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Habitat selection and time of breeding in the Great Blue Heron, (Ardea herodias) 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 , 1976 M . S c , Simon F r a s e r U n i v e r s i t y , 1980 A T H E S I S SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF SCIENCE ( D e p a r t m e n t o f Z o o l o g y ) 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 r e q u i r e d s t a n d a r d THE UNIVERSITY 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. The University of British Columbia Vancouver, Canada Department Date DE-6 (2/88) ABSTRACT This t h e s i s examines the causes and consequences of habi ta t s e l e c t i o n and t iming of breeding of the Great Blue Heron (Ardea herodias). My general hypothesis was that the durat ion of low t ides and seasonal abundance of prey s t rong ly in f luenced the l o c a t i o n of c o l o n y - s i t e s ; t iming of the breeding season; habi ta t s h i f t s ; and the use of space by foraging herons of d i f f e r e n t age- and s e x - c l a s s e s . I s tudied Great Blue Herons along the P a c i f i c coast of Canada for f i v e breeding seasons and four w in ters . Breeding herons were studied at a colony of 85 to 100 p a i r s on Sidney Island near the town of Sidney, and per iod ic v i s i t s were made to about 40 other co lon ies around the S t r a i t of Georgia , B r i t i s h Columbia. At Sidney, I s tudied the foraging behaviour, food a v a i l a b i l i t y , habi ta t use and reproduct ive success in d e t a i l . At other c o l o n i e s , I recorded the reproduct ive success of herons, located t h e i r main feeding areas and searched fo r nests of a predator , the Bald Eagle . In the non-breeding season, I invest iga ted the foraging behaviour, d i s p e r s i o n pattern and habi ta t s h i f t s of j u v e n i l e and post -breeding adult herons in the Fraser River d e l t a . I hypothesized that heron c o l o n y - s i t e s were located near food suppl ies or away from predators . Twenty-nine of 33 c o l o n y - s i t e s were located within 6 km of t h e i r main feeding s i t e . The number of heron p a i r s was s l i g h t l y greater where eagles nested in high abundance than where eagle abundance was low, contrary to the hypothesis that breeding herons avoid areas with act ive eagle n e s t s . I hypothesized that herons began breeding in spr ing s h o r t l y a f t e r females acquired enough food energy to make eggs, or so ch icks were in nests when food was most p l e n t i f u l to t h e i r parents . Egg- lay ing began about 9 days a f te r a i i female 's d a i l y food intake crossed an energy threshold of 1715 kJ /day , whereas the peak a v a i l a b i l i t y of food energy to adul ts occurred about 35 days before the peak food demands of t h e i r c h i c k s . Food intake rates by adults increased gradua l ly in March and A p r i l with the increas ing durat ion of low t i d e s and the inshore movement of f i s h e s . Adult food intake rates reached a peak in May when sea perch were most abundant, and diminished through June and J u l y . Most j u v e n i l e and adult female herons foraged on beaches from February to October and in marshlands and grasslands from November to January. Some males returned to t e r r i t o r i e s along r iverbanks in August and remained there u n t i l the s t a r t of the next breeding season in March. I tested the hypothesis that herons leave foraging habi ta ts in autumn when they can no longer catch enough food or when in te r fe rence from c o n s p e c i f i c s reduced foraging intake rates below a threshold required to maintain t h e i r energy balance. In October and November adul ts moved to marshlands and j u v e n i l e s moved to grasslands when they could no longer maintain d a i l y energy balance on beaches as a r e s u l t of d e c l i n i n g durat ion of low t ides and food intake r a t e s . Interference competi t ion was too infrequent to expla in habi ta t s h i f t s by adult or j u v e n i l e herons in autumn. i i i AT BLUE HERON CArdea herodias) i v TABLE OF CONTENTS A b s t r a c t i i F r o n t i s p i e c e i v L i s t o f Tables v i i i L i s t F i g u r e s x Acknowledgements x i i CHAPTER ONE. INTRODUCTION 1 The Theory o f Ha b i t a t S e l e c t i o n 1 The Timing o f Breeding 3 The Study Species 4 Aims o f the Th e s i s 5 CHAPTER TWO. STUDY SPECIES, STUDY AREA AND METHODS STUDY SPECIES 6 D i s t r i b u t i o n 6 Food 6 Breeding B i o l o g y 6 Non-breeding Season 8 Age-cl a s s D e s c r i p t i o n s 8 STUDY AREA 10 Sidney I s l a n d . 10 Fr a s e r R i v e r D e l t a 10 GENERAL METHODS 13 D i s p e r s i o n i n the Breeding Season 13 D i s p e r s i o n i n the Non-breeding Season .... 14 Foraging 16 Oceanic T i d e s 17 Time o f Breeding 17 Reproductive Success 17 Sex o f Adu l t Herons 18 CHAPTER THREE. THE EFFECT OF PREDATORS AND FOOD AVAILABILITY ON COLONY-SITE SPACING IN GREAT BLUE HERONS . 20 P r e d i c t i o n s 21 METHODS 21 Prey A v a i l a b i l i t y 21 Dist a n c e between C o l o n y - s i t e s and Feeding S i t e s 22 Distance between Heron C o l o n y - s i t e s and Eagle Nests 22 Eagle Nest Abundance 22 RESULTS 23 Dist a n c e between C o l o n y - s i t e s and Feeding S i t e s 23 Predat i o n 23 DISCUSSION 30 V a l i d i t y o f Assumptions . . . 30 Food and Predators as Determinants o f C o l o n y - s i t e L o c a t i o n ... 31 F l i g h t D istance and Reproductive Success 32 SUMMARY 33 v Table o f Contents (continued) CHAPTER FOUR. SEASONAL PATTERNS OF HABITAT USE BY FORAGING GREAT BLUE HERONS 34 P r e d i c t i o n s 35 I n t r a s p e c i f i c f o r a g i n g competition 35 Foraging times and f o r a g i n g success 35 STUDY AREA AND METHODS 36 H a b i t a t use o f Age- and Sex-classes 36 Breeding season d i s p e r s i o n 36 Non-breeding season d i s p e r s i o n 36 H a b i t a t S e l e c t i o n 37 I n t r a s p e c i f i c f o r a g i n g competition 37 I n t e r f e r e n c e 37 T e r r i t o r i a l i t y 37 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 ... 38 Beaches 38 Maintenance energy . 38 Inge s t i o n r a t e s 38 Foraging time 39 Grasslands 39 Inge s t i o n r a t e s 39 Ha 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 40 RESULTS 40 Spacing o f Age- and Sex-classes 40 Breeding season 40 Non-breeding season 42 Ha b i t a t S e l e c t i o n 44 I n t r a s p e c i f i c f o r a g i n g competition 44 I n t e r f e r e n c e 44 T e r r i t o r i a l i t y 48 Foraging success 52 Beaches 52 Foraging time 52 Grasslands 52 Ha 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 54 DISCUSSION . 57 V a l i d i t y o f Assumptions 57 Beach-foraging herons 57 G r a s s l a n d - f o r a g i n g herons 57 Year-round Foraging D i s p e r s i o n o f Age- and Sex- c l a s s e s 58 H a b i t a t S e l e c t i o n 59 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 59 T e r r i t o r i a l i t y 61 SUMMARY 62 CHAPTER FIVE. TIME OF BREEDING IN GREAT BLUE HERONS 63 STUDY AREA AND METHODS 64 R e l a t i v e A v a i l a b i l i t y o f Energy to Ad u l t Herons 64 Number o f f i s h i n the lagoon 65 Energy estimates i n prey . 65 Nesting stages 66 vi Table o f Contents (continued) Duration o f low t i d e s • 66 Estimated Energy Consumption by A d u l t s 66 A d u l t consumption 66 time of Breeding 67 Energy t h r e s h o l d f o r l a y i n g 67 Energy consumption by e g g - l a y i n g females 68 Food demands o f heron c h i c k s 68 RESULTS 68 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 68 Seasonal Energy Consumption by A d u l t s , Chicks and Egg-laying Females 71 A d u l t s • . • 71 Chicks 71 Egg-laying females 71 DISCUSSION 78 V a l i d i t y o f Assumptions 78 Time o f Breeding 79 Future D i r e c t i o n s 80 SUMMARY 81 CHAPTER SIX. GENERAL DISCUSSION 82 Choice o f C o l o n y - s i t e 82 Colony Formation 83 H a b i t a t S e l e c t i o n 84 Time o f Breeding 84 I n t e r - r e l a t e d P a tterns of Ecology and Behaviour 85 A d u l t versus j u v e n i l e t a c t i c s 86 LITERATURE CITED 88 APPENDICES Appendix I 99 v i 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 of 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 of 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 of the F r a s e r River d e l t a d u r i ng 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 three 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 lengths 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 of 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 v i i i Table 5-3. E x t r a p o l a t e d minimum po p u l a t i o n s o f f i s h i n the p a r t i a l e n c l o s u r e i n Sidney lagoon 72 Table 5-4. Number of 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. 73 Table 5-5. 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 of the breeding season on Sidney I s l a n d i n 1987-88. N i s the number of herons watched. 74 Table 5-6. Numbers o f each of 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 . 77 i x LIST OF FIGURES F i g u r e 2-1. 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 by h e r o n s on S i d n e y I s l a n d F i g u r e 2-2. L o c a t i o n o f h a b i t a t s a n d t h r e e b r e e d i n g c o l o n y - s i t e s ( d i a m o n d s y m b o l s ) u s e d by h e r o n s i n t h e F r a s e r R i v e r d e l t a F i g u r e 2-3. C e n s u s r o u t e s f o l l o w e d t o l o c a t e h e r o n s d u r i n g t h e n o n - b r e e d i n g s e a s o n i n t h e F r a s e r R i v e r d e l t a F i g u r e 3-1. L o c a t i o n o f G r e a t B l u e H e r o n 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 and 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 B a l d E a g l e n e s t i n g a b u n d a n c e . D a s h e d l i n e s e n c l o s e a r e a s w here 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 ) F i g u r e 3-2. 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 F i g u r e 3-3. Mean b r o o d s i z e (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 h e l d one o r more f l e d g l i n g ( B ) , i n t e n h e r o n c o l o n i e s v e r s u s d i s t a n c e , t o t h e m a j o r f e e d i n g s i t e i n 1988. L a r g e d o t i n A = 2 c o l o n i e s ; a l l o t h e r d o t s r e p r e s e n t 1 c o l o n y - s i t e F i g u r e 4-1. 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 b e t w e e n A u g u s t a n d A p r i l 1986-87. Numbers a b o v e months 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 F i g u r e 4-2. 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 on 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 lo 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 F i g u r e 4-3. 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 meet 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 30 November 1987-1988 11 12 15 24 25 26 43 45 53 x 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 Figure 4-5. Numbers of adult and juvenile Great Blue Herons counted each month on road surveys of the Fraser River delta Figure 5-1. Index of food ava i lab i l i ty during the breeding season of the Great Blue Heron. Percent ava i lab i l i ty is estimated from the product of numbers of minutes low tides (<1.7 m) expose the foraging habitat and the size of the f ish population converted into units of energy. Vertical l ines are standard errors Figure 5-2. Numbers of days that available energy exceeded an estimated energy threshold for egg laying and the estimated dates when clutches were la id in 1987 and 1988 55 56 75 76 xi ACKNOWLEDGEMENTS Several people a s s i s t e d me du r i n g t h i s study. Jamie Smith encouraged the development of 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 Ric 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 at 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 P h i l 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 c o n t i n u i n g 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 t h a t 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 o f oceanic t i d e s pervades the l i v e s o f many b i r d s using seashores by determining when they w i l l feed, nest and migrate. The d i s c o v e r y of d i o x i n contamination of Great Blue Herons (Ardea herodias) 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 o f 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 . T h i s 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, c o m p e t i t i v e 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 o f the h a b i t a t ( C a t t e r a l l et a l . 1989). I t 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 r e p r o d u c t i v e success (e.g Krebs 1971, Whitham 1980, A l a t a l o et al. 1985). I n d i v i d u a l s 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 g e n e r a t i o n s . H a b i t a t 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 t h a t i n d i v i d u a l s t h a t are e q u a l l y s k i l l e d and have complete i n f o r m a t i o n 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 g r e a t e s t , and thus should experience the same average r a t e of r e t u r n ( F r e t w e l l and Lucas 1970, F r e t w e l l 1972). 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 t h a t the best competitors l i m i t settlement of i n f e r i o r competitors to s m a l l e r s i t e s i n good 1 h a b i t a t or to s i t e s i n poor h a b i t a t . T h i s r e s u l t s i n unequal rewards between i n d i v i d u a l s . Brown (1969) proposed a s i m i l a r e x p l a n a t i o n 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 t o 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 ' t h a t 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 po p u l a t i o n s 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 p o p u l a t i o n d e n s i t i e s i n h a b i t a t s t h a t 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 al. 1985, Richner 1986), but only the l a t t e r model e x p l a i n s the presence of n o n - t e r r i t o r i a l f l o a t e r s (e.g. Krebs 1971). A l l th r e e 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 t h a t 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 t h a t 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 et 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 the 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 t h a t 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 r e t u r n s t o zero (McNamara 1982). T h i s theory e x p l a i n s changes i n h a b i t a t use by animals without i n v o k i n g density-dependent i n t e r a c t i o n s . 