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Reproductive success, food supply, and the evolution of clutch-size in the Glaucous-winged gull Ward, John Gordon 1973

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ci REPRODUCTIVE SUCCESS, FOOD SUPPLY, AND THE EVOLUTION OF CLUTCH-SIZE IN THE GLAUCOUS-EINGED GDLL by JOHN GORDON WARD B.S.A., U n i v e r s i t y of Guelph, 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIRHENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Zoology We accept t h i s t h e s i s as conforming to the req u i r e d standard THE UNIVERSITY OF June, BRITISH 1973 COLUMBIA In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of ZOOLOGY The University of B r i t i s h Columbia Vancouver 8, Canada Date July 30, 197 3 ABSTRACT -Lack (1947) proposed that, in nidicolous birds, clutch-size has evolved to correspond, on average, to the most productive brood size. The limit i s normally set by the maximum number of young the adults can adequately feed to fledging. Recent studies using gulls to test Lack's hypothesis have shown that the most common and most productive clutch-size do not coincide. Recent increases in human refuse may have been a factor in these results. In this study, the Glaucous-winged Gull Larus glancescens was used to test Lack's hypothesis. Experiments were designed to test the possible effects of refuse on the birds' capabilities of raising extra young. Both normal (1-3 chicks) and supernormal (4-6 chicks) broods were set up on both a colony (Mandarte Island) where refuse was used by the gulls and on colonies (Cleland Island and islands (QCI) in the Queen Charlotte Islands, B.C.) where refuse was not used. The results did not support Lack's hypothesis. Chicks grew better on Cleland and QCI, where only natural food was used, than on Mandarte where both refuse and natural food was used. On Cleland, chicks in a l l brood sizes reached an average weight of 1000 g (adult weight) before fledging. On Mandarte, the maximum weight was significantly below 1000 g for most brood sizes._ Numbers of chicks fledged for each brood size increased with increasing brood size on a l l the colonies. Post-fledging survival rates indicated that on Cleland, chick survival was similar for normal and i i i supernormal broods. On Mandarte, chick survival was better for the normal broods than for supernormal broods. The contribution to future breeding populations by individuals from different brood sizes was highest for a brood of six on Cleland, but was highest for a brood of three on Mandarte. The results are contrary to what Lack predicted both because a supernormal brood on Cleland was the most productive and because on Mandarte refuse did not give the birds an advantage over those colonies where refuse was not available. On Cleland, Pacific Sandlance was the predominant food. On Mandarte Pacific Herring was the predominant food. On Mandarte in 1971, I found that refuse formed up to 25 percent of the chick diet even though significantly more time was required by the adults when foraging for refuse as opposed to natural foods. The duration of the average foraging trip increased with chick age but this was due to an increased use of refuse by the adults as the chicks got older. Reasons for the high success on Cleland and QCI are discussed including both the p o s s i b l i l i t y of a recent change in the abundance of sandlance and the possible influence of reproductive effort on adult mortality. A winter study was carried out in south-western British Columbia in order to assess the use made of refuse sites during the winter by the Glaucous^winged Gull. I found that up to 65,000 glaucous-winged gulls wintered in the lower mainland region of British Columbia and that between 70 and 90 percent of these birds were using refuse sites. iv Relatively few birds were using the intertidal zone, possibly because i t is not exposed to any extent during daylight hours. The numbers of gulls in this area are discussed in relation to known information on the total population along the west coast of North America. V TABLE OF CONTENTS Chapter Page I A Introduction 1 B Study Area 8 C Chick Addition Experiment - Rationale ........ 11 II Chick Growth And Survival A Introduction 14 B Growth Rates and Asymptotic Weights .... 14 C Chick Hortality and Fledging Success ......... 33 D Post-fledging Success ........................41 E Summary 49 III Food Utilized By Breeding Gulls A Introduction 51 B Diet of Adult Gulls Prior To Chick Hatching .. 52 C Chick Regurgitations ......................... 54 D Feeding Observations on Mandarte in 1971 ..... 57 E Discussion: Evolution of Clutch-Size ......... 75 F Summary ...................................... 83 IV Use Of Refuse Dumps By The Glaucous-Winged Gull A Introduction 85 B Daily Activity During the winter ............. 86 C Numbers of Gulls and Their Use of Refuse Dumps 90 D Discussion ...................................104 E Summary ...................................... 108 Page LITERATURE CITED .................................109 APPENDICES ...115 / v i i LIST OF TABLES Table Page 1 Numbers of d i f f e r e n t brood s i z e s s et up on the c o l o n i e s i n the d i f f e r e n t years. ............. 15 2 Average age (days) (± SE) at which the asymptotic weight was a t t a i n e d by c h i c k s i n the d i f f e r e n t s i z e d broods. 