine and s h o r e l i n e b i r d s i n the S t r a i t o f G e o r g i a , B r i t i s h Columbia. Can. W i l d l . S e r v . S p e c . P u b l . , Ottawa. Pp:  Henny, C . J . 1972. An a n a l y s i s o f the p o p u l a t i o n dynamics o f s e l e c t e d avian s p e c i e s w i t h s p e c i a l r e f e r e n c e to changes d u r i n g the modern p e s t i c i d e e r a . U . S . F i s h and W i l d l . S e r v . , W i l d l . Research Report 1. Washington, D . C . Henny, C . J . and M.R. B e t h e r s . 1971. P o p u l a t i o n e c o l o g y o f the Great Blue Heron, w i t h s p e c i a l r e f e r e n c e t o western Oregon. Can. F i e l d - N a t u r a l . 85:205-209. Holmes, W.N. and E.M. Donaldson. 1969. The body compartments and the distribution of e l e c t r o l y t e s . IN: W.S. Hoar and D . J . R a n d a l l . Fish p h y s i o l o g y . V o l . I., Academic P r e s s , N.Y. Pp. 1-89. Hughes, G.W. 1985. The comparative ecology and - e v i d e n c e partitioning i n two p h o l i d i d f i s h e s ( P i s c e s : P h o l i d i d a e ) B r i t i s h Columbia e e l g r a s s b e d s . Can. J . Z o o l . 6 3 : 7 6 - 8 5 . H u t c h i n s o n , I. 1982. V e g e t a t i o n - e n v i r o n m e n t r e l a t i o n s I s l a n d , Richmond, B . C . Can. J . B o t . 60:452-462.  for from  resource southern  i n a b r a c k i s h marsh, L u l u  H u t c h i n s o n , I., A . Campbell P r e n t i c e and G. B r a d f i e l d . 1989. A q u a t i c p l a n t r e s o u r c e s o f the S t r a i t o f G e o r g i a . IN: K. Vermeer and R.W.. B u t l e r ( e d s . ) . The e c o l o g y and s t a t u s o f marine and s h o r e l i n e b i r d s i n the S t r a i t o f G e o r g i a , B r i t i s h C o l u m b i a . Can. W i l d l . S e r v . S p e c . P u b l . , Ottawa. K a h l , M.P. 1964. Food e c o l o g y o f the F l o r i d a . E c o l . Monogr. 3 4 : 9 7 - 1 1 7 .  92  Wood  Stork  {Mycteria  americana)  in  Kaufmann, G.W. and E . T . Cawley. 1986. N e s t i n g success and f e e d i n g h a b i t s o f Great Blue Herons on the M i s s i s s i p p i R i v e r near C a s s v i l l e , W i s c o n s i n and East Dubuque, I l l i n o i s . P r o c . Iowa A c a d . S c i . 93:184-187. K e l s a l l , J . P . and K. Simpson. 1980. A t h r e e - y e a r study o f the Great Blue Heron i n southwestern B r i t i s h C o l u m b i a . P r o c . C o l . W a t e r b i r d G r p . (1979) 3 : 6 9 74. K e n d e i g h , S . C . 1970. Energy requirements bird. Condor 7 2 : 6 0 - 6 5 .  f o r existence  in relation  K i n g , J . R . 1973. E n e r g e t i c s o f r e p r o d u c t i o n . IN: D . S . F a r n e r b i o l o g y o f b i r d s . N a t i o n . A c a d . S c i . Washington, D . C .  to s i z e o f  (ed.).  Breeding  K r e b s , C . J . 1989. E c o l o g i c a l methodology. Harper & Row, New Y o r k . K r e b s , J . R . 1971. T e r r i t o r y L. E c o l o g y 5 2 : 2 - 2 2 .  and breeding d e n s i t y i n t h e Great T i t , Parus major.  K r e b s , J . R . 