2 Gathering evidence that habi tat s h i f t s by b i rds are re la ted to food a v a i l a b i l i t y has been hampered by the d i f f i c u l t y of fo l lowing mobile animals from summer to winter habi ta ts and by measuring the a v a i l a b i l i t y of t h e i r food. For example, S u l l i v a n (1989, 1990) showed that the d i s p e r s i o n and surv iva l of adul t and immature Yellow-eyed Juncos (Junco phaeonotus) soon a f te r the breeding season was p o s i t i v e l y re la ted to t h e i r a b i l i t y to acquire food. The Timing o f Breeding by B i r d s Most b i rds breed during a season when food i s r e l a t i v e l y p l e n t i f u l (Moreau 1950, Lack 1950). However, the mechanisms that determine p r e c i s e l y when birds begin to breed are l e s s c l e a r . Lack (1954, 1966) proposed that b i rds time t h e i r breeding seasons so that nes t l ings are present when food fo r the parents i s most abundant. However, l a t e r s tudies showed that c lu tches l a i d on the average date were not , as predic ted by Lack 's (1954) hypothes is , always the most product ive (Perr ins 1965, Cave 1968, Perr ins and Birkhead 1983). An a l t e rna te explanat ion i s that the amount of food a v a i l a b l e to the l a y i n g female determines when she w i l l lay her eggs (Perr ins 1965, 1970). P e r r i n s ' hypothesis p red ic ts that females should begin to lay as soon as t h e i r foraging s i t e s al low them to make eggs as well as maintain themselves. Many food add i t ion s tudies with female b i rds ear ly in the breeding season support P e r r i n s ' view (see reviews by Daan et al. 1988, D i j k s t r a et al. 1990). Evidence that n e s t l i n g s hatch a f te r t h e i r parents ' food supply peaks is mostly weak because of methodological problems in est imat ing the a v a i l a b i l i t y of food . Most s tudies have used measures of food abundance as an index of food a v a i l a b i l i t y (see Daan et al. 1988). Although t h i s seems i n t u i t i v e l y reasonable , the conclus ions of these studies are e q u i v o c a l . Daan et al. (1988) pieced together s tudies of 5 songbirds and 2 raptors to conclude that four 3 species had n e s t l i n g s at a time when t h e i r food suppl ies were increas ing in abundance, and three while food abundance was waning. A second methodological problem in r e l a t i n g food a v a i l a b i l i t y to the demands of the n e s t l i n g s is e s t a b l i s h i n g i f and when a c r i t i c a l time occurs fo r the food-gather ing parent . In some s p e c i e s , the peak demand fo r food occurs during the n e s t l i n g stage (see review by Martin 1987) while in others i t occurs a f t e r the young leave the nest (Weathers and S u l l i v a n 1989, S u l l i v a n 1989). The Study Species The ideal animal fo r studying habi tat s e l e c t i o n and time of breeding is a year-round res ident that i s h ighly v i s i b l e , has recognizable age- and s e x - c l a s s e s whose reproduct ive performance can be documented, whose prey a v a i l a b i l i t y and food consumption can be measured, and i s easy to catch and mark fo r l i f e . The Great Blue Heron on the coast of B r i t i s h Columbia approximates t h i s i d e a l . The species is sedentary and conspicuous on beaches, in marshlands and in neighbouring g rass lands . Juven i l es are e a s i l y d i s t i n g u i s h e d from adults by plumage d i f f e r e n c e s (Palmer 1962, Hancock and Kushlan 1984), and most adul ts can be sexed from a d is tance by estimates of culmen lengths (But ler et al. 1990). Most Great Blue Herons in B r i t i s h Columbia nest in co lon ies where t h e i r reproduct ive success can be r e a d i l y documented (Simpson 1984, But le r 1989). Herons in B r i t i s h Columbia catch small f i s h on beaches near c o l o n y - s i t e s (Krebs 1974, Simpson 1984) where prey populat ions and adult consumption can be estimated (Simpson 1984). 4 Aims of the Thesis In this thesis I describe seasonal changes in habitat use by different age- and sex-classes and test the 3 sets of alternative hypotheses that: (i) herons locate colony-sites near food supplies or away from predators (Chapter 3 ) , ( i i ) foraging herons move between habitats in autumn because of interference from foraging conspecifics or are forced to move because of insuff ic ient foraging time during tidal cycles (Chapter 4 ) and, ( i i i ) female herons begin to lay eggs after a threshold of available energy for egg production has been passed or match the period of peak food demands of their chicks (Chapter 5 ) . In Chapter 6 , I examine the year-round relationships between habitat use, dispersal and foraging eff iciency of age- and sex-classes. 5 CHAPTER TWO. STUDY SPECIES, STUDY AREA AND METHODS STUDY SPECIES D i s t r i b u t i o n - The Great Blue Heron i s the l a r g e s t and most widespread ardeid in North America (Hancock and Kushlan 1984). It breeds across the cont inent from the A r c t i c C i r c l e in Alaska to Mexico and on the Galapagos Islands (American O r n i t h o l o g i s t s ' Union 1983). The Great Blue Heron migrates from most of i t s northern range in winter except on the coasts of B r i t i s h Columbia and Alaska where i t res ides year - round. Food - Great Blue Herons inhabi t seashores, r i v e r s and lakes where they wade in search of f i s h during day and night (Brandman 1976, Black and Col lupy 1983). Great Blue Herons mostly hunt by standing in water where they wait for prey to pass wi th in s t r i k i n g range ("Stand and wait" in Kushlan 's (1976a) terminology) or "Walk Slowly" (Kushlan 1976a) u n t i l prey i s encountered. I r a r e l y saw the 28 other feeding techniques descr ibed by Kushlan (1976a). Most f i s h are caught between the f i n e l y serrated open upper and lower mandibles, and f i s h are immobilized by shaking. Large prey with dangerous spines are sometimes taken ashore where they are speared with the b i l l u n t i l the spines re lax or break (Forbes 1982, pe rs . o b s . ) . Great Blue Herons a lso eat mammals, i n s e c t s , c rustaceans , gastropods, amphibians, r e p t i l e s , b i rds and car r ion (Kushlan 1978). Breeding b i o l o g y - The breeding b io logy of the Great Blue Heron has been documented across North America inc lud ing A lber ta (Vermeer 1969)', Nova Scot ia (McAloney 1973, Quinney and Smith 1979, Quinney 1983), Quebec (DesGranges and Laporte 1979, DesGranges et al. 1979), Oregon (Henny and Bethers 1971, Werschkul et al. 1977, Engl ish 1978, Blus et al. 1980), Washington (Calambokidi et al. 1985), Montana (Parker 1980), C a l i f o r n i a (Prat t 1970, 1972 a , b , Brandman 1976, Prat t and Winkler 1985), and F l o r i d a (Powell 1983, Powell 6 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 the 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 B r i t i s h 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 1968, Paine 1972, Krebs 1974). The breeding b i o l o g y o f se v e r a l c o a s t a l c o l o n i e s were compared by 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 the group o f herons that gather t h e r e (Kushlan 1986). B r i e f l y , male Great Blue Herons on the coast 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 or e a r l y March. They s e t t l e on p r e vious y e a r s ' nests or 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 f o r t h e i r a t t e n t i o n . 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 the afternoon u n t i l by l a t e March or 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 appa 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 the 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 the mated p a i r ; males incubate mostly during the day and females at n i g h t (Paine 1972, Brandman 1976, Kaufmann and Cawley 7 1986). Incubation lasts about 27 days (Vermeer 1969) and the eggs hatch asynchronously (Mock 1985). Clutch sizes average from five eggs in Alberta (Vermeer 1969) to fewer than three in Florida (Powell and Powell 1986). In coastal Br i t ish Columbia the average clutch size is 4.1 eggs (Butler 1989). Nestlings are fed on f ish caught near the colony-site during the day (Simpson 1984, pers. obs. ) . Sibl ing aggression results in the reduction in the size of some broods (Mock 1985). On average, about 2.5 young reach independence per successful nesting attempt in Br i t ish Columbia (Forbes et a7. 1985a, Butler 1989). Non-breeding Season - Less is known about the Great Blue Heron outside i ts breeding season. Band recovery data indicate a southward movement of post-breeding herons in northern populations excluding coastal Bri t ish Columbia and Washington (Henny 1972, Byrd 1978). Mortality estimates based on band recovery data show about three-quarters of the juveniles die before their f i r s t birthday (Henny 1972, Bayer 1981). Juvenile Great Blue Herons are less ef f ic ient foragers than adults after the close of the breeding season (Quinney and Smith 1980). Some post-breeding adults hold exclusive feeding terr i tor ies (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 Mil l stein et a7. (1970) for the Gray Heron (A. cinerea). The classes used in this thesis are defined below. Chick: A heron between hatching and fledging is called a chick. This period lasts about two months, usually May and June; Fledgling: Chicks are called fledglings from when they f i r s t leave the nest in late June or early July, until about two weeks later when no trace 8 of the neossopt i l e down remains. This 'down' i s most apparent on the crown and on the f ace ; J u v e n i l e : F ledg l ings become j u v e n i l e s once they lose the neossopt i l e down in mid -Ju ly and remain in t h i s c l a s s u n t i l the next breeding season which begins in March. They can be recognized in the f i e l d by a s l a t e - g r e y crown, the absence of body plumes, and very short or no o c c i p i t a l plumes; Y e a r l i n g : Juven i l es become y e a r l i n g s about 11 months a f t e r hatching with the commencement of the adult breeding season in March. They remain in t h i s age -c lass fo r twelve months. Year l ings can be d is t ingu ished by a small (c .3 cm) white crown patch , grey forehead and the presence of o c c i p i t a l plumes and by a chestnut -co loured bend of the wing. However, they lack the long adult body plumes, and some wing cover ts are edged with brown l i k e j u v e n i l e ' s ; A d u l t : Adul ts have a white crown, white or white f lecked with grey forehead, and black post o r b i t a l s t r i p e s fus ing at the p o s t e r i o r end in to long plumes. Long plumes cascade over the back and splay out from the chest and b e l l y most no t iceab ly in the breeding season. The gray body plumage has no brown edging. In the breeding season, the b i l l approaches an orange-yel low hue (co lo r 18, Smithe 1975). 9 STUDY AREA I studied Great Blue Herons mostly in the Gulf Islands and in the Fraser River delta in the southwest corner of Br i t ish Columbia, Canada. The maritime climate ensures a mild winter and cool summer, despite the high latitude (48-49°N). Infrequent v is i t s were made to about 40 colonies around the Strait of Georgia and two areas were studied in de ta 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 of the town of Sidney and 23 km ENE of V ic tor ia , Br i t ish Columbia. Sidney Island is one of a dozen large Canadian Gulf (San Juan) Islands that l i e between Vancouver Island and the mainland of Br i t ish Columbia. Sidney Island is mostly covered in second growth Douglas Fir (Pseudotsuga menziesii) and mature Arbutus menziesii. The herons nested in Red Alder (Alnus rubra) trees. Lit toral d r i f t has created a lagoon (Fig. 2-1) (100 ha) in which most female herons fed each day. the f i r s t part of the lagoon exposed by fa l l ing tides is a saltmarsh community dominated by Salicornia virginica and Distichlis spicata. Herons used this marsh as a loafing s i te . Adjacent to the saltmarsh is 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 ha). Fraser River Delta - I studied herons during autumn and winter mostly in the Fraser River delta immediately south of Vancouver, Br i t ish Columbia (Fig. 2-2). the Fraser is the largest estuary on the Pacif ic coast of Canada. A general description is given by Butler and Campbell (1987). 10 Figure 2-1. Location of the breeding colony-site and habitats used by herons on Sidney Island. 11 Figure 2-2. Locat ion of habi ta ts and three breeding c o l o n y - s i t e s (diamond symbols) used by herons in the Fraser River d e l t a . 12 B r i e f l y , most of the 680 km2 d e l t a above high t i d e has been diked for a g r i c u l t u r a l , r e s i d e n t i a l and i n d u s t r i a l use. An extensive network of d i tches and sloughs dra ins much of the d e l t a . Vegetable crops are grown in f i e l d s in summer and f i e l d s e i t h e r l i e fa l low or grow co ld-hardy crops in winter . Outside the d i k e s , a 600 to 1000 m wide band of brackish marshland extends between the channels of the Fraser River ( F i g . 2 -2) . From the western shore o f Point Roberts east in to Boundary Bay l i e extensive sand and mud beaches over 4 km wide at t h e i r greatest width ( F i g . 2 -2) . These beaches support f l o u r i s h i n g beds of ee lgrass (Z. japonica and Z . marina) ( F i g . 2 -2) . GENERAL METHODS Great Blue Herons were studied year-round between 3 A p r i l 1986 and 31 August 1990. I s tudied herons f o r f i v e breeding seasons, (3 A p r i l to 15 August 1986, 21 March to 24 September 1987, 21 February to 18 August 1988, 17 March to 15 August 1989, 1 A p r i l to 31 August 1990) on Sidney Is land. Herons were watched at a l l times of the year in the Fraser River de l t a but most of ten from l a t e summer to ea r l y spr ing (15 August to 15 March 1986-90). D i s p e r s i o n i n the Breeding Season - Nesting c o l o n y - s i t e s of Great Blue Herons reported by Forbes et al. (1985b) were v i s i t e d by volunteers and me in 1987 and by A. Breaul t (pers . comm.) in 1988-89. C o l o n y - s i t e s unknown to Forbes et a7. (1985b) were found by contact ing n a t u r a l i s t s and through pub l ic requests in l o c a l newspapers. I examined how herons chose a s i t e fo r t h e i r colony by comparing the r e l a t i v e importance of food suppl ies and the presence of predators , notably the Bald Eagle (Haliaeetus leucocephalus). 13 Dispersion in the Non-breeding Season - The d i s p e r s i o n pattern of herons outs ide the breeding season was determined in two ways. Every two weeks 2 observers independently mapped a l l herons seen from a Cessna 185 a i r c r a f t flown above the beaches and marshlands of the e n t i r e Fraser River de l ta ( F i g . 2 -3) . The maximum number of herons overa l l in each about 4 km segment of the route was considered as the best index of the number present . The a i rp lane flew at an a l t i t u d e of about 90 m and an a i rspeed of about 140 km/h. The f l i g h t co inc ided with the lowest mid-winter daytime t i d e (3 m) between 1 September 1986 and 31 March 1987 and 1 September 1987 and 30 A p r i l 1988. The f l i g h t path fol lowed the t i d e l i n e in one d i r e c t i o n and the top of the beach in the opposi te d i r e c t i o n . This allowed both observers to record herons over the e n t i r e beach. The e n t i r e survey took about 70 minutes. From 17 J u l y 1987 to 31 March 1988 and from 5 J u l y to 16 September 1988 I drove a 89.4 km route once a week that crossed the farmlands behind the dikes at Boundary Bay and the mouth of the Fraser River ( F i g . 2-3) . Two observers searched fo r herons in the d i tches and f i e l d s on opposite s ides of the road. Each heron was assigned an age using plumage characters (see A g e - c l a s s Descr ip t ions ) and i t s l o c a t i o n was noted on a map. Data from these road censuses were used to estimate habi tat use and seasonal mor ta l i t y of herons during the non-breeding season. Deta i led methods are provided in fo l lowing chapters . 