25 3 Average growth r a t e s (g/day) (±SE) and sample s i z e (n) f o r c h i c k s ranked according to growth r a t e s i n the i n d i v i d u a l broods. ........ 26 4 Average asymptotic weight (grams) (±SE) and sample s i z e (n) f o r c h i c k s ranked by growth r a t e as shown i n Table 3. 28 5 Average growth r a t e s (g/day) (±SE) f o r the d i f f e r e n t brood s i z e s with respect to the breeding season. ................................. 30 6 N e s t l i n g m o r t a l i t y r a t e s f o r Mandarte i n 1969 and Cl e l a n d i n 1970. The number of c h i c k s dying during each age i n t e r v a l i s expressed as a percent of the t o t a l number of c h i c k s at the beginning of the i n t e r v a l . ...... 34 7 Percentage of deaths due to d i f f e r e n t f a c t o r s on Mandarte i n 1969 and Cleland i n 1970. 36 8 Percentage s u r v i v a l r a t e s of f i r s t year b i r d s as r e l a t e d t o brood s i z e . These s u r v i v a l values are based on the number of l i v e s i g h t i n g s of b i r d s away from the colony. .......................................... 42 9 Average asymptotic weight (grams) (±SE) of a l l i n d i v i d u a l s f o r d i f f e r e n t brood s i z e s compared with the average asymptotic weight of those i n d i v i d u a l s seen a l i v e away from the colony f o r Mandarte i n 1969 and 1971. ........ 44 10 T h e o r e t i c a l c o n t r i b u t i o n to fu t u r e breeding populations f o r broods one to s i x on Mandarte i n 1962, 1969, and 1971 and Clel a n d i n 1970. , 45 vxix T a b l e Page 11 P e l l e t s c o l l e c t e d on Mandarte i n 1969 and C l e l a n d i n 1970 c l a s s i f i e d as t o whether garbage o r non-garbage i t e m s . A more complete l i s t i n g o f s p e c i e s o f n a t u r a l food i s found i n appendix 4. 53 12 Average d u r a t i o n of f o r a g i n g absences (±SE) f o r a d u l t s w i t h both normal and supernormal broods. 60 13 Change i n l e n g t h o f average f o r a g i n g t r i p s (minutes) (±SE) w i t h c h i c k age f o r both normal and s u p e r n o r m a l broods on Mandarte i n 1971. 62 14 Average number (±SE) o f f o r a g i n g t r i p s made per day by a d u l t s w i t h r e s p e c t t o the age o f t h e c h i c k s and t h e brood s i z e 65 15 Average d u r a t i o n (minutes) (±SE) o f f o r a g i n g t r i p s f o r d i f f e r e n t f o o d t y p e s w i t h r e p e c t t o brood s i z e on Mandarte i n 1971. ............... 68 16 Roost c o u n t s f o r t h e w i n t e r seasons between 1969 and 1972 f o r t h e l o w e r mainland a r e a o f B.C. 92 17 Numbers of g u l l s counted a t o r near r e f u s e s i t e s 94 18 Comparison o f r o o s t and r e f u s e s i t e c e n s u s e s . These f i g u r e s do not i n c l u d e the numbers o f g u l l s f o r -the r o o s t s and t h e r e f u s e s i t e s on B u r r a r d I n l e t . ................... 97 19 Numbers of b i r d s a l o n g the i n t e r t i d a l c e nsus a r e a s (shown i n f i g . 14) and t h e age c o m p o s i t i o n o f tho s e b i r d s . ...................... 99 20 P e r c e n t a g e age c o m p o s i t i o n o f g u l l s a t t h r e e d i f f e r e n t r e f u s e s i t e s . .....................102 LIST OF FIGURES Figure Page 1 Hap of the west coast of B r i t i s h Columbia, Canada showing the l o c a t i o n of the three d i f f e r e n t s i t e s on which the experiments t e s t i n g Lack's hypothesis were performed. ........ 9 Map showing the l o c a t i o n of the winter study area which included Vancouver, B r i t i s h Columbia and the surrounding m u n i c i p a l i t i e s . .................................. 12 Average growth curve f o r a brood of three c h i c k s on Mandarte, C l e l a n d , and QCI. The s t r a i g h t e s t p o r t i o n of the curves was taken to be between 6 and 26 days. Some of the 5% confidence l i m i t s are put i n to show the amount of v a r i a t i o n a s s o c i a t e d with each curve. ........................................... 17 Graph showing l i n e s obtained when R i c k l e f s * conversion f a c t o r s f o r a l o g i s t i c and Gompertz curve are a p p l i e d to the growth curve f o r a brood of three c h i c k s from -Cleland i n 1970. The conversion l i n e f o r the von B e r t a l a n f f y form i s curved upwards even more than the Gompertz. .....................19 Average growth rates (between 6 and 26 days of age) f o r d i f f e r e n t s i z e d broods on Mandarte, C l e l a n d and QCI. The values f o r the average growth r a t e and t h e i r standard e r r o r s are i n appendix 1 21 Average asymptotic weights of the d i f f e r e n t brood s i z e s on Mandarte and C l e l a n d . The values and standard e r r o r s are i n appendix 2. 23 Percentage f l e d g i n g success f o r the d i f f e r e n t brood s i z e s on Mandarte, C l e l a n d , and QCI 37 8 Absolute numbers of c h i c k s fledged f o r the d i f f e r e n t brood s i z e s on Mandarte and C l e l a n d . 40 X Figure Page 9 Percentage composition of food types found i n chick r e g u r g i t a t i o n s on Mandarte, C l e l a n d , and QCI. ....55 10 Change i n average t o t a l time spent f o r a g i n g each day by a p a i r with respect to the age of the c h i c k s and the va r i o u s brood s i z e s 64 11 Change i n frequency of food types i n the d i e t of the c h i c k s with respect to t h e i r age and brood s i z e (Mandarte, 1971). 