1974. C o l o n i a l n e s t i n g and s o c i a l f e e d i n g as s t r a t e g i e s f o r e x p l o i t i n g food r e s o u r c e s i n , t h e g r e a t blue heron {Ardea herodias). Behav. 51:99-131. K r e b s , J . R . , D.W. Stephens and W . J . S u t h e r l a n d . 1984. P e r s p e c t i v e s i n optimal f o r a g i n g : what a r e t h e q u e s t i o n s . IN: A . H . Brush and G . A . C l a r k , J r . P e r s p e c t i v e s i n o r n i t h o l o g y . Cambridge U n i v . P r e s s . Pp. 165-215. K u s h l a n , J . A . 1976a. Feeding behaviour o f North American h e r o n s . Auk 9 3 : 8 6 - 9 4 . K u s h l a n , J . A . 1976b. S i t e s e l e c t i o n f o r n e s t i n g c o l o n i e s by the American White I b i s Eudocimus albus i n F l o r i d a . I b i s 118:590-593. K u s h l a n , J . A . 1977. P o p u l a t i o n 94:114-122.  energetics  o f the American  White  I b i s . Auk  K u s h l a n , J . A . 1978. Feeding e c o l o g y o f wading b i r d s . IN: A . Sprunt IV, J . C . Ogden and S . W i n k l e r . Wading b i r d s . N a t i o n a l Audub. S o c . R e s . R p t . No. 7, N.Y. Pp:249-298. K u s h l a n , J . A . 1986. C o l o n i e s , s i t e s , and s u r v e y s : the t e r m i n o l o g y waterbird studies. C o l o n i a l Waterbirds 9:119-120. L a c k , D. 1950. The b r e e d i n g seasons o f European b i r d s . L a c k , D. 1954. The n a t u r a l Oxford.  regulation  o f animal  of colonial  I b i s 92:288-316.  numbers.  Clarendon  Press,  L a c k , D. 1966. P o p u l a t i o n s t u d i e s o f b i r d s . Clarendon P r e s s , O x f o r d . L a c k , D. 1968. E c o l o g i c a l a d a p t a t i o n s  f o r breeding i n b i r d s . Methuen, London.  L i n d e n , M. and A . M o l l e r . 1989. Cost o f r e p r o d u c t i o n and c o v a r i a t i o n h i s t o r y t r a i t s i n b i r d s . Trends i n E v o l . and E c o l . 4:367-371.  93  of l i f e  M a r i o n , L. 1989. T e r r i t o r i a l f e e d i n g and c o l o n i a l breeding are not m u t u a l l y e x c l u s i v e : the case o f the Grey Heron (Ardea cinerea). J . Anim. E c o l . 58:693-710. M a r t i n , T . E . 1987. Food as a p e r s p e c t i v e . Ann. Rev. E c o l .  limit Syst.  on breeding 18:453-487.  birds:  McAloney, K. 1973. The b r e e d i n g b i o l o g y o f the Great Blue I s l a n d , Nova S c o t i a . C a n . F i e l d - N a t u r a l . 87:137-140. McNamara, Biol.  J . M . 1982. Optimal 21:269-288.  patch  use i n a s t o c h a s t i c  a  life-history  Heron  on Tobacco  environment.  Popul.  M e y e r r i e c k s , A . J . 1960. Comparative breeding b e h a v i o r o f f o u r s p e c i e s o f North American h e r o n s . P u b l . N u t t a l l O r n i t h o l . Club No. 2. Cambridge, Mass. M i l l s t e i n , P. L e S . , I. P r e s t t and A . A . B e l l . Grey Heron. Ardea 58:171-257.  1970. The b r e e d i n g  c y c l e o f the  Mock, D.W. 1976. P a i r f o r m a t i o n d i s p l a y s o f the Great Blue Heron. Wilson B u l l . 88:185-230. Mock, D.W. 1985. S i b l i c i d a l N a t u r a l . 125:327-343.  brood r e d u c t i o n :  the p r e y - s i z e  hypothesis.  