14 F i g u r e 2-3. Census routes f o l l o w e d to l o c a t e herons d u r i n g 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 location, the tide height (Department of Fisheries and Oceans 1986, 1987, 1988, 1989) and the age of the heron (juvenile or adult). Over the next ten minutes I recorded the time of each strike to the nearest second, the type of f ish caught (e.g. gunnel, sculpin, etc.) and i ts 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). A female Great Blue Heron in Br i t ish Columbia has an average culmen length of 123.9 mm (S.E.=5.1, n=29, Simpson 1984) so these increments represent about 31 mm. Observer bias using this method was tested and accounted for during the analysis (see Chapter 5). Prey populations in the lagoon on Sidney Island were sampled using a 2.4 x 18 m beach seine with a 6 mm mesh in April to July 1987-88 during the breeding season of the Great Blue Herons. Fish caught in seine hauls were identi f ied using Hart's (1973) descriptions. The entire catch was emptied into buckets and counted later . Samples of the catch were weighed with Pesola spring balances, measured (total length) on a f ish ruler and then released. These data were used to estimate the seasonal occurrence of major f ish 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, especially Microtus townsendii, caught in grasslands (Taitt and Krebs 1983). To assess dai ly and seasonal use of grasslands, I counted the number of herons on Alaksen National Wildl i fe Area f ie lds on Westham Island, where large numbers of M. townsendii occur (Beacham 1980, Taitt et al. 1981, Tait t and Krebs 16 1983), from 13 November 1986 to 23 March 1987, and from 23 October 1987 to 12 February 1988. Oceanic tides - Tides in the Strait of Georgia expose and cover the intert idal foraging sites of herons twice about every 25 h. The highest tide each day is followed about 8 h later by the lowest t ide. An intermediate high and low tide complete the 25 h cycle. Within a year the lowest tides change gradually from midday in June to midnight in December. The maximum tidal amplitude is about 4.5 m. The number of minutes that low tides uncovered eelgrass beds (<1.7 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 cal l ing soon after hatching (Brandman 1976; pers. obs. ) . Once the f i r s t chick was heard, I made nearly dai ly v is i t s to gather shells in the colony. 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 shell was found below a nest as the date of hatching. 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 fledglings raised in nests at colony-sites: on Sidney Island, near the town of Crofton, in the University of Br i t ish Columbia Endowment Lands, on Point Roberts (in Washington), and beside the Nicomekl River near Crescent Beach (Fig. 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 Br i t ish Columbia coast in 1987. In 1988-89, these and other colony-sites were vis i ted by me and A. Breault (pers. comm.) to record nesting data. 17 I numbered a l l nest t rees at Sidney with a f e l t pen in March and ear ly A p r i l 1987-90 before the eggs were l a i d . I l a t e r attached numbered aluminum p la tes to a l l t rees with n e s t s . Herons are of ten very s e n s i t i v e to humans wi th in s igh t or earshot of t h e i r nests ea r l y in the season (eg. Vos et al. 1985). To reduce t h i s d isturbance I r e s t r i c t e d most colony v i s i t s in 1987 to wi th in about 50 m of the edge of the colony u n t i l the eggs hatched. A black polyethylene p l a s t i c b l i n d was b u i l t on the ground at the southeast corner of the colony in 1988 and 1989. Access was gained v i a a 100 m covered walkway beginning at the beach. This b l i n d was used to observe herons at t h e i r nests and had no not iceab le e f f e c t on the sett lement or nest ing success of herons. About one month a f te r hatching the s u r v i v i n g ch icks were counted d i r e c t l y in e a s i l y observed n e s t s , or when they became ac t ive during feedings at obscured n e s t s . For many n e s t s , two observers used vantage points on opposite s ides of the nest and a count was made using b i n o c u l a r s . These counts were taken about once each week u n t i l the ch icks became f l e d g l i n g s . Herons were considered to have used a l l nests that held eggs or c h i c k s , or had hatched eggshe l ls or feces on the ground below them. Sex of Adult Herons - From 15 October 1988 to 30 A p r i l 1989 I drove to vantage points along the Fraser River and major roost and feeding s i t e s to assign the age and sex to herons. I a lso o p p o r t u n i s t i c a l l y assigned the age and sex o f other herons found in the d e l t a over the same p e r i o d . The sex was determined using the ' g r a d i c u l e method' (But ler et al. 1990). Th is technique est imates culmen lengths against a g rad icu le sca le mounted in the eye-piece of a t e l e s c o p e . The d is tance between the heron and the te lescope i s measured using a tape-measure, and the g r a d i c u l e - s c a l e measure i s converted in to actual beak l eng ths . The accuracy of t h i s method depends on several fac tors inc lud ing the angle of the culmen r e l a t i v e to the s i g h t - l i n e through the 18 telescope and the distance between heron and observer. Nevertheless, the method allowed me to estimate the sex of about three-quarters 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, I examine factors affecting how Great Blue Herons select habitats. In this chapter I focus on the relationship between the distr ibut ion of foraging habitat and presence of predators on the spacing of heron colony-si tes. Lack (1954, 1968) postulated that food ava i lab i l i ty was more important than predators in determining the location of most breeding colonies of birds. Several studies have shown that colonies are located near food supplies (see Perrins and Birkhead 1983, Kushlan 1976b, Gibbs et al. 1987) as predicted by Lack (1954, 1968). However, predators are often d i f f i c u l t to locate or their populations reduced so their impact on colony-site spacing is poorly understood. Predators cause colony-site abandonments in herons (Dusi and Dusi 1968, Simpson 1984) and might affect the spacing of their colony-sites. The Great Blue Heron in Br i t ish Columbia is suitable for exploring the relationship between food ava i lab i l i ty , distr ibution of predators and colony- si te spacing because: (i) colony-sites can be located by following herons returning from feeding areas, ( i i ) the major predator in Br i t ish Columbia is the Bald Eagle (Haliaeetus leucocephalus) (Vermeer et al. 1988, Norman et al. 1989) which nests in conspicuous places, and ( i i i ) most female herons feed on intert idal beaches near the colony-sites during the breeding season (Chapter 4) and only females provision chicks under 3 weeks of age. The hypotheses I tested were that herons located colony-sites: (i) near food supplies or, alternately ( i i ) away from predators. 20 P r e d i c t i o n s Food s u p p l i e s ( i ) More herons should breed near habi ta ts with high prey a v a i l a b i l i t y than near habi ta ts with low prey a v a i l a b i l i t y ; ( i i ) Fledged brood-s i ze should d e c l i n e , and the number of deserted nests should inc rease , with d is tance between the food supply and the colony- s i t e ; ( i i i ) The number of breeding herons should be p o s i t i v e l y cor re la ted with the area of foraging hab i ta t ; Predators ( iv ) Fewer herons should breed in areas f requent ly used by eagles than areas in f requent ly used by eag les ; (v) Fledged brood-s izes and the number of successfu l nest ing pa i rs should be greater in co lon ies outs ide eagle t e r r i t o r i e s than ins ide eagle t e r r i t o r i e s . METHODS Great Blue Herons were studied at c o l o n y - s i t e s around the S t r a i t of Georg ia , B r i t i s h Columbia ( F i g . 3 -1) . General methods used t o : estimate f ledged b r o o d - s i z e s , estimate the number of nests used at a c o l o n y - s i t e , and loca te c o l o n y - s i t e s are given in Chapter 2. Prey A v a i l a b i l i t y I assumed that shallow beaches with ee lgrass (Zostera marina) beds, kelp (mostly Nereocystis luetkeana and Laminaria spp.) beds and marshes had high prey a v a i l a b i l i t y whereas other habi ta ts had low prey a v a i l a b i l i t y . This assumption i s d iscussed l a t e r . A l l ee lgrass beds, kelp beds and marshes 21 >10 ha in area in the S t r a i t or Georgia were located from searches and publ ished maps (Hutchinson 1982, Hutchinson et al. 1989). Distance between colonies and feeding sites Most breeding herons feed at one major s i t e near t h e i r c o l o n y - s i t e (Chapter 4 ) . Herons depart ing from 22 c o l o n y - s i t e s were fol lowed to t h e i r major feeding l o c a t i o n when t h e i r nests held eggs and small c h i c k s . The d is tance between the c o l o n y - s i t e and centre of the feeding s i t e was measured on 1:50,000 sca le maps. Distance between heron colonies and eagle nests I assumed that the proximity of Bald Eagle nests was a good index of predat ion pressure at heron c o l o n i e s . Th is assumption i s d iscussed l a t e r . Eagle nests were found by ground (6 cases) and a i r searches (113 cases) (Vermeer et al. 1989) and by pub l i c request (74 c a s e s ) . Bald Eagle feeding t e r r i t o r i e s extend outwards fo r up to 3 km around t h e i r nests (Stalmaster 1987). There fore , heron c o l o n y - s i t e s >3 km from an eagle nest were assumed to be outs ide an e a g l e ' s t e r r i t o r y and were considered to be ' f a r ' from predators , whereas 'near ' c o l o n y - s i t e were located <3 km from an eagle nest and assumed to be wi th in an e a g l e ' s t e r r i t o r y . Eagle-nest abundance I assumed that eagles occurred in high abundance where 3 or more nest ing t e r r i t o r i e s were contiguous ( i . e . nests were <6 km apart on average) and occurred in low abundance where t h e i r nest ing t e r r i t o r i e s were not contiguous ( i . e . nests where >6 km apart on average). 22 RESULTS Distance between colony-sites and feeding sites Colony-sites were near feeding grounds; the average distance was 2.3 km (S.D.=1.3, N=22). Of 33 colony sites used in 1987-89, a l l but four small (5-40 pairs) colonies were near shallow beaches with kelp beds, eelgrass beds or marshes (Fig. 3-1). The feeding sites of these four colonies were unknown. However, a l l of them were within 2 km of the Strai t of Georgia where lone herons foraged. Three apparently suitable feeding areas had no colonies nearby during this study but have been used in the past (Forbes et al. 1985b). The number of breeding pairs correlated posit ively and s igni f icant ly with the area of the feeding sites used by each colony (Fig. 3-2). Brood size and nesting success were not s igni f icant ly correlated with the f l ight distance between the colony-site and feeding area (Fig. 3-3), although brood sizes were weakly negatively correlated with f l ight distance as predicted. These findings support the prediction that colonies are sited near feeding areas with abundant prey, and the number of breeding pairs is posit ively correlated with the area of foraging habitat. However, my results f a i l to support the prediction that reproductive success is related to the f l ight distance between colony-sites and feeding areas. Predation There was no support for the hypothesis that colony-sites were located far from eagle nests (Fig. 3-1). Four-hundred and ninety-seven herons nested in 23 Figure 3 -1 . Locat ion of Great Blue Heron c o l o n y - s i t e s (c losed c i r c l e s ) , kelpbeds, ee lgrass beds and marshes >10 ha (arrow) in r e l a t i o n to Bald Eagle nest ing abundance. Dashed l i n e s enclose areas where eagle nests averaged l e s s than 6 km apart (high abundance) and dotted l i n e s enclose areas where eagle nests averaged fewer than one every 6 km (low abundance). 24 1200' 1000 JS 800 ' < UJ 600- < 400H 200- y m 19.9 + 0.2x r 2 =0.796 50 100 150 2 0 0 250 300 NUMBER O F B R E E D E R S Figure 3-2. Numbers of breeding pairs of herons and the area of their foraging sites in 1988. 25 UJ N CO o o o cc 00 2- 2 4 6 DISTANCE (km) 100 co co 80, UJ' o- O 60 » 40, 204 B " I I " 'n 2 . 4 6 DISTANCE, (km) Figure 3-3. Mean brood size (A) and percentage of occupied nests that held one or more f ledgling (B), in ten heron colony-sites versus distance to the major feeding si te in 1988. Large dot in A = 2 colony- s i tes ; a l l other dots represent 1 colony-si te. 26 19 co lon ies in areas where eagles occurred in high abundance, and 515 herons nested in 13 co lon ies in low abundance eagle areas . More important ly , the proport ion of c o l o n y - s i t e abandonments was a c t u a l l y s l i g h t l y lower in h igh- abundance eagle areas than in low abundance areas (Table 3 -1 ) . At one s i t e in V i c t o r i a , a p a i r of eagles nested in the c o l o n y - s i t e and a second p a i r nested l e s s than 1 km away. These eagles r e g u l a r l y attacked the herons in 1988, 1989 and 1990. The colony abandoned i t s eggs in 1990. Eagles attacked the Sidney colony in a l l 5 years of t h i s study. The colony abandoned the s i t e twice dur ing the egg stage in 1989 and once in 1990. At l e a s t 3 adult herons were k i l l e d by eagles in the colony in 1989 and 5 in 1990. In 1989, many herons were seen on nearby is lands where few had been seen in 1987-88 so I presume the herons d id not attempt to nest a t h i r d t ime. In 1990, the Sidney herons abandoned a f t e r one attempt and renested about 6 km away on Vancouver Is land. Eagles a lso attacked herons in 7 of 21 other c o l o n y - s i t e s in 1988 and in 9 of 23 c o l o n y - s i t e s in 1989. Although eagles sometimes cause abandonments of e n t i r e c o l o n y - s i t e s , there i s no evidence that the proximity of nest ing eagles a f fec ted the average reproduct ive success in heron co lon ies that ra ised f l e d g l i n g - a g e d c h i c k s . The mean brood s i z e was near ly i d e n t i c a l in co lon ies near and f a r from eagle nests (Table 3 -2 ) . More important ly , the mean percentage of successfu l nest ing pa i rs was near ly i d e n t i c a l in co lon ies near and fa r from nest ing eagles (Table 3-2) . 27 Table 3 -1 . Number of successfu l and f a i l e d heron c o l o n y - s i t e s where eagles nested in high (nests <6 km apart) and low (nests >6 km apart) abundance in 1987-89. Eagle-nest abundance Success F a i l High 23 5 Low 17 6 28 Table 3-2. Percentage of successfu l (>1 f l e d g l i n g ) nest ing p a i r s and mean brood s i z e s of c o l o n y - s i t e s that produced f l e d g l i n g s near (<3 km) and far (>3 km) from an occupied Bald Eagle nest in 1989. Near Far Mean SD N Mean SD N Percent success 81.1 11.9 12 85.1 13.7 7 Brood s i z e 2.6 0.4 12 2.6 0.4 7 29 DISCUSSION Val id i ty of assumptions This' analysis was based on two key assumptions. F i r s t , I assumed that prey "avai labi l i ty was greater on beaches with kelp beds, eelgrass beds and marshes than on beaches without these habitats. This assumption is supported by several distr ibution studies of prey ,species eaten by herons in the Strait of Georgia (see Hughes 1985, Gordon and Levings 1984, Hay et a / . 