'n 1 i s t o t a l on which percentages based. Percentage values i n appendix 3. ................. 67 12 Numbers of fora g i n g t r i p s f o r d i f f e r e n t food types c l a s s i f i e d according to the time of day the a d u l t s returned (Mandarte, 1971). ........ 74 13 Change i n chick s u r v i v a l to breeding (b) with respect to brood s i z e . The curve f o r adul t m o r t a l i t y (g) with respect t o brood s i z e i s a h y p o t h e t i c a l one which would be required i n order f o r a brood of three c h i c k s t o be optimum. (from Charnov and Krebs, i n prep) ..,.80 14 Map of winter study area showing l o c a t i o n of refuse dumps, roost s i t e s , and f l i g h t l i n e s between refuse s i t e s and r o o s t s . Refuse s i t e s : 1 - West Vancouver (closed Oct. 31/69); 2 - North Vancouver; 3 - Barnet; 4 Terra Nova; 5 - Leeder; 6 - Port Mann; 7 Burnaby (closed Oct. 31/69); 8 Richmond; 9 - Vancouver. 87 15 Rate of departure from r o o s t s i n morning and ra t e of a r r i v a l at r o o s t s i n evening f o r Dec 20/68 89 x i ACKNOWLEDGEMENTS I wish to express my g r a t i t u d e to my s u p e r v i s o r , Dr. R. H. Drent, f o r h i s advice and d i s c u s s i o n during the course of t h i s study. Dr. j . R. Krebs acted as supervisor during the w r i t i n g of t h i s t h e s i s and o f f e r e d much h e l p f u l c r i t i c i s m . Dr. C. J . Krebs and Dr. D. McPhail c r i t i c a l l y read the manuscript. F i n a n c i a l a s s i s t a n c e was received i n the form of an NRC and CWS s c h o l a r s h i p to myself and an NRC research grant to R. H. Drent. I would l i k e to thank Mr. I . R. MacGregor f o r the band s i g h t i n g s which g r e a t l y improved part of t h i s study. The as s i s t a n c e by B. Baker, M. Easton, M. Shepherd, C. Shepherd, and K. Summers during the summer months are g r a t e f u l l y acknowledged. The help of Mr. N. Seymour of the Department of F i s h e r i e s i n Tofino and of the Tofino L i f e b o a t S t a t i o n was very much appreciated. Mr. F. Beban and Mr. M. Halbauer k i n d l y o f f e r e d us the use of the f a c i l i t i e s at t h e i r logging camp on Talunkwan I s l a n d i n the Queen C h a r l o t t e I s l a n d s , B.C.. The a u t h o r i t i e s f o r the various refuse s i t e s permitted me f r e e access to the dumping area f o r observing and counting g u l l s . I would a l s o l i k e t o thank the 'McLorys* and the " I n k s t e r s 1 f o r t h e i r kind h o p i t a l i t y during the summers. The East Saanich Indian band k i n d l y rented Mandarte I s l a n d to the Department of Zoology. I g r a t e f u l l y acknowledge the help of the many people who helped at various times with the ' g u l l \ x i i counts•. Last, but not least, I would like to acknowledge the understanding and encouragement given by my wife during the course of this study. 1 CHAPTER I A I n t r o d u c t i o n The study described i n t h i s t h e s i s had two aims. The f i r s t , which was experimental i n approach, involved the t e s t i n g of Lack*s (1947) hypothesis on the e v o l u t i o n of c l u t c h - s i z e i n n i d i c o l o u s b i r d s . The second aim, which was of a more d e s c r i p t i v e nature, d e a l t with the u t i l i z a t i o n of garbage by winter populations of the Glaucous-winged G u l l Larus glaucescens i n south-western B r i t i s h Columbia. In recent years the study of f a c t o r s involved i n the determination of c l u t c h - s i z e i n b i r d s has received much a t t e n t i o n , with the i n i t i a l impetus coming from the e a r l y work of David Lack. Researchers have looked at both the proximate (immediate causal) and u l t i m a t e (evolutionary) f a c t o r s determining c l u t c h - s i z e , e s p e c i a l l y i n the n i d i c o l o u s s p e c i e s . In t h i s study I was concerned with the u l t i m a t e f a c t o r s i n f l u e n c i n g the e v o l u t i o n of c l u t c l t - s i z e i n a n i d i c o l o u s s p e c i e s . Lack (1947) postulated that i n n i d i c o l o u s b i r d s , the most common c l u t c h - s i z e has evolved to correspond with that brood s i z e from which, on average, the most s u r v i v i n g young are produced. The l i m i t to the number of young produced i s normally set by the amount of food the parents can bring to the n e s t l i n g s . Thus Lack considered the u l t i m a t e f a c t o r determining c l u t c h - s i z e to be the a b i l i t y of the a d u l t b i r d s to provide food f o r the young. 