Amer.  Monaghan, P. 1980. Dominance and d i s p e r s a l between f e e d i n g s i t e s i n the H e r r i n g Gui (Larus argentatus) . Anim. Behav. 28:521-527. Moreau,  R . E . 1950. The. breeding  seasons o f A f r i c a n > b i r d s .  I b i s 92:223-267.  Morse, D . H . 1985. H a b i t a t s e l e c t i o n i n North American p a r u l i d w a r b l e r s . I N : M . L . Cody ( e d . ) . H a b i t a t s e l e c t i o n i n b i r d s . Academic P r e s s , N . Y . , Pp. 131-157. Moul, I . E . 1990. Environmental contaminants, d i s t u r b a n c e and b r e e d i n g f a i l u r e at a Great Blue Heron c o l o n y on Vancouver I s l a n d . U n p u b l . MSc. t h e s i s , U n i v . B r i t . C o l . , Vancouver. Murton, R . K . and N . J . Westwood. Oxford.  1977. Avian breeding c y c l e s . Clarendon P r e s s ,  Noordwijk, A . J . v a n , J . H . van Balen and W. S c h a r l o o . 1981. G e n e t i c v a r i a t i o n i n the t i m i n g o f r e p r o d u c t i o n i n the Great T i t . O e c o l o g i a 49:158-166. Norman, D . M . , A . M . B r e a u l t and I . E . Moul. p r e d a t i o n at Great Blue Heron c o l o n i e s .  1989. Bald Eagle i n c u r s i o n s and C o l o n i a l Waterbirds 12:143-230.  O r e l l , M. and M. Ojanen. 1983. Timing and l e n g t h o f the b r e e d i n g season o f the Great T i t , Parus major and Willow T i t , P. montanus near O u l u , n o r t h e r n F i n l a n d . Ardea 71: 183-198.  94  O r i a n s , G . H . and N . E . Pearson. 1979. On the theory o f c e n t r a l p l a c e f o r a g i n g . IN: D . J . Horn, R.D. M i t c h e l l and G.R. S t a i r s ( e d s . ) . A n a l y s i s o f E c o l o g i c a l Systems. Ohio S t a t e U n i v . P r e s s , Columbus, O h i o , Pp:155-177. P a i n e , J . M . 1972. A study o f c o l o n i a l n e s t i n g i n the Great Blue Heron (Ardea herodias). U n p u b l . B . S c . t h e s i s , U n i v . B r i t . C o l . , Vancouver, Palmer, R . S . 1962. Haven.  Handbook o f North American b i r d s . Yale U n i v e r s i t y P r e s s , New  Parker, G.A. and W . J . Sutherland. 1986. Ideal free distribution i n d i v i d u a l s d i f f e r i n c o m p e t i t i v e a b i l i t y : phenotype-1imited ideal models. Anim. Behav. 34:1222-1232.  when free  P a r k e r , J . 1980. Great Blue Herons (Ardea herodias) i n northwestern Montana: n e s t i n g h a b i t a t and the e f f e c t s o f human d i s t u r b a n c e . Unpubl. M . S c . t h e s i s . U n i v . Montana. P a r t r i d g e , L. and P. Green. 1985. I n t r a s p e c i f i c f e e d i n g s p e c i a l i z a t i o n s and p o p u l a t i o n dynamics. IN: R.M. S i b l y and R.H. S m i t h . B e h a v i o u r a l e c o l o g y . B l a c k w e l l S c i . P u b l . London. Pp. 207-226. P e r r i n s , C M . 1965. P o p u l a t i o n f l u c t u a t i o n s and c l u t c h - s i z e Parus major L. J . Anim. E c o l . 34:601-647. Perrins,  CM.  1970.  The t i m i n g  P e r r i n s , C M . and T . R . B l a c k i e , London.  of b i r d s '  Birkhead.  