1989). I feel less confident with the assumption that Bald Eagle nesting abundance was a good measure of predation pressure in heron colonies. F i rs t , my analysis of heron nesting success might have been more sensitive i f I had used more than 2 categories of eagle nest abundance. Second, raccoons {Procyon lotor), Red-tailed Hawks (Buteo jamaicensis), and Turkey Vultures (Cathartes aura) also occasionally prey upon heron eggs and chicks in Br i t ish Columbia (Simpson et al. 1987, A. Breault, pers. comm., pers. obs.) but I did not measure their impact. 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 led 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 Apri l 1987 when female herons were laying or incubating eggs in most colony-si tes, over one-third (293 out of 797) of the eagles counted from aircraf t in the Southern Gulf Islands were immature-plumaged birds. Moreover, the abundance of eagles in this part of Br i t ish Columbia might be so great that herons can not avoid them. There was s l ight ly more than one eagle per km of coastline on average (797 eagles along 750 km of shoreline) in the southern Gulf Islands in 1987 (data in Vermeer et al. 1989). 30 Food and Predators as Determinants of Colony L o c a t i o n Several s tud ies have shown that wading b i rds (Kushlan 1976b, Custer and Osborne 1978) inc lud ing herons (Fasola and Barb ie r i 1977, Gibbs et al. 1987, Simpson et al. 1987) nest near product ive foraging areas but my study is the f i r s t to my knowledge that shows that the d i s t r i b u t i o n of heron c o l o n y - s i t e s i s unrelated to the dens i ty of breeding eag les . Many heron c o l o n y - s i t e s were located near eagle nests presumably because both species have s i m i l a r habi tat needs. Eagles in B r i t i s h Columbia eat a wide assortment of prey (mostly vertebrates) caught along seacoasts of which herons are of minor consequence (Vermeer et al. 1989). Heron c o l o n y - s i t e s are located near foraging habi ta ts ( F i g . 3-1) with an abundance of small f i s h (Hughes 1985, Hay et al. 1989, Chapter 5) . These b i o l o g i c a l l y productive hab i ta ts a lso a t t r a c t the prey species taken by eag les . Gibbs et al. (1987) suggested that the spacing of heron c o l o n y - s i t e s in Maine resu l ted from d ispersers s e t t l i n g outside the foraging range of other c o l o n y - s i t e s where they would get f u l l access to unexploi ted resources. According to t h e i r model, new co lon ies would s e t t l e at the edges of the foraging ranges of neighbouring co lon ies once the best s i t e s were f u l l . Even tua l l y , c o l o n y - s i t e s would become evenly-spaced along the coas t . Th is model assumes that herons deplete food suppl ies near t h e i r co lony- s i t e s (Gibbs et al. 1987). If herons deplete l o c a l food suppl ies then spacing should be a funct ion of colony s i ze s ince large co lon ies require more food than small ones. However, c o l o n y - s i t e s were evenly spaced in t h e i r study even though the number of breeding pa i rs ranged from 4 to 252 herons. I agree with Gibbs et al. (1987) that food supply determines the spacing of c o l o n y - s i t e s but I doubt that herons in general deplete t h e i r food suppl ies to a measureable extent . Herons in my study foraged mostly near the colony- 31 s i t e (Chapter 4) and had i n s u f f i c i e n t time during low t i d e s to deplete t h e i r food supp l ies markedly (Chapter 5) . Flight Distance and Reproductive Success Adult herons must increase t h e i r p rov is ion ing rates to r a i s e large broods to f l e d g i n g age ( S u l l i v a n 1988). Thus, adul ts should increase t h e i r foraging time and/or reduce t h e i r own energy requirements by decreasing t rave l time between the c o l o n y - s i t e and foraging area (Orians and Pearson 1979, Bryant and Turner 1982). Therefore , i t makes i n t u i t i v e sense fo r herons to nest as c lose as p o s s i b l e to food s u p p l i e s . A l l c o l o n y - s i t e s were located too c lose to food suppl ies to reveal any d i f f e r e n c e s in t h e i r average f ledged brood s i z e s . However, the power of the s t a t i s t i c a l t e s t s to detect a s i g n i f i c a n t d i f f e r e n c e here i s low because of small sample s i z e s of c o l o n y - s i t e s d is tan t from feeding grounds. Moreover, herons should avoid poorer s i t e s i f bet ter ones are ava i l ab le near food s u p p l i e s . It i s unclear how fa r herons would have to nest from t h e i r feeding s i t e s before d i f f e r e n c e s in reproduct ive success should be de tec tab le . Herons carry large amounts of food each t r i p and make few t r i p s to feed young. Marion (1989) reported ind iv idua l Gray Herons feeding up to 38 km from c o l o n y - s i t e s in France but he d id not r e l a t e f l i g h t d is tances to reproduct ive success . Simpson (1984) showed that Great Blue Herons feeding near a c o l o n y - s i t e on the B r i t i s h Columbia coast had higher reproduct ive success than d i s t a n t - f e e d i n g b i r d s , but he d id not know how far d i s t a n t - f e e d e r s t r a v e l l e d (pers. comm.). Moreover, l o c a l - f e e d i n g herons supplemented t h e i r d ie t by making regular t r i p s to tanks holding l i v e herr ing (Clupea harengus) in Simpson's (1984) study. 32 I conclude that average nest ing success in co lon ies i s unrelated to the d is tance to the feeding s i t e over the d is tances I measured. SUMMARY 1) Great Blue Heron c o l o n y - s i t e s (n=22) in the S t r a i t of Georgia were located wi th in 6 km of major foraging areas. 2) The number o f breeding p a i r s , reproduct ive success , or number of abandonments of heron c o l o n y - s i t e s were s i m i l a r in low and high densi ty eag le -nes t ing areas. P red ic t ions from the hypothesis that predators a f fec t the l o c a t i o n of heron c o l o n y - s i t e s were not supported but more d i r e c t evidence of e f f e c t s o f predators , inc lud ing Bald Eag les , i s required to t e s t the hypothesis f u r t h e r . 3) The spacing of heron c o l o n y - s i t e s in B r i t i s h Columbia i s best explained by the d i s t r i b u t i o n of shallow beaches with kelp beds, ee lgrass beds and marshes where most herons foraged in summer. 33 CHAPTER FOUR. SEASONAL PATTERNS OF HABITAT USE BY FORAGING GREAT BLUE HERONS In the previous chapter I showed that the spacing of heron c o l o n y - s i t e s was expla ined by the d i s t r i b u t i o n of food s u p p l i e s . In t h i s chapter , I descr ibe the habi ta t use by foraging herons and r e l a t e seasonal s h i f t s in t h i s d i s t r i b u t i o n to i n t r a s p e c i f i c competi t ion and food a v a i l a b i l i t y . One view of habi ta t s e l e c t i o n p o s i t s that as the dens i ty of foragers i n c r e a s e s , the best competitors e s t a b l i s h exc lus ive feeding t e r r i t o r i e s or dominate weaker i n d i v i d u a l s who then move to marginal hab i ta ts (Fretwel l and Lucas 1970, Fretwel l 1972, Sutherland and Parker 1985, Chapter 1) . Another view p o s i t s that an i n d i v i d u a l ' s foraging s k i l l mostly determines where i t w i l l forage , independently of the dens i ty of competitors (Stephens and Krebs 1986, Chapter 1) . Tests of the foraging competi t ion and food a v a i l a b i l i t y hypotheses have been hampered by d i f f i c u l t i e s in t rack ing mobile animals between habi tats through the y e a r . Th is i s p a r t i c u l a r l y important s ince foraging s k i l l s (see review by Burger 1988) and competi t ive s k i l l s (Par t r idge and Green 1985, Goss- Custard and d i t Dure l l 1987a,b) improve with age in many animals. Great Blue Herons in B r i t i s h Columbia lend themselves well to s tudies of forag ing habi ta t s e l e c t i o n because: (1) they feed year-round in open habi ta ts where i n t e r a c t i o n s can be observed and food ingest ion rates can be estimated (Chapter 5 ) , (2) some defend feeding t e r r i t o r i e s (Brandman 1976, Bayer 1978) while others feed alone or in groups (Krebs 1974, Kushlan 1978), and (3) j u v e n i l e s are l e s s p r o f i c i e n t at foraging than adul ts (Quinney and Smith 1980). The aim of t h i s chapter i s to r e l a t e the d i s p e r s i o n of age- and sex -c lasses of herons across habi ta ts to the presence of i n t r a s p e c i f i c competitors and a v a i l a b i l i t y of foraging t ime. F i r s t , I descr ibe the year-round use of 34 habi ta ts by heron age- and s e x - c l a s s e s . Next, I compare the hypotheses that forag ing herons move between habi ta ts in autumn because of : ( i ) in ter ference from foraging c o n s p e c i f i c s (Sutherland and Parker 1985), or ( i i ) i n s u f f i c i e n t forag ing time ( S u l l i v a n 1990) dur ing low t i d e . F i n a l l y , I d iscuss how the forag ing s k i l l of j u v e n i l e and adult herons a f f e c t s t h e i r s u r v i v a l . Predictions Intraspecific foraging competition ( i ) J u v e n i l e herons should depart from foraging habi ta ts when in ter ference from adul ts reduces energy ingest ion rates below the d a i l y energy maintenance requirement; ( i i ) Feeding t e r r i t o r i e s should be defended by adult males because they are l a r g e r and heavier (Simpson 1984) than adult females and j u v e n i l e s , and thus bet ter able to exclude other age- and s e x - c l a s s e s . Foraging time and foraging success ( i ) Herons should leave foraging habi ta ts when there i s too l i t t l e time per t i d e c y c l e to catch enough food there to meet t h e i r d a i l y energy maintenance requirement (McNamara 1982); ( i i ) J u v e n i l e s are l e s s p r o f i c i e n t foragers and should therefore vacate forag ing habi ta ts before adults in autumn. 35 STUDY AREA AND METHODS Great Blue Herons were studied mostly on Sidney Island during the breeding season and in the Fraser River d e l t a during the non-breeding season. I descr ibe the study areas and general methods in Chapter 2, and a lso descr ibe there how I: i ) estimated t i d e he ights , d i e t s and foraging r a t e s , i i ) determined use of foraging habi ta ts by breeding a d u l t s , i i i ) counted herons in marshlands, on beaches, in g rass lands , and along r i v e r s , and iv) assigned age and sex to herons. I estimate the energy in heron prey and the d a i l y energy maintenance needs of herons in Chapter 5. Habitat use of Age- and Sex-classes Breeding season dispersion - The d i r e c t i o n s of a l l incoming and outgoing forag ing f l i g h t s by breeding adults from the Sidney c o l o n y - s i t e were recorded from dawn to dusk during 19 low t ides and 17 high t ides on 16 days at 3 times in the season. I observed b i rds on 4 days when most nests held eggs (12 A p r i l - 4 May) and 4 days when nests held small ch icks (10-31 May). When nests held large ch icks (16 June-10 J u l y ) , I recorded f l i g h t s during f i v e low t ides and one high t i d e . Bearings of major f l i g h t d i r e c t i o n s o f f the i s l a n d were drawn on a map and des t ina t ions v i s i t e d about once every two weeks to search for feeding herons. A r r i v a l and departure d i r e c t i o n s at these s i t e s confirmed that the herons came from Sidney Is land. Non-breeding season dispersion - I censused herons in potent ia l habi ta ts from an a i r p l a n e , car and boat. These covered a l l heron habi ta ts although census e f f o r t s were uneven. I consider these habi ta ts were most important to herons but i t remains open that some herons might have been overlooked in other h a b i t a t s . 36 Herons feed on beaches, in marshlands and grasslands and along r i v e r banks in the Fraser River d e l t a . Herons were p lo t ted on maps during aer ia l censuses of the Fraser River d e l t a beaches and marshlands ( F i g . 2-3) between September 1986 and February 1987. Nearest-neighbour d is tances were l a t e r measured from these maps. A 300 m buf fer zone was drawn around the census areas to reduce edge-e f fec t b ias (Krebs 1989). An index of aggregation and tes ts of s i g n i f i c a n c e fo r i t s dev ia t ion from randomness were der ived fo r marsh and beach habi ta ts (pp. 126-129 in Krebs 1989). H a b i t a t S e l e c t i o n I n t r a s p e c i f i c f o r a g i n g competition I n t e r f e r e n c e - I used i n t e r f e r e n c e , inc lud ing t e r r i t o r i a l behaviour, as an i n d i c a t o r of i n t r a s p e c i f i c competi t ion fo r food and foraging s i t e s . I noted the frequency and durat ion of chases, d i s p l a y s and f i g h t s of foraging herons on beaches in the 1987 and 1988 breeding season at Sidney and from 1 August- 3 November 1987-89 in the Fraser River d e l t a (see Chapter 2) . The e f f e c t of t h i s in te r fe rence was quan t i f i ed by est imat ing the reduct ion in Metabolized Energy (ME,see Chapter 5) to a foraging heron r e s u l t i n g from i n t e r a c t i o n s . T e r r i t o r i a l i t y - The smal lest t e r r i t o r y defended by 32 herons on the Oregon coast was about 200 m long (Bayer 1978) and about 300 m in my study area (6 c a s e s ) . I assumed that other feeding t e r r i t o r i e s ex is ted in my study area i f : ( i ) s i n g l e herons repeatedly used a s i t e avoided by neighbouring herons (4 c a s e s ) , ( i i ) a heron I d is turbed returned to the same s i t e and avoided areas used by other herons (3 c a s e s ) , or ( i i i ) a to ta l of 200 m or more of continuous unoccupied habi ta t extended on one or both s ides of a s o l i t a r y heron (25 c a s e s ) . 37 Habi tats and the l o c a t i o n s of herons along the banks of the Fraser River ( F i g . 2-3) were mapped from a boat on 10 January 1990. Habitat segments l e s s than 200 m long were e l iminated from the ana lys is because they were u n l i k e l y to be large enough to hold a heron t e r r i t o r y (see Bayer 1978). Relative ava i lab i l i ty of foraging time Re la t i ve a v a i l a b i l i t y of foraging time i s the proport ion of the to ta l a v a i l a b l e time required by adult and j u v e n i l e herons to meet t h e i r d a i l y maintenance energy needs on beaches and in grasslands in the non-breeding season. I assumed that adult and j u v e n i l e s required the same amount of maintenance energy because they have s i m i l a r body masses. Beaches - I estimated the r e l a t i v e a v a i l a b i l i t y of forag ing time to herons forag ing on beaches in September-November using the formula: F i = + <vyj x i o o M where Fj. i s the percentage of the low t i d e foraging per iod required by a heron to meet i t s d a i l y maintenance energy in each 24 h p e r i o d , M i s the amount of energy (kJ) required by herons f o r d a i l y energy maintenance, 1̂  and In are the respec t ive ingest ion rates during day and n igh t , and l"d and T n are the respec t ive number of minutes of low t ides a v a i l a b l e f o r foraging during day and n igh t . Each of these terms in the equation i s now explained in d e t a i l . Maintenance energy (M) - I estimated the amount of energy needed fo r maintenance (=1560 kJ) using methods out l ined in Chapter 5. I assumed that adul t and j u v e n i l e herons had s i m i l a r energy maintenance needs because they have s i m i l a r body masses. Ingestion rates (I) - Ingestion rate i s the amount of energy (kJ) consumed per minute of forag ing by an adult and j u v e n i l e heron. It was estimated by 38 m u l t i p l y i n g foraging rates of adult and j u v e n i l e herons (see below) by the average amount of energy in t h e i r respect ive d ie ts (see Methods in Chapter 5) . ' Foraging rates of adult and j u v e n i l e herons on beaches on the Fraser River 3 d e l t a were estimated on 33 days between 1 August and 3 November 1987-89. The mean capture ra te during high t i d e s at Sidney in May-June was 3.6 times f a s t e r - dur ing the day (x=2.6 m i n s / f i s h , S .E .