2 The most s u c c e s s f u l of the s t u d i e s i n v e s t i g a t i n g Lack's hypothesis have been those which took an experimental approach i n order to see i f the most common c l u t c h - s i z e corresponded to / the most productive one. In the experimental s t u d i e s , broods, which were l a r g e r than the most common one and which as a r u l e r a r e l y occurred n a t u r a l l y , were set up by adding e x t r a n e s t l i n g s to broods j u s t . a f t e r the eggs hatched. Success of these supernormal broods was measured by both how many i n d i v i d u a l s fledged from the supernormal broods and how many of these i n d i v i d u a l s survived to breeding as compared to normal broods. I f more young fledge from supernormal broods than from normal broods, then i t i s necessary to obtain p o s t - f l e d g i n g m o r t a l i t y r a t e s before an adequate d e c i s i o n can be made concerning Lack's hypothesis. Studies on the f o l l o w i n g species have g e n e r a l l y supported Lack's hypothesis:.European S t a r l i n g Sturnus v u l g a r i s (Lack, 1948), Great T i t Parus major (Lack, Gibb, and Owen, 1957; P e r r i n s , 1965), S w i f t AJJUS apus (Lack and Lack, 1952; Lack and Owen, 1955; P e r r i n s , 1964), Pied F l y c a t c h e r F i c e d u l a hypoleuca (Haartman, 1967; Klomp, 1970), Black-faced Dioch Quelga guelea (Ward, 1965), Snow Bunting Plectorophenax n i v a l i s (Hussel, 1972), Laysan A l b a t r o s s Diomedea immutabilis (Rice and Kenyon, 1962), Hanx Shearwater P u f f i n u s p u f f i n u s ( H a r r i s , 1966), Rhinocerus Auklet Cerorhinca mcnocerata (Summers, 1970), Pigeon Guillemot Cgpphus columba (Koelink, 3 1972), Common P u f f i n F r a t e r c u l a a r c t i c a ( N e t t l e s h i p , 1972), Brown Booby Sula leucogaster (Dorward, 1962), and the Bed-footed Booby Sula s u l a (Nelson, 1966). In these species the most common c l u t c h - s i z e corresponded to the maximum number of young that the a d u l t s could adequately feed to f l e d g i n g . In some of these s t u d i e s ( eg. a l b a t r o s s and shearwater s t u d i e s ) the broods d i d not have to be followed beyond f l e d g i n g because the normal broods were obviously producing f a r more i n d i v i d u a l s , and i n b e t t e r weight c o n d i t i o n , than were the supernormal broods. In others ( eg. Great T i t ) the s u r v i v a l of young had to be followed a f t e r f l e d g i n g i n order to e s t a b l i s h that the normal c l u t c h - s i z e s were i n f a c t the most productive ones. Other s t u d i e s d i d not appear to support Lack's hypothesis i n that the most productive c l u t c h - s i z e d i d not correspond to the most common one. Hountford (1968) explained these r e s u l t s i on the b a s i s that the s i z e of c l u t c h produced by a genotype i s not a c c u r a t e l y determined, but w i l l vary amongst i n d i v i d u a l s of the same genotype. Thus the numbers of d i f f e r e n t c l u t c h - s i z e s produced by a genotype w i l l form some frequency d i s t r i b u t i o n . The number of i n d i v i d u a l s produced from each c l u t c h w i l l vary depending on the s i z e of the c l u t c h . As a r e s u l t , the number of o f f s p r i n g produced by a genotype i n any year w i l l then be a product of the numbers of d i f f e r e n t c l u t c h - s i z e s l a i d by that genotype and the f l e d g i n g success as s o c i a t e d with each of the c l u t c h - s i z e s . Mountford suggested that the most productive and most common c l u t c h s i z e may or may not correspond depending on the shape of these two d i s t r i b u t i o n s . However, at present there are no data supporting Mountford's m o d i f i c a t i o n of Lack's hypothesis. Studies on the Pied F l y c a t c h e r (Curio, 1958; Campbell i n Lack, 1966), the C o l l a r e d F l y c a t c h e r F i c e d u l a a l h i c o l l i s ( L o h r l , 1957) and the Heron Ardea c i n e r e a (Owen, 1960) present c i r c u m s t a n t i a l evidence which may support Mountford's i d e a . In the f l y c a t c h e r s t u d i e s the most productive c l u t c h - s i z e was l a r g e r than the most common c l u t c h - s i z e . In the heron study the most productive c l u t c h - s i z e was smaller than the most common. In a l l these s t u d i e s the r e s u l t s included data on the po s t - f l e d g i n g s u r v i v a l of the young. The r e s u l t s of s e v e r a l other s t u d i e s do not appear to support Lack's hypothesis even with the i n c l u s i o n of Mountford's i d e a . In these s t u d i e s the most productive c l u t c h - s i z e was l a r g e r "than any normally found. Nelson (1964) found that the North A t l a n t i c Gannet Sula bassana, which normally l a y s only one egg, could both incubate two eggs and fledge two young. In a d d i t i o n , more young were produced by the experimental broods of two young than by normal broods of one. The d i f f e r e n c e i n f l e d g i n g weight between the two brood s i z e s was s m a l l enough to preclude any d i f f e r e n c e s i n p o s t - f l e d g i n g m o r t a l i t y . Robertson (1971) found that the Double-crested Cormorant Phalacrocorax a u r i t u s could s u c c e s s f u l l y r a i s e up to s i x young. Normally they only r a i s e 5 a maximum of four young. In a d d i t i o n , the p o s t - f l e d g i n g s u r v i v a l was as high f o r c h i c k s from supernormal broods as f o r those from normal broods. Both the Gannet (Nelson, 1966) and the Double-crested Cormorant are at present i n c r e a s i n g i n numbers. In the case of the Gannet, the species i s pr e s e n t l y recovering from e a r l i e r depredations by man. Lack (1966) argued t h a t , because the Gannet i s not pr e s e n t l y i n balance with i t s food supply, they are e a s i l y able to f i n d enough food f o r more than one young. This