1983.  i n the Great  breeding s e a s o n s . I b i s Avian ecology.  Tertiary  Tit,  112:242-255. Level  Pimm, S . L . , M . L . Rosenzweig and W. M i t c h e l l . 1985. Competition s e l e c t i o n : f i e l d t e s t s o f a t h e o r y . Ecology 66:798-807.  and  Biol.,  food  P o w e l l , G . V . N . 1983. Food a v a i l a b i l i t y and r e p r o d u c t i o n by Great White Herons Ardea herodias: a food a d d i t i o n s t u d y . C o l o n i a l Waterbirds 6:138-147. P o w e l l , G . V . M . and A . H . P o w e l l . 1986. Reproduction by Great White Herons Ardea herodias i n F l o r i d a Bay as an i n d i c a t o r o f h a b i t a t q u a l i t y . B i o l . Conserv. 36:101-113. P r a t t , H.M. central  1970. Breeding b i o l o g y o f Great C a l i f o r n i a . Condor 72:407-416.  Blue Herons and Common Egrets  in  P r a t t , H.M. 1972a. N e s t i n g s u c c e s s ' o f Great Blue Herons and Common Egrets Audubon Canyon Ranch i n 1971. Amer. B i r d s 26:699-702.  at  P r a t t , H.M. 1972b. N e s t i n g success o f Common Egrets and Great the San F r a n c i s c o Bay r e g i o n . Condor 74:447-453.  in  Blue Herons  P r a t t , H.M. 1980. D i r e c t i o n s and t i m i n g o f Great Blue Heron f o r a g i n g f l i g h t s from a C a l i f o r n i a c o l o n y : i m p l i c a t i o n s f o r s o c i a l f a c i l i t a t i o n o f food f i n d i n g . Wilson B u l l . 92:489-496.  95  P r a t t , H.M. and D.W. W i n k l e r . 1985. C l u t c h s i z e , timing o f l a y i n g and r e p r o d u c t i v e success i n a c o l o n y o f Great Blue Herons and Great E g r e t s . Auk 102:49-63. P u l l i a m , H.R. and T . S . C a r a c o . 1984. L i v i n g i n g r o u p s : i s t h e r e an optimum group size? IN: J . R . Krebs and N . B . D a v i e s . Behavioural Ecology. B l a c k w e l l . Pp. 122-147. Quinney, T . E . 1982. Growth, d i e t , Wilson B u l l . 94:571-577.  and m o r t a l i t y o f n e s t l i n g Great Blue Herons.  Quinney, T . E . 1983. Comparison o f Great Blue Heron, Ardea herodias, r e p r o d u c t i o n at Boot I s l a n d and o t h e r Nova S c o t i a c o l o n i e s . C a n . F i e l d N a t u r a l . 97:275-278. Quinney, T . E . and P . C . S m i t h . 1979. R e p r o d u c t i v e s u c c e s s , growth o f n e s t l i n g s and f o r a g i n g behaviour o f t h e Great Blue Heron (Ardea herodias herodias / . . ) . C a n . W i l d l . S e r v . c o n t r a c t r e p t . n o . KL229-5-7077, Ottawa. Quinney, T . E . and efficiency of 58:1168-1173.  P . C . Smith. 1980. Comparative foraging a d u l t and j u v e n i l e Great Blue H e r o n s .  R i c h n e r , H. 1986. Winter f e e d i n g Anim. Behav. 34:881-886.  strategies  of  individually  behaviour and Can. J . Z o o l .  marked  herons.  Simpson, K. 1984. F a c t o r s a f f e c t i n g r e p r o d u c t i o n i n Great Blue Herons herodias). U n p u b l . M . S c . t h e s i s , U n i v . B r i t . C o l . , 90pp.  (Ardea  Simps'on, K . , J . N . M . Smith and J . P . K e l s a l l . 