=0 .3 , N=81) than at night (x=9.4 m i n s . / f i s h , S .E .=2 .1 , N=9). I f f i s h caught during the day and night are the same s i z e , then herons ingested about 3.6 times more energy per minute while forag ing dur ing the day as at night from August to November in the Fraser River d e l t a . Th is assumption i s d iscussed l a t e r . I assumed that herons caught the same species of f i s h during night and day on the Fraser River d e l t a because sea perch and s c u l p i n s were caught in beach seines during day and night in ee lgrass beds in the Yaquina estuary , Oregon (Bayer 1985b). Foraging time (T) - The number of minutes that low (<1.7 m) t i d e s uncovered the ee lgrass beds each 24h from 1 September to 30 November- 1987 was used as an estimate of the t o t a l .amount of a v a i l a b l e foraging time on beaches. Grasslands - I estimated the r e l a t i v e a v a i l a b i l i t y of forag ing time of herons feeding on Townsend's voles (Microtus townsendii) in grasslands in November - January using the formula: F t =IiWl x 1 0 0 M where F̂ . i s the percentage o f the day required by a heron to meet i t s d a i l y energy maintenance need, M i s the estimated amount o f energy needed fo r maintenance (=1560 kJ) using methods out l ined in Chapter 5, I i s the ingest ion ra te (kJ) of herons eat ing v o l e s , and T i s the number of d a y l i g h t hours per day. How I determined ingest ion rates i s now explained in d e t a i l . Ingest ion r a t e s (I) - Ingestion rates (1^) during the day were estimated by m u l t i p l y i n g the number of vo les caught per minute of grass land foraging by the 39 amount of energy (kJ) contained in an average v o l e . I watched 54 herons hunt voles in grasslands between 18 November 1985 and 17 January 1987. I estimated the average amount of metabol izable energy a v a i l a b l e to a heron eat ing voles by conver t ing the mean weight (56g,SD = 12) of 61 voles caught into energy un i ts (kJ ) . Voles were caught in l i v e - t r a p s on 2 December 1986 in grasslands used by herons on Westham Island in the Fraser River d e l t a . I assumed that : (1) the average Townsend's vole contained the same amount of water (67%) and energy (23.5 kJ /g dry wt) as the Common vole (M. arvalis, Wijnandts' 1984), and (2) herons d igested 78% of the energy in each vole (Castro et al. 1989). There fore , the average metabolized energy a v a i l a b l e in a vole eaten by a heron was estimated to be (56g x 0.33 dry wt x 23.5 kJ /g dry wt x 0.78 d i g e s t i v e e f f i c iency=) 339kJ. These assumptions are d iscussed l a t e r . Habitat Sh i f ts , Dispersal and Mortality I assumed that the dec l ine in numbers of herons along an 89.4 km census route ( F i g . 2-3) approximated t h e i r death and d i s p e r s a l rate from the Fraser River d e l t a . This assumption i s d iscussed l a t e r . Four road censuses were conducted each month between August and February 1987-88, except in September (3 censuses) , November (5 censuses) and August (6 censuses) . Emaciated dead herons found by me or others in the Fraser River d e l t a that had no abdominal fa t s tores and depleted pectoral muscle mass were assumed to have s tarved . RESULTS Spacing of Age- and Sex-classes Breeding season - During low t i d e s , most adult herons foraged in nearby Sidney lagoon (Table 4 -1 ) . Herons f l y i n g o f f the i s l a n d fed in ee lgrass and kelp beds on neighbouring is lands within 10 km of the co lony. 40 Table 4 -1 . Percentage of f l i g h t s to and from the Sidney lagoon versus other s i t e s o f f the i s l a n d during low t i d e s (<1.7 m) and high t ides when the nests held eggs, small ch icks and la rge c h i c k s . Low t i d e s High t ides Stage Percent N~ Percent N Egg 94.1 271 27.2 213 Small ch ick 90.5 541 51.3 298 Large ch ick 62.6 460 10.3 78 41 During high t i d e s , herons with eggs and large ch icks in nests avoided the lagoon and flew mostly o f f the i s l a n d (Table 4-1) to beaches and es tuar ies an average of 14 km (SD=8, range=6-27, n=13) from the c o l o n y - s i t e . However, dur ing the small ch ick stage, about h a l f the f l i g h t s at high t i d e were to and from the lagoon (Table 4 -1 ) . Only females (18 cases) were recorded in the lagoon between 9 A p r i l and 31 May 1988. Males (2 cases) were f i r s t seen in the lagoon on 1 June in the company of females (11 c a s e s ) . Seven male herons and no females were i d e n t i f i e d o f f the i s l a n d in l a t e afternoon and ea r ly evening before 1 June. I conclude that when nests held eggs and small ch icks most females fed themselves and t h e i r ch icks on f i s h caught in the lagoon during the day when t i d e s were low and males fed themselves mostly o f f the i s l a n d in the afternoon u n t i l the fo l lowing morning when t ides were mostly h igh . When nests held la rge c h i c k s , both parents fed the ch icks f i s h caught in the lagoon dur ing low t i d e and o f f the i s l a n d during high t i d e s . Year l ing herons r a r e l y v i s i t e d Sidney Island during the breeding season. One was seen in the lagoon fo r about 3 weeks in l a t e June-Ju ly in 1987, 1988 and 1989. A y e a r l i n g occupied a nest in the colony in June-Ju ly 1989 but d id not f i n d a mate (I . Moul, pe rs . comm.). I found 1-8 yea r l ings among adul ts on f o r t n i g h t l y v i s i t s made in May-July 1988 to 4 es tuar ies and 6 beaches up to 27 km away from Sidney Island and on beaches and in marshlands in the Fraser River d e l t a . Non-breeding season - Adult female and j u v e n i l e herons in the Fraser River d e l t a s h i f t e d from feeding mainly (92%) on the beaches in August to feeding mainly (56%) in marshlands in January. Th is trend was reversed in spr ing ( F i g . 4 -1 ) . The number of herons counted on the beaches at low t i d e from August 1986 to A p r i l 1987 was s i g n i f i c a n t l y (t=7.6, p<0.001) and p o s i t i v e l y 42 100 20 MARSHLANDS no j data BEACHES t» eg to ca © ^ to 10 i » N co © •* N CU 10 N CO T - w to 10 N D J MONTH M Figure 4 -1. Percentage of herons using marshlands and beaches in the Fraser River d e l t a between August and A p r i l 1986-87. Numbers above months ind ica te the number of herons counted in that month. 43 c o r r e l a t e d with the number of hours of a v a i l a b l e foraging itime during low t i d e ( F i g . 4 -2 ) . The marshlands were seldom covered by more than 30 cm of water f o r longer than 2h each day. S i g n i f i c a n t l y more j u v e n i l e s than adults used grasslands ra ther than marshlands (Table 4 -2 ) . Herons were mostly clumped on the beaches and in marshlands between September and February (Table 4 -3 ) . In summary, herons used beach habi ta ts mainly from March to October , and i n c r e a s i n g l y used marshlands and grasslands from November to January a f t e r which they began to return to beaches. H a b i t a t S e l e c t i o n I n t r a s p e c i f i c f o r a g i n g competition I n t e r f e r e n c e - Most breeding herons fed peace fu l l y in Sidney lagoon during the day. I saw one i n t e r a c t i o n in over 2700 mins. of watching herons catch non-school ing f i s h . Interference increased when herons b r i e f l y f locked to pursue schools of Shiner Sea Perch (Cymatogaster aggregata). However, these in te r fe rences d id not r e s u l t in herons being chased from the lagoon. For example, on 28-29 June 1988 I recorded 41 i n t e r a c t i o n s between about 50 herons that pursued perch f o r 23 mins. (0.04 i n t e r f e r e n c e s / h e r o n / m i n . ) . D isp laced b i rds f lew a few meters away and began feeding on sea perch again about 30s l a t e r . The median capture rate in 27 of these groups was 0.2 sea perch per min. ( range=0.04-l .7 ) . Each sea perch conta ins an estimated 100.4 kJ of Metabol izeable Energy (ME) (Table 5-2, Chapter 5) . Therefore , the estimated ME intake rate f o r herons feeding on sea perch was (0.2 f i s h / m i n . x 100.4 k J / s e a perch=) 20.1 kJ /min . The estimated energet ic cost of an in te r fe rence was (0.5 min. to s t a r t foraging per in te r fe rence x 0.04 in te r fe rences /min x 20.1 kJ/min.=) 0.4 kJ/min/heron or 2% of the to ta l ME 44 5 0 0 o £ 30o o CC LL) CQ ID 100 100 2 0 0 3 0 0 N U M B E R O F F O R A G I N G , H O U R S Figure 4-2. Numbers of Great Blue Herons counted on beaches in the Fraser River de l t a in r e l a t i o n to the average number of foraging hours at low t ide (<2.3 m) from August 1986 to A p r i l 1987. 45 Table 4-2 . Number of adult and j u v e n i l e herons counted in grasslands and marshlands of the Fraser River d e l t a between J u l y 1987-March 1988 and July-September 1988. Adul t Juven i l e Grasslands 698 302 Marshlands 158 16 46 Table 4 -3 . Indices of aggregation of Great Blue Herons feeding in marshlands and on i n t e r t i d a l beaches of the Fraser River de l t a dur ing the non-breeding season. The s p a t i a l pattern ranges f r o m clumped (R approaches zero) to regu la r , (R approaches 2 .15) . When the pattern i s random, R=l (Krebs 1989). Index of Aggregation (R) Year Date Marshlands n Beach n 1986 30 Sept 0 .53** 69 0 .59** 224 29 Oct 0.96 73 0 .87** 164 12 Nov 0.98 105 0 .83** 96 27 Nov 1.25** 28 0.98 54 10 Dec 0 .80** 43 1.20* 22 1987 6 Jan 0 .81** 45 0 .73** 32 21 Jan 0.95 46 0 .57** 42 6 Feb 0.97 27 0 .74** 47 20 Feb 0 .63** 34 0 .61** 84 * = the s p a t i a l pattern i s s i g n i f i c a n t l y (p<0.05) and * * = h igh ly 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 feeding on sea perch. Therefore in ter fe rence had l i t t l e e f f e c t on the average ME intake rate of these herons although i t could have a f fec ted some non-average b i rds who got c o n t i n u a l l y d i s p l a c e d . Adul t and j u v e n i l e heron d i e t s on the Fraser River d e l t a were s i g n i f i c a n t l y d i f f e r e n t in August/ September (Table 4-4) because j u v e n i l e s ate fewer sea perch. Interference from foraging c o n s p e c i f i c s reduced the ME intake rates o f adul t and j u v e n i l e herons by about 1% of t h e i r average rate and t h e r e f o r e , in te r fe rence was unimportant in autumn (Table 4 -5 ) . I conclude that in te r fe rence by c o n s p e c i f i c s maintains ind iv idua l foraging d is tances but does not exp la in habi ta t s h i f t s among herons foraging on beaches. Territoriality - Adul ts held t e r r i t o r i e s along r iverbanks (n=38) on the Fraser R i v e r . The highest d e n s i t i e s of t e r r i t o r i a l herons occurred in r iver -edge marshes (Table 4 -6 ) . Nine t e r r i t o r i e s that could be viewed both day and night were occupied by s o l i t a r y herons every month of the year . Three out of 5 herons could be c o n f i d e n t l y sexed along r iverbanks and a l l were adult males. Nineteen out of 28 adul ts whose sex could be determined in marshlands during the non-breeding season were females, one was a male and the sex of 8 others could not be c o n f i d e n t l y i d e n t i f i e d . None were t e r r i t o r i a l . These values are s i g n i f i c a n t l y d i f f e r e n t from an expected even sex r a t i o in marshlands and along r iverbanks (F isher Exact Test X 2=11.8, p<0!003). These r e s u l t s support the p r e d i c t i o n that i t i s usua l ly adult male herons who defend feeding t e r r i t o r i e s . 48 Table 4-4 . Number of each prey species caught by adult and j u v e n i l e Great Blue Herons in August and September 1987, 1988 and 1990 (N=33 days ) . Prey spec ies Adult Juven i l e Number % Number % Perch 82 63.0 27 35.1 Scu lp ins 39 30.0 40 52.0 Gunnels 4 3.1 3 3.9 Others 5 3.9 7 9.0 Total 130 77 X 2=15.7, df=3, p=0.002 49 Table 4-5 . Da i l y Metabol izable Energy (ME) intake in kJ l o s t to in te r fe rence by c o n s p e c i f i c s in autumn. Adult Juven i le M e a n S . E . Mean S . E . Estimated ME ingested Mean no. f i s h caught /min. Mean M E / f i s h a Mean ME/min. 0.46 48.6 22.4 0.06 14.0 0.24 42.6 10.2 0.03 10.5 Cost of in te r fe rence No. i n t e r f e r e n c e s / m i n . ME l o s t to i n t e r f e r e n c e / m i n . 0.01 0.22 0.01 0.10 c a l c u l a t e d by weighting the average metabol izeable energy per species (Chapter 5) by the proport ion of the d i e t in Table 4-4. 50 Table 4-6 . Numbers and d e n s i t i e s of t e r r i t o r i a l Great Blue Herons in three habi ta ts along the r iverbanks on the Fraser River d e l t a . Length of habitat(km) Number Density (herons/km) Forest edge 17.1 7 0.41 Marsh with or without f o r e s t 19.2 25 1.30 I n d u s t r y / a g r i c u l t u r e with or without marsh 62.1 6 0.10 51 Foraging success Beaches - The mean daytime foraging rates of adul ts on the Fraser River d e l t a dec l ined s i g n i f i c a n t l y (t=4.9, r 2=0.24, p<0.001) between 1 August and 3 November. In August, adul ts needed an average of 1.9 mins (SE=0.5) to capture a f i s h versus 3.5 mins (SE=0.3) in September and 5.0 mins (SE=0.4) in October and e a r l y November. Juven i le foraging rates a lso d e c l i n e d , but not s i g n i f i c a n t l y (t=2.57, r 2=0.12, p=0.13). Juven i l es needed an average of 3.3 mins. (SE=0.4) to capture a f i s h in August, 4.7 mins (SE=0.5) in September, and 6.0 mins (SE=0.9) in October and ea r ly November. Juven i les took 74%, 34% and 20% more time to catch a f i s h in August, September and October, r e s p e c t i v e l y , than a d u l t s . Foraging time The average j u v e n i l e runs out of time (Ft>100%) to meet i t s d a i l y energy needs on beaches on 5 days in September, 7 days in October and 12 days in November ( F i g . 4 -3) . In c o n t r a s t , the average adult meets i t s d a i l y energy need dur ing low t i d e on beaches on a l l but 2 days in September, 4 days in October and 7 days in November ( F i g . 