argument could a l s o apply f o r the Double-crested Cormorant. The r e s u l t s of s e v e r a l s t u d i e s on g u l l s a l s o do not appear to support Lack's hypothesis. Coulson and White (1958) showed that the K i t t i w a k e B i s s a t r i d a c t y l a could r a i s e three young as w e l l as two, but that over 74 percent of the p a i r s l a i d two egg c l u t c h e s . They d i d not, however, have any data on p o s t - f l e d g i n g m o r t a l i t y . H a r r i s and Plumb (1965) showed that the Lesser Black-backed G u l l Larus fuscus could r a i s e more c h i c k s than normal. Again there were no p o s t - f l e d g i n g s u r v i v a l data. Vermeer (1963) found that the Glaucous-winged G u l l could r a i s e up to twice the normal number of c h i c k s with equal success. In a d d i t i o n the p o s t - f l e d g i n g s u r v i v a l was higher f o r the supernormal broods than f o r the normal broods. Lack (1966) argued t h a t these r e s u l t s d id not c o n t r a d i c t h i s hypothesis. In recent years there has been a lar g e increase i n the g u l l ' s food supply, mainly i n the form of human r e f u s e . This added food supply has enabled the b i r d s to feed more 6 young than normal. In the s t u d i e s of g u l l s discussed above, no data were given i n d i c a t i n g whether or not garbage was an important food source. In support of Lack's c r i t i c i s m , Spaans (1971) found that c h i c k s fed garbage i n a d d i t i o n to n a t u r a l food grew b e t t e r than c h i c k s fed only n a t u r a l food. Fordham (1970) and Hunt (1972) found that breeding success was b e t t e r on those i s l a n d s c l o s e to refuse sources. As can be seen from these s t u d i e s more in f o r m a t i o n i s needed, both on the u t i l i z a t i o n of garbage by g u l l s used to t e s t Lack's hypothesis, and on the p o s t - f l e d g i n g m o r t a l i t y of c h i c k s from d i f f e r e n t s i z e d broods. In the f i r s t part of t h i s i n v e s t i g a t i o n , the aim was to t e s t Lack's hypothesis i n a g u l l species i n order to r u l e out the a f f e c t of garbage i n the success of parents r a i s i n g a d d i t i o n a l c h i c k s . My study enlarged on Vermeer's work i n order to determine, (1) how important garbage i s as a food source i n feeding the c h i c k s , (2) whether the g u l l s could r a i s e e x t r a c h i c k s where Only n a t u r a l food was a v a i l a b l e and/or used, (3) whether the Glaucous-winged G u l l could r a i s e e x t r a c h i c k s i n more than one year and p l a c e , (*l) how w e l l the c h i c k s grew p r i o r to fledging,: and how w e l l they survived a f t e r f l e d g i n g . Recent i n c r e a s e s i n the amount of garbage discarded by man, besides i n f l u e n c i n g the success of brood manipulation experiments, may a l s o be an important f a c t o r i n the recent 7 increases of g u l l populations (Brown,; 1967; H a r r i s , 1972). The p r o v i s i o n of copious q u a n t i t i e s of refuse by man may have permitted more b i r d s t o s u r v i v e the winter than was p o s s i b l e i n the past. In recent years, s e v e r a l people have studied the use made of human refuse by g u l l s during the winter months. The more extensive among these s t u d i e s i n c l u d e the work on the Herring G u l l Larus argentatus by Drury (1963) on the New England coast of the U.S.A. and Spaans (1971) i n the Netherlands, and the work on the Dominican G u l l Larus dominicus by Fordham (1968 and 1970) i n New Zealand. Spaans showed that depending on the fo r a g i n g c o n d i t i o n s on the Wadden Sea, between 32 and 77 percent of the h e r r i n g g u l l s i n that region fed on the refuse s i t e s during the winter. On the east coast of the U.S.A. up to 70 percent of the h e r r i n g g u l l population used the refuse s i t e s . In New Zealand, up to 50 percent of the g u l l s were at or near refuse s i t e s and meatworks during the winter. In a l l of the above s t u d i e s the g u l l s d i d use n a t u r a l food sources, however, Spaans and Drury found that winter storms caused g u l l s which normally fed on the i n t e r t i d a l to switch to refuse s i t e s . Along the west coast of B r i t i s h Columbia and Alaska, g u l l s feeding on i n t e r t i d a l areas have the added disadvantage of the extreme low t i d e s g e n e r a l l y o c c u r r i n g at n i g h t . The advent of refuse s i t e s has provided a very a t t r a c t i v e feeding area f o r the Glaucous-winged G u l l and perhaps has co n t r i b u t e d t o t h e i r recent increase i n numbers. The aim of the second 8 part of t h i s study was to i n v e s t i g a t e t h i s use of refuse dumps by the Glaucous-winged G u l l along the west coast and s p e c i f i c a l l y i n the lower mainland region of B r i t i s h Columbia. B Study Area A large part of the study on the c l u t c h - s i z e question i n the Glaucous-winged G u l l was c a r r i e d out during the summer months (1969 - 1972) on t h e i r breeding grounds. In order to t e s t Lack's hypothesis as r i g o r o u s l y as p o s s i b l e , the study was c a r r i e d out on two d i f f e r e n t types of g u l l c o l o n i e s f o r more than one year. The f i r s t type included those c o l o n i e s r e l a t i v e l y c l o s e to lar g e garbage sources. Here i t was expected t h a t the g u l l s would use refuse to feed t h e i r young. The other type included those c o l o n i e s as f a r removed as p o s s i b l e from refuse sources. In t h i s case i t was hoped that g u l l s would only feed n a t u r a l foods to t h e i r young. The colony on Mandarte I s l a n d ( l a t . 