1987. C o r r e l a t e s and consequences o f c o l o n i a l i t y i n Great Blue Herons. C a n . J . Z o o l . 65:572-577. Smith, S . M . 1978. The "underworld" in a t e r r i t o r i a l s t r a t e g i e s f o r f l o a t e r s . Amer. N a t u r a l . 112:571-582. Smithe,  F . B . 1975. N a t u r a l i s t ' s  S t a c e y , P . B . and W.B. K o e n i g . Univ. Press. Stalmaster  sparrow:  C o l o r G u i d e . Amer. Mus. N a t . H i s t . 1990. C o o p e r a t i v e  breeding  in birds.  adaptive  N.Y. Cambridge  M.V. 1987. The B a l d E a g l e . U n i v e r s e , N.Y.  S t e p h e n s , D.W. and J . R . K r e b s . 1986. F o r a g i n g Princeton Univ. Press, Princeton, N . J .  theory.  Monogr.  Behav.  Ecol.,  S u l l i v a n , J . P . 1988. E f f e c t s o f food p r o v i s i o n i n g r a t e s and number f l e d g e d on n e s t l i n g a g g r e s s i o n i n Great Blue Herons. C o l o n i a l W a t e r b i r d s 11:220-226. S u l l i v a n , K . A . 1989. S t a r v a t i o n and p r e d a t i o n : a g e - s p e c i f i c mortality in j u v e n i l e Y e l l o w - e y e d Juncos (Junco phaeonotus). J . Anim. E c o l . 58:275-286. S u l l i v a n , K . A . 1990. D a i l y time a l l o c a t i o n among a d u l t and immature eyed j u n c o s over the b r e e d i n g s e a s o n . Anim. Behav. 39:380-387.  96  yellow-  S u t h e r l a n d , W . J . and G . A . P a r k e r . IN: R.M. S i b l y and R.H. Smith P u b l . , London. Pp:255-273.  1985. D i s t r i b u t i o n o f unequal c o m p e t i t o r s . ( e d s . ) . Behavioural E c o l o g y , B l a c k w e l l S c i .  Swennen, C. M.F. Leopold and L.L.M. o y s t e r c a t c h e r s , Haematopus ostralegus, Anim. Behav. 3 8 : 8 - 2 2 .  DeBruijn. 1989. Time-stressed can i n c r e a s e t h e i r i n t a k e r a t e .  T a i t t , M . J . , J . H . W . G i p p s , C . J . Krebs and Z . D u n d j e r s k i . 1981. The e f f e c t o f e x t r a food and c o v e r on d e c l i n i n g p o p u l a t i o n s of Hicrotus townsendii. Can. J . Z o o l . 59:1593-1599. T a i t t , M . J . and C . J . K r e b s . 1983. P r e d a t i o n , c o v e r , and food m a n i p u l a t i o n s d u r i n g a s p r i n g d e c l i n e o f Nicrotus townsendii. J . Anim. E c o l . 52:837- 848. Urhahn, H . J . M . 1968. Feeding e c o l o g y o f the Great Blue t h e s i s , Dept. Z o o l . , U n i v . o f B r i t . C o l . 43 pp.  Heron.  Vermeer, K. 1969. 83:237-242.  Can.  Great  Blue Heron c o l o n i e s  in A l b e r t a .  U n p u b l . BSc.  Field-Natural.  Vermeer, K. K . H . Morgan, R.W. B u t l e r and G . E . J . S m i t h . 1989. Population, n e s t i n g h a b i t a t and food o f Bald Eagles i n the G u l f I s l a n d s , J N : K. Vermeer and R.W. B u t l e r ( e d s . ) . The s t a t u s and ecology o f marine and s h o r e l i n e b i r d s i n the S t r a i t o f G e o r g i a , B r i t i s h Columbia. Can. W i l d l . S e r v . Spec. P u b l . Ottawa. Vessem J . van and D. D r a u l a n s . 1986. The a d a p t i v e s i g n i f i c a n c e breeding i n the Grey .Heron Ardea cinerea: inter.and v a r i a b i l i t y in breeding success. O r n i s Scand. 