4 -3 ) . Grasslands - An average heron required about 160 mins. to catch a vole (12 voles caught in 1938 mins.) between 18 November 1986 and 17 January 1987. An average vole contained an estimated 339 kJ of metabol izable energy, so a heron ingested about 127 kJ per hour (339 kJ per vole/160 mins.) while feeding on voles caught in g rass lands . There were about 9h of day l igh t each day in November and 8h of day l igh t in December and January at the l a t i t u d e of t h i s study ( 4 9 ° N ) . A heron preying only on voles ingested about 1144 kJ per day in November (127 kJ /h x 9h) and 1016 kJ per average day in December and January (127 kJ /h x 8h) . Therefore , a heron obtained an average of 73% 52 Figure 4-3. Estimated percent of ava i l ab le low-t ide (<1.7 m) foraging time required by adult (s t ipp led) and juven i le (open) herons to meet t h e i r estimated energy needs over 24 h (1560 kJ) each day in September to November 1987-1988. of i t s d a i l y need in November (1144 kJ/1560 kJ x 100%) and 65% in December and January (1016 kJ/1560 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 one - th i rd of a l l j u v e n i l e s were seen in grasslands in September- October and over three-quar ters in November ( F i g . 4 -4 ) . Most adul ts stayed on beaches u n t i l November. Between November and February adult females used marshlands ra ther than grasslands to a greater degree than j u v e n i l e s (X 2=32.1, p<0.001, Table 4 -2 ) . Propor t ionate ly more j u v e n i l e s than adults foraged during the day on 17 road-s ide censuses between November 1986 and February 1987. Ninety-n ine out of 139 (71.2%) j u v e n i l e herons were foraging compared to 218 out or 551 (39.6%) adults (p<0.001). J u v e n i l e s disappeared from the Fraser River d e l t a at a f a s t e r rate ( F i g . 4-5) and more starved to death than a d u l t s . Twenty-eight out of 46 (60.9%) j u v e n i l e herons starved to death compared to 3 out of 16 (18.8%) adul ts found between August and February (p<0.009). These f ind ings support the p red ic t ions that : ( i ) j u v e n i l e and adult herons depart from beaches when there i s too l i t t l e foraging time during low t i d e to meet t h e i r d a i l y energy need and, ( i i ) j u v e n i l e s vacate beaches before adul ts in autumn. 54 100 2B 37 li:::;:;:;:;:; 40 Z UJ O 50 CC UJ 0. 3 0 1 i t y i S 0 N D J F MONTH B 98 23 141 ••.-•••'•••'I 121 128 S O N D J F MONTH Figure 4-4. Percentages of j u v e n i l e (A) and adult (B) herons counted on grasslands ( s t i p p l e d bars) and beaches (open bars) dur ing the non-breeding season. Numbers above bars are number o f herons counted. 55 A S O N D J F MONTH Figure 4-5. Numbers o f adul t and j u v e n i l e Great Blue Herons counted each month on road surveys of the Fraser River d e l t a . DISCUSSION Validity of assumptions This ana lys is i s based on several key assumptions about herons foraging on beaches and in g rass lands . Beach-foraging herons - I assumed that : ( i ) herons can not change t h e i r food intake ra te at a s i t e (see Swennen et al. 1989), ( i i ) foraging time d ic ta ted by low t i d e s and foraging s k i l l were the only v a r i a b l e s that s i g n i f i c a n t l y a f fec ted d a i l y ingest ion r a t e s , and ( i i i ) herons caught three times as many f i s h during the day as at night from August to November.The food intake ra tes of herons are a lso a f fec ted by high winds (Bovino and Burt t 1979), but my study took place when winds were genera l ly l i g h t . I have l e s s confidence in my estimate that three times as many f i s h are caught dur ing the day as at n i g h t . My estimate i s based on a small n ight - t ime sample (N=9) taken in summer. However, most foraging i s done during daytime at t h i s time of year and i t i s reasonable to expect that herons catch fewer f i s h at night than during the day. It i s u n l i k e l y that herons caught l a r g e r f i s h at night than during the day because large f i s h move into deeper water dur ing low t ides when most herons feed . A l l of these assumptions cont r ibute add i t iona l e r ro r to my estimates of the amount of time herons need to balance t h e i r d a i l y energy budgets. However, these e r rors do not g r e a t l y a f f e c t my conc lus ions because j u v e n i l e s would requ i re 6-165% (median 83%) more time on 10 days in November to meet t h e i r d a i l y energy need. T ides were too high to al low herons to forage in ee lgrass on a fu r ther two days. Grassland-foraging herons - I assumed tha t : Townsend's voles eaten by herons in t h i s study weighed an average of 56g, contained the same amount of water and energy per gram as the Common vole in Wijnandts' (1984) study, 78% 57 energy a s s i m i l a t i o n e f f i c i e n c y by herons, and herons caught voles at a rate of one every 160 mins. Other s tud ies have found s i m i l a r body weights fo r Townsend's voles in the Fraser River d e l t a between November and January (Beacham 1980). Moreover, the water and energy content i s r e l a t i v e l y constant between rodents (Cummins and Wuycheck 1971, Zwarts and Blomert 1990) and the d i g e s t i v e e f f i c i e n c y of 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 from that of other b i rds (Castro et a / . 1989). Each of these estimates has a small assoc ia ted e r r o r . I have l e s s conf idence in my estimates of the rate that vo les were captured by herons. Herons could be watched c o n t i n u a l l y f o r about a h a l f - h o u r on average (X=36 mins, SD=22, N=54), before they flew or walked out of s i g h t . No heron could be watched long enough to determine the time in te rva l between prey captures . Other f a c t o r s , such as the a c t i v i t y of v o l e s , the amount of vegetat ion cover , and e s p e c i a l l y the degree of winter f l o o d i n g o f g r a s s l a n d s , a f f e c t the v u l n e r a b i l i t y of voles to predators , i n c l u d i n g the Great Blue Heron. For example, T a i t t and Krebs (1983) reported herons catching voles every 20 mins. in f i e l d s during heavy p r e c i p i t a t i o n between November 1980 and February 1981 compared to 160 mins. in my study of the same f i e l d when f lood ing was not widespread. Thus, the a v a i l a b i l i t y to herons of voles in grasslands i s probably more v a r i a b l e than the a v a i l a b i l i t y of f i s h on beaches. Year-round Foraging Dispersion of Age- and Sex-classes This i s the f i r s t study to my knowledge that descr ibes the year-round habi ta t use of Great Blue Heron age- and s e x - c l a s s e s . When nests held eggs and small ch icks and food a v a i l a b i l i t y in the lagoon was high (Chapter 5) , females fed c lose to the colony while males tended nests dur ing the day. 58 Other s tudies have shown that most herons feed near t h e i r c o l o n y - s i t e s ( e . g . Brandman 1976, Prat t 1980, Dowd and Flake 1985, Simpson et al. 1987) but the sex of those herons was not known. When t ides were high in l a t e afternoon or evening, females in my study exchanged dut ies with males who then l e f t the colony to feed alone on t e r r i t o r i e s up to 27 km away u n t i l they returned the next morning. Other s tudies of coasta l herons have shown that most p a i r s exchange dut ies during high t i d e (Paine 1972, Brandman 1976, Moul 1990). Both parents fed in the lagoon l e s s often when the ch icks were 3-4 weeks o ld than in e a r l i e r stages (Table 4 -1 ) . Th is s h i f t in use of foraging s i t e s co inc ided with a dec l ine in food a v a i l a b i l i t y in the lagoon (Chapter 5 ) . H a b i t a t S e l e c t i o n F oraging 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 - My study j o i n s several others that i n d i c a t e that foraging e f f i c i e n c y , d a i l y use of t ime, and d i s p e r s a l are c l o s e l y i n t e r r e l a t e d in b i rds ( e . g . Goss-Custard and d i t Dure l l 1984, Weathers and S u l l i v a n 1989, S u l l i v a n 1989, S u l l i v a n 1990). I showed that j u v e n i l e herons were l e s s e f f i c i e n t foragers (Table 4 -4 ) , required more foraging time to meet t h e i r energy needs ( F i g . 4 -3 ) , and departed from beach habi ta ts sooner in autumn than adul ts ( F i g . 4 -4 ) . I a lso ind ica ted that propor t ionate ly more j u v e n i l e s used grasslands than marshlands compared to adul ts (Table 4 -2 ) . L a s t l y , I showed that j u v e n i l e s disappeared from the study area at a higher rate than adul ts ( F i g . 4 -5 ) . Foraging theory p o s i t s that animals switch feeding s i t e s when t h e i r food intake d e c l i n e s to the average gain in the habi ta t (Charnov 1976). Th is theory p r e d i c t s that herons should assess t h e i r forag ing success on both beaches and in grasslands by sampling each h a b i t a t . Most adul ts 59 (94.5%, n=l10) and j u v e n i l e s (85%, n=35) roosted in marshes and f i e l d s during high t i d e s in autumn rather than foraging in g r a s s l a n d s , contrary to the p r e d i c t i o n of habi ta t sampling. There fore , I used a hunger threshold to p red ic t when herons departed beach habi ta ts in autumn. Herons are faced with a d e c l i n i n g durat ion of low day time t ides (Department of F i s h e r i e s and Oceans 1987, 1988) and d imin ish ing prey populat ions (Gordon and Levings 1984) on beaches through autumn. Adul ts could meet t h e i r d a i l y energy needs in about 55-85% of the a v a i l a b l e time dur ing most low t i d e s in September, October and November. Juven i l es requi red about 73-100% of the a v a i l a b l e time on most days over the same per iod and had i n s u f f i c i e n t time on many days ( F i g . 4 -3 ) . Adul t and j u v e n i l e herons store body fa t (unpubl. data) presumably on days when t i d e s remain low fo r many hours to insure against p red ic tab le per iods of shortage. This opt ion ceased f o r j u v e n i l e s on beaches a f t e r about mid-October when they could hardly meet t h e i r d a i l y energy needs on most days ( F i g . 4 -3 ) . Most j u v e n i l e s f lew to grasslands a f t e r October ( F i g . 4-4) where they acquired about 73% of t h e i r d a i l y energy need each day in November and 65% of t h e i r d a i l y energy need each day in December and January. Presumably they met t h e i r energy needs by foraging in marshlands during low t i d e s at n igh t . These f i n d i n g s suggest that herons leave foraging habi ta ts when t h e i r food energy po ten t ia l dec l ines below a c r i t i c a l l eve l (McNamara 1982). Indiv idual d i f f e r e n c e s among j u v e n i l e s at the leve l of foraging e f f i c i e n c y I descr ibed can g r e a t l y a f f e c t t h e i r surv iva l ( G i l l et a l . 1975, S u l l i v a n 1990). As winter approached, j u v e n i l e herons spent l e s s time on the beaches and more time in grasslands ( F i g . 4-4) where they hunted small mammals. I hypothesize that the low foraging success in grasslands l e f t 60 e s p e c i a l l y j u v e n i l e s emaciated and vulnerable to c o l l i s i o n s with v e h i c l e s , telephone wires and fences . It i s unclear why more j u v e n i l e s do not forage with adul ts in marshlands dur ing the day. Juven i les might lack the knowledge of the best forag ing times in the day (Draulans and Vessem 1985) or trade o f f low mean f o r high var iance intake rates in grasslands (Caraco et a7. 1980, Caraco 1981). J u v e n i l e s forage in marshlands at night (pers . obs. Richner 1986). Adul ts might leave marshlands on b lus tery days when t h e i r foraging success d e c l i n e s below some threshold (Bovino and Burt t 1979) or to e x p l o i t temporary bonanzas created when ra ins f lood voles from t h e i r underground burrows in g rass lands , or both. Territoriality Several s tud ies have shown that adult herons defend feeding t e r r i t o r i e s and that i n t e r a c t i o n s are infrequent (Bayer 1978, Cook 1978, Richner 1986, Draulans and Hannon 1988). Mine i s the f i r s t study to show that t e r r i t o r i a l herons were mainly and perhaps only males. Density-dependent habi ta t s e l e c t i o n models (Fretwel l and Lucas 1970, Fretwel l 1972) exp la in well the spacing of t e r r i t o r i a l herons. The greatest d e n s i t i e s occurred in marshes along r iverbanks where prey d e n s i t i e s are probably g r e a t e s t . Moreover, j u v e n i l e s were excluded from t e r r i t o r i e s in t h i s and other s tudies ( e . g . Bayer 1978, Richner 1986). These r e s u l t s are pred ic ted from the ideal despot ic d i s t r i b u t i o n model (Fretwel l and Lucas 1970, Fretwel l 1972). Future s tudies are needed to examine i f the average intake rates are greater in the high dens i ty habi ta ts than in low densi ty h a b i t a t s , as pred ic ted by the ideal despot ic d i s t r i b u t i o n . 61 SUMMARY 1) When nests held eggs and small c h i c k s , breeding female herons at Sidney mostly fed wi th in 2 km of the c o l o n y - s i t e dur ing the day when t i d e s were low and, breeding males mostly fed along beaches from 6- 27 km from the c o l o n y - s i t e dur ing l a te afternoon u n t i l the next morning when t i d e s were h igh . Breeding males and females fed wi thin 2 km of the c o l o n y - s i t e when ch icks were l a r g e . 2) The s h i f t in habi tat use by adult females and j u v e n i l e s from beaches to marshlands and grasslands in autumn was best explained by a shortage of foraging time during low t ides coupled with shr ink ing prey popu la t ions . 3) Adul t males spent the non-breeding season in t e r r i t o r i e s along r iverbanks and t h e i r spacing was predic ted well by the ideal despot ic d i s t r i b u t i o n (Fretwel l and Lucas 1970). 62 CHAPTER FIVE. TIME OF BREEDING IN GREAT BLUE HERONS In the previous chapters , I examined how herons se lec ted breeding and foraging h a b i t a t s . Here, I focus on the r e l a t i o n s h i p between food a v a i l a b i l i t y and time of breeding. Lack (1954) postulated that natural s e l e c t i o n favours adul ts whose n e s t l i n g s are present when food i s most a v a i l a b l e to the parents . This hypothesis p r e d i c t s that ea r l y and l a te nesters should fare l e s s well than those nest ing on the average date . Several s tud ies have supported Lack 's (1954) hypothesis (see Perr ins and Birkhead 1983), but ea r l y c lu tches are of ten the most product ive ( e . g . Cave 1968, Davies and Lundberg 1985, but see Noordwijk 1983). Per r ins (1965, 1970) proposed that food shortages during egg- lay ing prevent most females from breeding e a r l i e r so that young would be in the nest a f t e r the parents ' food suppl ies had peaked. Several s tud ies suggest that females breed when food becomes p l e n t i f u l (Drent and Daan 1980, Daan et a l . 1988) but i t i s not c l e a r whether most young are in nests when food is increas ing or decreasing in abundance (see Daan et al. 1988). Tests of Lack 's (1954) and P e r r i n s ' (1965, 1970) hypotheses have been hampered by methodological problems. Food abundance, ra ther than food a v a i l a b i l i t y , has been used as an index of food supply in most s tudies (Daan et a / . 1988). Moreover, these hypotheses assume that food i s in short supply to the egg- lay ing female (Perr ins 1965, 1970) or adul ts with n e s t l i n g s (Lack 1954), although many studies have found that breeding b i rds are not short of food (see reviews by Martin 1987, Linden and Mol ler 1989, Chapter 1 ) . 