48° 38' N, long. 123° 17* W) ( f i g . 1) was s e l e c t e d to f u l f i l the requirements of the f i r s t type of colony f o r three reasons. (1) I t i s a lar g e colony with over two thousand breeding p a i r s . Although the colony at present appears to be f u l l , the reproductive r a t e s do not d i f f e r from those o c c u r r i n g when the colony was expanding. (2) I could compare my data with t h a t of Vermeer who used the same colony. (3) I t was r e l a t i v e l y c l o s e to sources of r e f u s e . The i s l a n d , f u l l y described by Drent et 9 Figure 1: Map of the west coast of B r i t i s h Columbia, Canada showing the l o c a t i o n of the three d i f f e r e n t s i t e s on which the experiments t e s t i n g Lack's hypothesis were performed. 10 al - . (1964), i s lo c a t e d 4 1/2 miles ESE of Sidney, B r i t i s h Columbia, and only 30 miles from the Vancouver garbage dump. The colony on Cleland I s l a n d ( l a t . 49° 10* N, long. 126° 06• W) ( f i g . 1) was s e l e c t e d to f u l f i l the requirements of the second type of colony. The i s l a n d , described by Campbell and S t e r l i n g (1967), i s lo c a t e d eight miles WNB of Tofino on the west coast of Vancouver I s l a n d . again t h i s colony i s l a r g e with more than 1500 breeding p a i r s of g u l l s . No informat i o n i s a v a i l a b l e as to whether i t i s expanding. Although the colony i s not e n t i r e l y removed from p o t e n t i a l garbage sources such as the salmon f i s h i n g i n d u s t r y , i t i s the best that can be obtained anywhere along the west coast and s t i l l be r e a d i l y a c c e s s i b l e . The nearby T o f i n o garbage dump i s very s m a l l and seldom used by g u l l s during the summer (Campbell, personal communication). In the summmer of 1972, the study was c a r r i e d out on the northern part of the B r i t i s h Columbia coast. In t h i s region the c o l o n i e s are g e n e r a l l y small ( <100 breeding pairs) and i f expanding appear to be doing so at a very slow r a t e . Three s m a l l c o l o n i e s ( l a t . 52° 55» N, long 131° 34» H) ( f i g . 1) were s e l e c t e d i n the Queen C h a r l o t t e I s l a n d s , B r i t i s h Columbia, and are l o c a t e d approximately 25 miles SSE of Sandspit. The c o l o n i e s are lo c a t e d on three s m a l l rocky i s l e t s ( c o l l e c t i v e l y abbreviated to QCI i n the text) and together contained a breeding population of only 135 p a i r s . 11 These c o l o n i e s a l s o f u l f i l l e d the requirements of being f a r removed from a l l garbage sources except the f i s h i n g i n d u s t r y . The second part of the study, as mentioned i n the i n t r o d u c t i o n , was c a r r i e d out during the winter months (1968 -1972) i n the lower mainland region of B r i t i s h Columbia. The boundaries of the study area, as shown i n f i g . 2, incorporated the c i t y of Vancouver and a number of the surrounding m u n i c i p a l i t i e s . This study area only comprises a sm a l l part of the winteri n g range of the Glaucous-winged G u l l . Some info r m a t i o n i s a v a i l a b l e from other s t u d i e s done along the west coast of B r i t i s h Columbia and Alaska and w i l l be incorporated i n the d i s c u s s i o n i n chapter f o u r . , C Chick A d d i t i o n Experiments - Rationale In many of the b i r d s p e cies used to t e s t Lack*s hypothesis, the broods have been formed by adding c h i c k s at the time of hatching. This procedure has avoided problems in v o l v e d i n o b t a i n i n g f e r t i l e eggs i n s i m i l a r stages of in c u b a t i o n . The c h i c k a d d i t i o n s have been done with the knowledge that the parent b i r d s could have e f f e c t i v e l y incubated that many eggs. Various s t u d i e s on passerines have confirmed that c l u t c h - s i z e was not determined by the number of eggs the b i r d could e f f e c t i v e l y incubate (studies l i s t e d i n Klomp, 1970). The Gannet, u n l i k e the passerine b i r d s which have one l a r g e brood patch, uses i t s f e e t i n order to incubate 12 Figure 2: Map showing the location of the winter study area which included Vancouver, British Columbia and the surrounding municipalities. 