17:356-362. Vos,  of colonial intra-colony  D . K . , D.A. Ryder and W.D. G r a u l . 1985. Response o f b r e e d i n g Great Blue Herons to human d i s t u r b a n c e i n n o r t h c e n t r a l C o l o r a d o . C o l o n i a l Waterbirds 8:13-22.  W a l s b e r g , G . E . 1983. A v i a n e c o l o g i c a l e n e r g e t i c s . IN: D . S . F a r n e r , J . R . King and K . C . Parkes ( e d s ) . A v i a n B i o l o g y . V o l . 7, Academic P r e s s . N.Y. Pp:161220. Weathers, W.W. and K . A . S u l l i v a n . 1989. J u v e n i l e f o r a g i n g p r o f i c i e n c y , p a r e n t a l e f f o r t and a v i a n r e p r o d u c t i v e s u c c e s s . E c o l . Monogr. 59:223-246. Weibe, J . P . 1968. The r e p r o d u c t i v e c y c l e o f the v i v i p a r o u s Cymatogaster aggregata. Gibbon's. Can. J . Z o o l . 46:1221-1234. Werschkul, D . F . , reproductive 58:7-12.  sea  perch,  E. McMahon and M. L e i t s c h u h . 1977. O b s e r v a t i o n s on the e c o l o g y o f the Great Blue Heron i n western Oregon. M u r r e l e t  Whitham, T . G . 1980. The theory o f h a b i t a t s e l e c t i o n : examined u s i n g Pemphigus a p h i d s . Amer. N a t u r a l . 115:449-466.  97  and  extended  W i j n a n d t s , H. 1984. Ardea 7 2 : 1 - 4 2 .  E c o l o g i c a l e n e r g e t i c s o f the  W i t t e n b e r g e r , J . F . 1981.  Long-eared Owl  (Asio  otus).  Animal s o c i a l b e h a v i o r . Duxbury P r e s s , B o s t o n , Mass.  W i t t e n b e r g e r , J . F . J r . and G . H . Hunt. 1985. The a d a p t i v e s i g n i f i c a n c e o f c o l o n i a l i t y i n b i r d s , i n : D . S . F a r n e r , J . R . King and K . C . P a r k e s . A v i a n b i o l o g y , V o l . V I I I . p p : 2 - 7 8 , Academic P r e s s , N.Y. Zar,  J . H . 1984.  Biostatistical  a n a l y s i s . 2nd e d . P r e n t i c e - H a l l ,  N.J.  Z w a r t s , L. and A - M . B l o m e r t . 1990. S e l e c t i v i t y o f Whimbrels f e e d i n g on f i d d l e r c r a b s e x p l a i n e d by component s p e c i f i c d i g e s t i b i l i t i e s . Ardea 78:193-208.  98  Appendix  I.  R e g r e s s i o n equations o f c a t c h per u n i t e f f o r t (Y) versus c u m u l a t i v e c a t ch (X) and e x t r a p o l a t e d minimum p o p u l a t i o n i n the p a r t i a l e n c l o s u r e f o r f o u r f r e q u e n t l y eaten prey s p e c i e s eaten by Great Blue Herons i n Sidney lagoon i n 1988. 3  Correlation coefficient  Extrapolated population  Stage  Regression  Eggs  Y = 45.28 - 0.12X  -  0.72  349  48  Y = l o g 2.6 - 0.001X  -  0.96  3258  6  Large c h i c k s  Y = 183.67 - 0.64X  -  0.99  287  5  Fledglings  Y = 14.73 - 0.30X  -  0.76  49  6  Small  chicks  a -  Pholis ornata, griseolineatus.  Gasterosteus  aculeatus,  b -  does not i n c l u d e mobile f i s h e s , a l s o eaten by many h e r o n s .  Leptocottus  principally  99  armatus,  Cymatogaster  b  S.E.  Signathus  aggregata,  

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