63 A s u i t a b l e species in which to compare t iming of breeding i s one in which food a v a i l a b i l i t y and prey consumption can be measured d i r e c t l y . The Great Blue Heron i s a s u i t a b l e species because: ( i ) i t eats small f i s h whose populat ions can be sampled with beach-seine n e t s , and ( i i ) i t s rate of consumption of f i s h can be estimated (Simpson 1984, Bayer 1985). The aim of t h i s chapter i s to r e l a t e the seasonal a v a i l a b i l i t y of food energy to the time of breeding of herons. F i r s t , I estimate the r e l a t i v e a v a i l a b i l i t y of prey energy to adult herons when they have eggs, small ch icks and la rge c h i c k s . I then examine i f c r i t i c a l food shortages occur . Next, I compare the hypotheses that : ( i ) females begin to lay eggs a f te r a threshold of a v a i l a b l e energy f o r egg production has been passed (Perr ins 1965, 1970) and, ( i i ) ch icks are in the nest when food f o r adul ts i s most p l e n t i f u l (Lack 1954). STUDY AREA AND METHODS Great Blue Herons were studied on Sidney Island (Figure 2-1) . General methods used t o : estimate foraging r a t e s , sample prey popu la t ions , determine when eggs were l a i d , and estimate reproduct ive success , are ou t l ined in Chapter 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 Ad u l t Herons Rela t ive a v a i l a b i l i t y of food energy i s the biomass of f i s h present in the lagoon expressed in un i ts of energy (kJ ) , m u l t i p l i e d by the durat ion of low (<1.7 m) t i d e s during each stage of the breeding season. It was estimated from the formula: ME S =((F S ) (W S ) -C)T S 64 where ME$ is the amount of available energy in nesting stage S; F $ is the estimated number of the f ish in the partial enclosure in nesting stage S, w"s is the average weight in grams of f ish caught in the partial enclosure in nesting stage S; C is a constant mult ipl ier for metabolizeable energy content of f ish equal to 4.76 kJ/g dry weight, and T $ is the average number of minutes of low tide foraging in nesting stage S. Each of these terms i n the equation is now explained in de ta i l . Number of f ish 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 held eggs, small chicks and large chicks (see Nesting stages, below) in 1988. A 1.5 x 18 m (6 mm stretched mesh) beach seine was then quickly and repeatedly hauled toward the opening. On each haul, the contents were quickly emptied into an empty bucket until most f ish had been caught. These data (Appendix I) were used in Leslie and Ricker models (pp. 162-166 in Krebs 1989) to estimate the number of f ish in the partial enclosure. The Shiner Sea Perch (Cymatogaster aggregata) and Tube-snout (Aulorhynchus flavidus) avoided the partial enclosure so I estimated their numbers by assuming that the proportion of a l l species caught in 19 seine- hauls outside the partial enclosure each month was the same as in the partial enclosure prior to i ts insta l la t ion. This assumption is discussed la ter . Energy estimates in prey (W,C) - Al l f ish caught in the partial enclosure were measured and samples were weighed to derive length-weight equations 65 f o r each s p e c i e s . The mean weight of a l l f i s h caught dur ing each nest ing stage (see below) was converted in to un i ts of energy as f o l l o w s . I assumed tha t : ( i ) a l l f i s h contained 71% water (Holmes and Donaldson 1969), ( i i ) the a s s i m i l a t i o n e f f i c i e n c y of herons was 77% (Castro et al. 1989) and, ( i i i ) each gram dry-weight of f i s h contained 21.3 kJ (Cummins and Wuycheck 1971). These assumptions are d iscussed l a t e r . Nesting stages (S) - I estimated each nest ing stage from the median date (14 May) that the f i r s t hatched eggshel l was found below each nest . The ch ick stage was d iv ided in h a l f so that most nests held small chicks from 14 May to 8 June, and large chicks from 9 June to 1 J u l y . Incubation requ i res about four weeks (Vermeer 1969, Brandman 1976). There fore , the incubation stage was back-dated 28 days from the median hatching date to inc lude the per iod 15 A p r i l to 13 May. Egg-laying was estimated to occur from 1-14 A p r i l . Duration of low tides (T) - Foraging occurred while t i d e s were low (<1.7 m) at Sidney lagoon. The number of minutes of low t i d e a v a i l a b l e to herons each day o f the breeding season was generated by computer using t i d a l data housed at the T ida l O f f i c e of the Ins t i tu te of Ocean Sc iences . Estimated Energy Consumption by Adults Adult consumption - The length of each f i s h caught by herons was estimated as a propor t ion of the heron's culmen length (Chapter 2) . The most s i g n i f i c a n t bias in t h i s method i s the observer ' s a b i l i t y to estimate the length of the prey (Bayer 1985). Observer p r e c i s i o n was tested by showing a range of s i z e s of the four major prey species held between the mandibles 66 of a dead heron to observers using te lescopes 65-100 m away. S i g n i f i c a n t d i f f e r e n c e s d id not occur between observers (X 2=1.86, p=0.8, d . f .=4 ) . Both c o r r e c t l y i d e n t i f i e d a l l f i s h and both underestimated the lengths of 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 f i s h length estimates by one s i z e - c l a s s to reduce observer bias and then converted each f i s h in to energy un i ts fo l lowing the methods ou t l ined 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 icon i i fo rmes requ i re about 30% more energy above maintenance costs to fuel t h e i r a c t i v i t i e s (Kushlan 1977). The estimated maintenance cost from Kendeigh's (1970) equations fo r a caged 2100 g heron i s 1200 kJ and her a c t i v i t y costs an add i t iona l estimated 360 kJ (30% of 1200 kJ) fo r a to ta l d a i l y energy need of 1560 k J . Twenty-seven heron eggs c o l l e c t e d in B .C . near the beginning of the l a y i n g per iod contained a mean of 4.77 g of fa t and 3.03 of carbohydrate and pro te in (P. Whitehead, unpubl. da ta ) . Each gram of fa t contains about 39 kJ of energy while a gram of carbohydrate and prote in holds about 18 k J . There fore , there are about 240 kJ [(4.77 g x 39 kJ/g)+(3.03 x 18 kJ)] in an average heron egg. The e f f i c i e n c y of convert ing metabolized energy in to eggs i s estimated to be about 70% (King 1973) so an average female musters about 312 kJ per egg (240kJ+(0.30x240 k J ) ) . Since herons lay eggs in two day i n t e r v a l s , a female requi res about h a l f the 312 kJ per day (Murton and Westwood 1977) or 155 k J . She a lso requi res energy f o r ovogenesis f o r a few days p r i o r to egg formation (Murton and Westwood 1977). There fore , a female heron must exceed a threshold of about 1715 kJ (1560 k J / d f o r maintenance +155 k J / d f o r each egg) fo r at l e a s t 10 days (Murton and Westwood 1977) to complete her 4 egg c l u t c h where one egg i s l a i d every two days. 67 Energy consumption by eg g - l a y i n g females - A v a i l a b l e energy to females before and while 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 during 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 to 14 May. Food demands of heron c h i c k s - Si 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, pers. comm.). They were moved i n t o outdoor a v i a r i e s at about 3 weeks of age. I used three-day running averages o f the weight of f i s h eaten to estimate the age when the maximum food demand 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 1982) 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, pers. comm.). RESULTS R e l a t i v e A v a i l a b i l i t y of Food Energy i n the Lagoon Herons caught tube-snouts (Aulorhynchus fiavidus), s t i c k l e b a c k s (Gasterosteus acuieatus), sea perch (Cymatogaster aggregata), p i p e f i s h (Signathus griseolineatus), s c u l p i n s (Leptocottus armatus) and gunnels (Pholis ornata and P. laeta). The average l e n g t h , weight and estimated energy contained 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 lengths of f i s h caught in beach seines A p r i l - J u l y 1987- 88 and t h e i r estimated weights from length-weight regress ion equat ions. Total length (mm) Weight (g) Mean SD N Mean wt(g) SE Pholis ornata A p r i l 87.0 28.1 18 1.8 0.2 May 80.8 19.7 141 1.3 0.1 June 99.0 17.2 92 2.6 0.1 J u l y 94.3 16.5 73 2.3 0.1 Gasterosteus aculeatus A p r i l 60.8 6.1 27 1.8 0.8 May 67.2 5.5 98 2.6 0.4 June 70.4 4.4 42 3.0 0.7 J u l y 73.8 4.8 12 3.5 1.2 Leptocottus armatus A p r i l 64.1 15.2 21 2.9 3.2 May 66.4 24.5 47 3.1 1.7 June 90.9 38.5 66 6.4 1.5 J u l y 95.8 22.9 58 7.6 1.5 Cymatogaster aggregata May 110.0 34.2 15 24.9 6.7 June 104.1 12.1 119 21.1 2.4 J u l y 52.1 5.3 50 12.0 3.7 Sygnathus griseolineatus A p r i l 138.0 58.0 7 3.9 0.7 May 156.7 48.2 118 4.4 0.2 June 173.1 39.2 31 4.8 0.4 J u l y 173.1 39.2 31 4.8 0.4 Aulorhynchus flavidus A p r i l 158.8 47.3 5 35.3 7.5( 69 Table 5-2. Estimates of Metabol izable Energy (ca lcu la ted from weights from Table 5-1) a v a i l a b l e to herons from the major prey species each summer month in the lagoon at Sidney. Metabol izeable energy (kJ) Mean SE Pholis ornata A p r i l 8.6 0.2 May 6.2 0.1 June 12.4 0.1 J u l y 11.0 0 . 1 , Gasterosteus aculeatus A p r i l 8.6 1.0 May ' 12.4 0.5 June 14.3 0.9 J u l y 16.7 1.5 Leptocottus armatus A p r i l 13.8 4.1 May 14.8 2.2 June 30.5 1.9 J u l y 36.2 1.9 Cymatogaster aggregata May 118.5 8.6 June 100.4 3.1 J u l y 57.1 4.7 Synathus griseolineatus A p r i l 18.6 0.9 May 20.9 0.2 June 22.9 0.5 J u l y a 22.9 0.5 Aulorhynchus flavidus A p r i l 168.0 9.7 a - no p i p e f i s h were caught in seines in J u l y so I assumed those caught by herons were the same s i z e as in June. 70 Numbers of prey i n d i v i d u a l s were greatest by f a r when herons had small ch icks in nests (Table 5-3) . Low t i d e s exposed the ee lgrass habi ta t for the longest per iod when large chicks (9 June-1 Ju ly ) were in nests (Table 5-4) . However, the r e l a t i v e a v a i l a b i l i t y of food energy was greatest when small ch icks were present (Figure 5-1) . Seasonal Energy Consumption by A d u l t s , Chicks and E g g - l a y i n g Females A d u l t s - Adul t herons ingested near ly four times more food energy per day when small ch icks were in t h e i r nests (Table 5-5) than when they were l a y i n g (p<0.001), or incubat ing eggs (p<0.001), and near ly three times as much as when r a i s i n g large ch icks (p<0.05, K r u s k a l l - W a l l i s ANOVA and mu l t ip le comparison t e s t ; p. 200, Zar 1984). The surge in energy consumption resu l ted from the large number of sea perch in the d i e t (Table 5-6) . Chicks - The greatest food demands by 6 cap t ive - rea red ch icks occurred at about 37 days of age. The median hatching date at Sidney was 14 May. There fore , the estimated peak food demands f e l l 37 days l a t e r on about 20 June when la rge ch icks were in n e s t s , and when ingest ion rates were r e l a t i v e l y low. E g g - l a y i n g females - Egg lay ing began when the females' estimated d a i l y energy ingest ion exceeded t h e i r energy threshold f o r egg- lay ing f o r 7 days in 1987 and 9 days in 1988 (Figure 5-2) . Moreover, a v a i l a b l e energy f e l l below the egg- lay ing threshold fo r 10 continuous days in l a t e February-ear ly March compared to only 1-2 days in ear ly May (Figure 5-2) . 71 Table 5-3. Extrapolated minimum populat ions of f i s h in the p a r t i a l enclosure in Sidney lagoon. Extrapolated Per iod populat ion S . E . Incubation 360 48 Small ch icks 3297 6 Large ch icks 403 5 a - See Appendix I. 72 Table 5-4. Number of low (<1.7 m) t i d e minutes per average day in which herons could forage in the lagoon ee lgrass beds during the breeding season. Number of minutes Breeding stage per average day Egg lay ing 151 Incubation 176 Small ch ick 208 Large ch ick 216 73 Table 5-5. Estimated Metabolized Energy (ME) intake (kJ) per average day by an adult heron averaged over four periods of the breeding season on Sidney Island in 1987-88. N i s the number o f herons watched. 4S Breeding per iod Egg- lay ing Incubation Small ch ick Large c h i c k SE N x SE N x SE N x SE N ME intake rate (kJ/min) 7.7 3.7 27 6.8 1.1 155 20.5 3.7 48 7.4, 0.7 109 Mins. a v a i l a b l e to forage per day 151 176 208 216 Estimated 1163 555 1197 194 4264 764 1598 151 intake/day X LU a 1.0 1 • 0.5 5 < < > < SMALL CHICK INCUBATION i n LARGE CHICK Figure 5-1 . Index o f food a v a i l a b i l i t y dur ing the breeding season of the Great Blue Heron. Percent a v a i l a b i l i t y i s estimated from the product of numbers of minutes when low t i d e s (<1.7 m) exposed the foraging habi ta t and the s i z e of the f i s h populat ion converted in to un i ts of energy. V e r t i c a l l i n e s are standard e r r o r s . 75 1987 Number of clutches Figure 5-2. Numbers o f days that a v a i l a b l e energy exceeded an estimated energy threshold fo r egg lay ing and the estimated dates when c lu tches were l a i d in 1987 and 1988. 76 Table 5-6. Numbers of each of the main prey species eaten by Great Blue Herons during the 1987-88 breeding season on Sidney Is land . Courtship Egg Small ch ick Large ch ick Total Species N % N % N % N % N °/ 0 Gunnel 20 41.7 188 40.9 56 34.6 175 65.3 439 46. ,8 S t i ck leback 0 0 42 9.1 10 6.2 8 3.0 60 6. .4 Scu lp in 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 P i p e f i s h 0 0 4 0.9 9 5.5 1 0.4 14 1. .5 Tubesnout 11 22.9 1 0.2 0 0 1 0.4 13 1. .4 Unknown 15 31.2 127 27.6 10 6.2 19 7.0 171 18, .2 48 460 162 268 938 77 In summary: ( i ) energy demands of the ch icks occurred several weeks a f t e r the peak in a v a i l a b i l i t y to t h e i r parents , and ( i i ) female herons l a i d t h e i r eggs soon a f t e r the estimated egg- lay ing threshold had been exceeded. I conclude that my f ind ings best support the energy threshold hypothesis (Perr ins 1965, 1970). DISCUSSION V a l i d i t y o f Assumptions In t h i s a n a l y s i s I assumed tha t : ( i ) the r e l a t i v e abundance of f i s h in the p a r t i a l enclosure represented the f i s h populat ion elsewhere in the lagoon, ( i i ) the propor t ion of tube-snouts and sea perch caught in seine hauls in the lagoon c l o s e l y r e f l e c t e d t h e i r abundance r e l a t i v e to other species in the lagoon, ( i i i ) each f i s h contained 71% water and 21.3 kJ per gram dry weight of which 77% was ass imi la ted by herons, ( iv ) I found eggshe l ls below nests on the day they hatched, and (v) female herons required 1560 kJ per day to maintain themselves and an extra 155 kJ each day to produce an egg. Departures from a l l of these assumptions cont r ibute add i t iona l e r ro r to the est imates of energy a v a i l a b i l i t y and consumption. However, these er rors do not a l t e r my conclus ions because the r e l a t i v e abundance of prey populat ions was more than e i g h t - f o l d greater when small ch icks were in nests than when large ch icks were present (Table 5-3) . Other s tud ies of i n t e r t i d a l f i s h populat ions in B r i t i s h Columbia have found s i m i l a r seasonal patterns in abundance (Weibe 1968, K e l s a l l and Simpson 1980, Hughes 1985). The addi t iona l e r rors from est imat ing water and energy content of f i s h , a s s i m i l a t i o n rates and maintenance energy requirements of herons are probably a lso low (see Kahl 1964, Dunn 1975, Kushlan 1977, 1978, Castro et a / . 