13 i t s s i n g l e egg. Helson (1964) showed that even with t h i s method of i n c u b a t i o n the b i r d could s t i l l e f f e c t i v e l y incubate two eggs. The above type of assumption i s not p o s s i b l e with the Glaucous-winged G u l l or any of the other Larus g u l l s having a c l u t c h of three eggs. These b i r d s have three i n d i v i d u a l brood patches which permit e f f e c t i v e i n c u b a t i o n of only three eggs at any one time. The a d d i t i o n of e x t r a eggs u s u a l l y r e s u l t s i n some or a l l of the eggs being unincubated f o r varying i n t e r v a l s of time. The f i n a l outcome i s a g r e a t l y lowered hatching success f o r supernormal c l u t c h e s r e l a t i v e to normal s i z e d ones (Vermeer, 1963;' Parsons, 1971). fl somewhat d i f f e r e n t assumption then has t o be made i n the a d d i t i o n of e x t r a c h i c k s i n these species. For the purposes of t h i s study, i t was assumed that the present brood patch arrangement i s not i n i t s e l f an u l t i m a t e f a c t o r determining c l u t c h - s i z e , but has evolved to correspond to the three egg c l u t c h . I f i t were advantageous to have a l a r g e r c l u t c h , a l a r g e r brood patch area could have evolved. 14 CHAPTER I I Chick Growth And S u r v i v a l A I n t r o d u c t i o n The p o s s i b i l i t y e x i s t s that garbage has formed part of the ch i c k d i e t f o r those s t u d i e s t e s t i n g Lack's hypothesis using g u l l s . This food source may have had a considerable i n f l u e n c e orj the success of these experiments. In t h i s chapter, I describe the r e s u l t s of experiments designed to t e s t the success of the Glaucous-winged G u l l i n r a i s i n g normal and experimentally enlarged broods i n two d i f f e r e n t types of s i t u a t i o n s . I n one case only n a t u r a l food was found i n the c h i c k s * d i e t v In the other, garbage made up an appreciable part of the d i e t . The c r i t e r i a used to measure success i n c l u d e d growth r a t e s , maximum weight a t t a i n e d ( r e f e r r e d to as asymptotic weight), m o r t a l i t y r a t e s p r i o r to f l e d g i n g , f l e d g i n g success, and p o s t - f l e d g i n g s u r v i v a l . B Growth Rates and Asymptotic Heights Methods: Large numbers of d i f f e r e n t brood s i z e s ranging from one to s i x c h i c k s were set up during t h i s study (Table 1). VermeerJs data (personal communication) from Mandarte Is l a n d showed t h a t a l a r g e percentage of the l a r g e r broods l o s t at l e a s t one c h i c k through m o r t a l i t y before the c h i c k s fledged. As a r e s u l t a l a r g e number of broods were reguired 15 Table 1 Numbers of Different Brood Sizes Set Up on The Colonies in the Different Years. Brood Size Location' J and Year 1 2 3 4 5 6 Mandarte 1969 63 56 48 50 36 53 Cleland 1969 12+ 16+ 18+ 14+ 17+ 16+ Cleland 1970 152 94 101 52 37 40 Mandarte 1971 153 80 58 40 34 38 QCI 1972 31 18 30 5 14 i i +• These values are the numbers of broods for which weights were available. The actual number set up was larger. 16 i n order that I would be able to weigh complete broods i n the o l d e r age c l a s s e s . I defined a complete brood as one co n t a i n i n g the same number of c h i c k s as I had i n i t i a l l y set i t up with. i n a d d i t i o n , the lar g e sample f o r each brood s i z e provided data f o r determining f l e d g i n g success. Broods were formed e i t h e r by removing or adding c h i c k s three days or l e s s o l d , or by l e a v i n g the broods as they hatched. Chicks were added to a nest w i t h i n a day a f t e r the l a s t egg i n that p a r t i c u l a r nest had hatched. The adu l t b i r d s d i d not appear to d i s t i n g u i s h between t h e i r own and strange c h i c k s , ftlso Tinbergen (1953) found that a d u l t h e r r i n g g u l l s d i d not d i f f e r e n t i a t e between t h e i r own and strange c h i c k s u n t i l t h e i r own c h i c k s were over four or f i v e days of age. On Mandarte i n 1969 and on Cleland i n 1969 and 1970, c h i c k s were weighed from hatching u n t i l f l e d g i n g . On Mandarte i n 1971, c h i c k s were only weighed from 30 days of age to fledging.: T h i s provided asymptotic weights without the disturbance caused by handling young c h i c k s i n order to obtain growth r a t e s . On QCI i n 1972 only weights of c h i c k s between hatching and 26 days of age were obtained. In a l l years c h i c k s were weighed at two day i n t e r v a l s when p o s s i b l e . The general shape of the growth curve obtained when weight was p l o t t e d against age i s shown i n f i g . 3. R i c k l e f s (1967) presented a method f o r converting t h i s sigmoid-shaped growth curve i n t o a s t r a i g h t l i n e . The slope of t h i s l i n e was 17 5 10 15 20 25 30 35 40 AGE {days) Fig. 3: Average growth curve for a brood of three chicks on Mandarte, Cleland, and QCI. The straightest portion of the curves was taken to be between 6 and 26 days. Some of the 5% confidence limits are put in to show the amount of variation associated with each curve. 18 a measure of ithe rate of growth at the i n f l e c t i o n point of the growth curve. In h i s technique, B i c k l e f s assumed that the growth curve iwas approximated by one of three general curves: the l o g i s t i c , Gompertz, or von B e r t a l a n f f y . As i l l u s t r a t e d i n f i g . 