1989). For. example, a sample of 12 prey f i s h e s in my study contained 71-73% water and 3 seaperch 78 contained about 3 kJ l e s s energy than my estimate (118 kJ) using values from the l i t e r a t u r e . I have l e s s conf idence with my estimates of energy consumption ear ly and l a t e in the nest ing season. My energy threshold estimate f o r egg- lay ing using publ ished values (1715 kJ) was greater than my f i e l d estimate (=1163 kJ) of energy ingest ion by l a y i n g females but the e r ro r around my estimate was great (Table 5-5) . Moreover, ex istence energy of herons held in outdoor cages at the U n i v e r s i t y o f B r i t i s h Columbia i s about 270 kJ lower than the 1200 kJ maintenance energy estimate der ived from Kendeigh's (1970) equations (D. Bennet, unpubl. da ta ) . Thus, my estimates of energy required for maintenance and the threshold fo r egg lay ing are probably s l i g h t l y higher than what female herons r e q u i r e . A l s o , a large change in the p o s i t i o n of the threshold i s requi red to make small change to the number of days when food a v a i l a b i l i t y exceeds the threshold in Figure 5-2. Time of Breeding My r e s u l t s are best explained by P e r r i n s ' (1965, 1970) hypothesis that : ( i ) energy a v a i l a b i l i t y determines when females w i l l l ay eggs and, ( i i ) ch icks are in the nest a f t e r food a v a i l a b i l i t y to t h e i r parents has peaked. However, most females l a i d eggs several days a f t e r the estimated threshold had been crossed (Figure 5-2) . Some of t h i s v a r i a b i l i t y probably arose from females being in d i f f e r e n t body cond i t ion (see Drent and Daan 1980, Perr ins and Birkhead 1983). Thus, the Great Blue Heron faces an impossible opt imizat ion problem: once females cross the threshold f o r egg l a y i n g , i n s u f f i c i e n t time remains f o r the ch icks to match t h e i r food demands with the peak in the food supply . Chicks are fed only by the female when food i s most a v a i l a b l e and by both parents a f t e r food a v a i l a b i l i t y peaks. 79 It i s u n l i k e l y that photoperiod or a i r temperature are s i g n i f i c a n t in f luences (see r e f s . in Ore l l and Ojanen 1983) on when herons breed because l a y i n g dates are not synchronized between nearby c o l o n y - s i t e s . For example, in three years of t h i s study, 3 co lon ies in the Fraser River d e l t a (30-40 km to the north) and a colony in V i c t o r i a (25 km to the southeast) hatched eggs 2-3 weeks e a r l i e r than Sidney. Moreover, a colony near Crofton (35 km to the west) hatched eggs in the same week as Sidney. I hypothesize that the asynchrony in l a y i n g dates between co lon ies r e s u l t s from d i f f e r e n c e s in inshore movements of l o c a l f i s h popula t ions . The d a i l y durat ion of low t i d e s determines how much food herons acquire on beaches throughout the year (Chapter 4, Figure 5-2) . In Chapter 4, I proposed that female herons might store energy on winter days when t ides were very low to safeguard against p red ic tab le per iods of food s c a r c i t y . Here, I propose that as per iods of food s c a r c i t y become l e s s frequent in s p r i n g , female herons devote l e s s foraging time to maintaining t h e i r energy balance, and more time to the breeding e f f o r t . Future D i r e c t i o n s Future work might examine the strength of the r e l a t i o n s h i p between egg- lay ing date and food a v a i l a b i l i t y to the female. Food manipulat ions of w i ld and capt ive herons o f f e r the best prospects . The bonanza in food energy a v a i l a b l e when sea perch are p l e n t i f u l in May might al low females to recover body cond i t ion from egg- lay ing and c h i c k - r e a r i n g and begin t h e i r feather moult. Sea perch are abundant between May and September in ee lgrass beds. 80 SUMMARY 1) The greatest amount of energy a v a i l a b l e and consumed by adul ts occurred in May when young chicks were in the nest . 2) The greatest energy demands of ch icks occurred in l a t e June about one month a f t e r energy was most a v a i l a b l e to t h e i r parents . 3) The t iming of breeding in Great Blue Herons in B r i t i s h Columbia i s best explained by the a v a i l a b i l i t y of energy to the egg- lay ing female (Perr ins 1965, 1970) ra ther than to the demands of the ch icks (Lack 1954). 81 CHAPTER SIX. GENERAL DISCUSSION The aims of t h i s t h e s i s were to consider the f a c t o r s that determine where herons loca te t h e i r breeding c o l o n y - s i t e s , how age- and s e x - c l a s s e s use forag ing hab i ta ts through the year , and to examine the r e l a t i o n s h i p between food a v a i l a b i l i t y , habi tat s e l e c t i o n and time of breeding. I d iscuss and in tegrate my main conclus ions now. Choice o f C o l o n y - s i t e Three main conclus ions emerged from t h i s study. F i r s t , colony spacing is best explained by the d i s t r i b u t i o n of feeding s i t e s and not by the presence of a key predator the Bald Eagle (Chapter 3 ) . Twenty-nine of 33 co lon ies were near r i c h i n t e r t i d a l feeding areas. The average d is tance to the feeding grounds o f a sample of 22 co lon ies was 2.4 km (Chapter 3 ) . In c o n t r a s t , the number of breeding pa i rs of herons, the number of successfu l nests and the s ize of broods d id not vary s i g n i f i c a n t l y with eagles nest d e n s i t i e s . Herons might be unable to avoid eag les , my ana lys is might be too coarse or the power of the t e s t might have been to weak, to detect a s i g n i f i c a n t d i f f e r e n c e . Several s tud ies have shown that the number of breeding wading b i rds i s p o s i t i v e l y re la ted to the area of t h e i r feeding habi ta t ( e . g . Werschkul et al. 1977, Kushlan 1978, Chapter 3 ) . Others have shown that colony spacing in continuous habi ta t i s p o s i t i v e l y re la ted to the number of breeding pa i rs (Fasola and Barb ie r i 1977). However, I d isagree with Gibbs et al. (1987) assumption that heron breeding habi ta ts in general are near t h e i r car ry ing c a p a c i t y . In my study, three foraging areas used by breeding herons in the past were not used during t h i s study (Chapter 3 ) . Forbes et al. (1985a) found that reproduct ive success d id not vary s i g n i f i c a n t l y with the number of breeding p a i r s in a co lony , and I found no evidence that food was l i m i t i n g at 82 the colony leve l among breeding herons (Chapter 5 ) . At the ind iv idua l l e v e l , brood reduct ion adjusts the s i z e of 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 that the number of s u i t a b l e feeding t e r r i t o r i e s spaces breeding male herons through the habi ta t (see Marion 1989) and that t h e i r choice o f colony s i t e s i s constra ined by how f a r they w i l l f l y to nest . I assume that male t e r r i t o r y holders exclude a l l other males from s e t t l i n g in the habi ta t when t e r r i t o r i e s shr ink to a minimum s i z e (Fretwel l and Lucas 1970). Th is exp la ins the d i s t r i b u t i o n of co lon ies and the r e l a t i o n s h i p between colony s i z e and the area of foraging habi ta t without having to assume that regional populat ions are at t h e i r ca r ry ing capac i ty of t h e i r h a b i t a t . Colony Formation Predators have been suggested to play an important r o l e in the formation or maintenance of nest ing co lon ies of b i rds (Lack 1968, Forbes 1989) although not a l l others agree (see review by Forbes 1989). Egg and n e s t l i n g surv iva l i s of ten used as a measure of the e f fec t i veness of c o l o n i a l nest ing against predators (see reviews by Wittenberger 1981, Per r ins and Birkhead 1983, Wittenberger and Hunt 1985). For example, Vessem and Draulans (1986) ru led out c o l o n i a l nest ing as an an t i -p reda tor adaptation in Gray Herons because egg and ch ick s u r v i v a l d id not d i f f e r s i g n i f i c a n t l y in co lon ies of d i f f e r e n t s i zes and adul ts d id not mob predators . However, c o l o n i a l nest ing might have ar isen p r i m a r i l y to favour the surv iva l of adul ts (Forbes 1989). Herons that nest c lose to one another would increase t h e i r v i g i l a n c e and reduce the chances o f being caught by eagles (see Pul l iam and Caraco 1984). Th is hypothesis p r e d i c t s that heron nests should be more clumped in eagle areas than elsewhere 83 in t h e i r range. Thus, Lack 's (1968) hypothesis needs to be examined fo r herons from the perspect ive of adult s u r v i v a l . Habi tat S e l e c t i o n My second main conclus ion i s that herons in my study have a set of p re fer red forag ing habi ta ts that s h i f t s e a s o n a l l y , as d i c t a t e d by t i d e s and food . Because of a strong seasonal movement of small f i s h e s to inshore waters, shallow t i d a l lagoons, kelp beds and ee lgrass beds are the pre fer red feeding habi ta t f o r breeding female herons. I showed that the number of herons feeding on beaches was s i g n i f i c a n t l y co r re la ted with the number of hours of low t i d e s each month of the year (Chapter 4 ) . Moreover, j u v e n i l e s and l a t e r , post -breeding adul ts vacated beaches f o r marshlands and grasslands when t h e i r estimated energet ic needs could no longer be met on beaches (Chapter 4) . Great Blue Herons might have a f l e x i b l e s o c i a l system, l i k e some other b i rds (see Stacey and Koenig 1990), and behave d i f f e r e n t l y elsewhere in t h e i r range. Time o f Breeding My t h i r d main conclus ion i s that the time of breeding i s es tab l i shed by the seasonal a v a i l a b i l i t y of food to egg- lay ing females and not by the energet ic demands of t h e i r c h i c k s . Egg- lay ing began about 9 days a f te r an estimated energy threshold f o r egg production had been crossed by egg- lay ing females, whereas the greatest energy demands of the growing ch icks occurred about one month a f t e r the food energy a v a i l a b l e to the adul ts had peaked (Chapter 5 ) . The surge in food energy to females feeding in the lagoon occurred in May (Table 5-5) when sea perch a r r i ved in shallow waters. This surplus might be used to recover body cond i t ion and prepare to moult the f l i g h t f e a t h e r s . Herons elsewhere in B r i t i s h Columbia (Simpson 1984) and in 84 C a l i f o r n i a (Brandman 1976) have chicks in the nest when f i s h are abundant. However, mine i s one of the f i r s t s tudies in which food energy a v a i l a b i l i t y , ra ther than food abundance, has been shown to be re la ted to energy demands of egg - lay ing adul ts and growing ch icks (see Daan et al. 1988). My index of food a v a i l a b i l i t y i s more accurate than ind ices of food abundance used in other t iming of breeding s tudies because i t combines estimates of prey abundance with those of foraging time by herons. This i s important because the longest low t i d e forag ing per iods in ear ly June d id not co inc ide with the peaks in abundance o f a l l the major prey s p e c i e s . I assumed that a l l herons fed on the beach cont inuously through each low t i d e . Further s tudies might address how v a r i a t i o n in the amount of time i n d i v i d u a l s spend foraging during low t i d e a f f e c t s t h e i r t iming of breeding. I n t e r - r e l a t e d P atterns o f Ecology & Behaviour Empir ica l evidence from many scat tered sources i n d i c a t e that an i n d i v i d u a l ' s f i t n e s s i s re la ted to the combined e f f e c t s of i t s choice of h a b i t a t , spacing pa t te rns , d i s p e r s a l and foraging e f f i c i e n c y ( C a t t e r a l l et a l . 1989). The d i s t r i b u t i o n and a v a i l a b i l i t y of food resources i s the most important f a c t o r under ly ing the eco log ica l and behavioural r e l a t i o n s h i p s between age -c lasses of Yellow-eyed Juncos (Juncos phaeonotus) s h o r t l y a f te r the breeding season ( S u l l i v a n 1990) and S i l ve reyes (Zosterops lateralis) in winter ( C a t t e r a l l et a l . 1989). I now extend the examination of e c o l o g i c a l and behavioural r e l a t i o n s h i p s throughout the year using age- and s e x - c l a s s e s of 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 J u v e n i l e herons are poor ly su i ted to foraging on beaches in autumn and winter because of t h e i r low foraging e f f i c i e n c y and spend more time than adults hunting voles in grasslands (Chapter 4 ) . The resu l tan t pattern i s one of low food consumption, narrow l i m i t s on t h e i r use of hab i ta ts and high rates of m o r t a l i t y , compared to a d u l t s . J u v e n i l e herons might s p e c i a l i z e at catching small mammals in grasslands in winter and return to beaches the fo l lowing spr ing as y e a r l i n g s to feed on f i s h re turn ing to inshore h a b i t a t s . Age- re la ted s h i f t s in use of habi ta ts and forag ing s p e c i a l i z a t i o n are well documented in other animals (Part r idge and Green 1985). Through the summer and autumn y e a r l i n g s might improve t h e i r feeding s k i l l s so that they can catch the food necessary f o r good winter surv iva l and reproduct ion as 2 - y e a r - o l d s . Adul t females have high foraging e f f i c i e n c y (Chapter 4) which makes them well su i ted to foraging on beaches during the breeding season and into autumn, and in grasslands and marshlands in winter (Chapter 4 ) . As a r e s u l t , adult females have high food consumption, use of a wide range of h a b i t a t s , and low rates o f m o r t a l i t y compared to j u v e n i l e s . Some adul t male herons foraged on t e r r i t o r i e s through the year (Chapter 4) . During the breeding season, some males t r a v e l l e d up to 27 km each night between t e r r i t o r i e s and t h e i r nests (Chapter 4 ) . When nests held large c h i c k s , males foraged with females near the co lony . Post-breeding males returned to t h e i r t e r r i t o r i e s to spend the autumn and winter (Chapter 4 ) . Thus, males were mostly s o l i t a r y feeders on t e r r i t o r i e s . The foraging e f f i c i e n c y of t e r r i t o r i a l males i s unknown. My study concurs with those of S u l l i v a n (1989) and C a t t e r a l l et a l . (1989) that food a v a i l a b i l i t y i s the most important f a c t o r shaping the behaviour of 86 age- and s e x - c l a s s e s . In many s p e c i e s , dominance is an important feature of ind iv idua l f i t n e s s and surv iva l fo r a l l age- and sex c l a s s e s (e .g . Monaghan 1980, B i l d s t e i n 1983, Arcese and Smith 1985, C a t t e r a l l et al. 1989). 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Stage Regression Eggs Y = 45.28 - 0.12X Small ch icks Y = log 2.6 - 0.001X Large ch icks Y = 183.67 - 0.64X F ledg l ings Y = 14.73 - 0.30X C o r r e l a t i o n Extrapolated c o e f f i c i e n t populat ion S.E. - 0.72 349 48 - 0.96 3258 6 - 0.99 287 5 - 0.76 49 6 a - Pholis ornata, Gasterosteus aculeatus, Leptocottus armatus, Signathus griseolineatus. b - does not include mobile f i s h e s , p r i n c i p a l l y Cymatogaster aggregata, a lso eaten by many herons. 99

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