1, none of B i c k l e f s * s conversion t a b l e s f o r these forms were s u i t a b l e f o r the growth curves i n t h i s study. Both the l o g i s t i c and Gompertz conversion l i n e s are curved through much of t h e i r l e n g t h . This was B i c k l e f s c r i t e r i a f o r r e j e c t i n g any one of these forms as one approximating the shape of the growth curve i n question. Because of the complex nature of B i c k l e f s method, and i t s poor f i t , the method used by Spaans (1971) was used t o c a l c u l a t e the growth r a t e s i n t h i s study. This method was found to be simple to use and s t i l l produced s i m i l a r r e s u l t s . A l i n e a r r e g r e s s i o n of weight on age was c a l c u l a t e d f o r each brood s i z e using the weights of c h i c k s between 6 and 26 days of age. I t was during t h i s age i n t e r v a l that the s t r a i g h t e s t p o r t i o n of the growth curve occurred f o r a l l brood s i z e s . This method a l s o approximates the growth r a t e at the i n f l e c t i o n point and l i k e B i c k l e f s * method permits a comparison of growth r a t e s between brood s i z e s . This method has the added advantage of having the growth r a t e s i n the same u n i t s as the weights of the b i r d s . D i f f e r e n t growth r a t e s do not n e c e s s a r i l y imply d i f f e r e n t asymptotic weights as w i l l be shown i n the r e s u l t s below. I determined the mean asymptotic weight f o r each brood s i z e by 19 5 10 15 20 25 30 35 Chick Age (days) Fig. 4: Graph showing lines obtained vhen Ricklef's conversion factors for a logistic and Gompertz curve, are applied to the growth curve for a brood of three chicks from Cleland in 1970. The conversion line for the von Bertalanffy form is curved upwards even more than the Gompertz. 20 averaging the maximum weights a t t a i n e d by i n d i v i d u a l s i n that brood s i z e . Chicks were not used i f the l a s t weight obtained f o r ' them was higher than a l l previous weights. Although the chick may have reached an asymptote, the p o s s i b i l i t y remained that the b i r d was s t i L l growing. Asymptotic weight was u s u a l l y very s i m i l a r to the f l e d g i n g weight i n t h i s study and was used as a measure of c h i c k c o n d i t i o n at f l e d g i n g . Together the above two measures served as good comparative i n d i c a t o r s of how w e l l the a d u l t s were able t o feed c h i c k s i n d i f f e r e n t s i z e d broods on the d i f f e r e n t i s l a n d s . R e s u l t s : Growth r a t e s v a r i e d depending on the colony, brood s i z e , and year ( f i g . 5 ) . The growth r a t e s f o r Mandarte (1961) were c a l c u l a t e d from data i n Vermeer (1963). An a n a l y s i s of covariance was done to t e s t f o r d i f f e r e n c e s i n growth r a t e s between brood s i z e s f o r each of the i s l a n d - y e a r s ( i e . C l e l a n d i n 1970, e t c . ) . Ho s i g n i f i c a n t d i f f e r e n c e s were found. This was p a r t l y due to the larg e amount of variance associated with the growth r a t e f o r each brood s i z e . On Mandarte i n 1961 6 1969 and Cleland i n 1970 there was a trend f o r growth r a t e s to decrease with i n c r e a s i n g brood s i z e . In the case of Mandarte i n 1969 t h i s trend appeared to have some b i o l o g i c a l s i g n i f i c a n c e as i n d i c a t e d by the asymptotic weights presented below. A one-way a n a l y s i s of variance of the d i f f e r e n t growth r a t e s with cespect to the i s l a n d - y e a r showed s i g n i f i c a n t 21 i ~ 1 T r ~ i r 1 2 3 4 5 6 Brood S i z e Fig. 5: Average growth rates (between 6 and 26 days of age) for different sized broods on Mandarte, Cleland, and OCI. The values for the average grovjth rate and their standard errors are in appendix 1 . 22 d i f f e r e n c e s ( P < 0.01 ). As shown i n f i g . 5, Mandarte c h i c k s always had the poorer growth r a t e s . For those i s l a n d - y e a r s when growth was considered good (Cleland and QCI i n a l l y e a r s ) , the two chick broods appeared to grow f a s t e r than the one c h i c k broods. This d i f f e r e n c e , although i t was not s i g n i f i c a n t s t a t i s t i c a l l y , may have some b i o l o g i c a l meaning as discussed below. The average asymptotic weights of c h i c k s i n the d i f f e r e n t brood s i z e s f o r the d i f f e r e n t i s l a n d - y e a r s are shown i n f i g . 6. In both years on C l e l a n d , c h i c k s a t t a i n e d s i m i l a r asymptotic weights f o r both normal and supernormal Brood s i z e s . This weight was egual t o the average ad u l t weight of * 1000 g obtained from a sample of 50 a d u l t p a i r s weighed i n 1969 on Mandarte. Thus i n s p i t e of the trend f o r a s l i g h t l y slower growth ra t e i n the l a r g e r broods on Cleland i n 1970, a s i m i l a r average asymptotic weight was a t t a i n e d i n a l l the brood s i z e s . The few observations I had on the QCI c h i c k s i n 1972 i n d i c a t e d that the pattern of asymptotic weight with respect to brood s i z e would be s i m i l a r to that on C l e l a n d . A one-way a n a l y s i s of variance of asymptotic weights of c h i c k s f o r each of the d i f f e r e n t i s l a n d - y e a r s showed s i g n i f i c a n t d i f f e r e n c e s ( P < 0.01 ) between broods on Mandarte and on C l e l a n d , . In c o n t r a s t to Cleland the asymptotic weights of c h i c k s on Mandarte appeared to decrease with i n c r e a s i n g brood s i z e i n both years. These d i f f e r e n c e s 23 Cleland (1969) A n o r m a l b r o o d s • s u p e r n o r m a l b r o o d s 1 0 1 Cleland (1970) 91 8 r 2 .Mandarte (1969) Mandarte (1971) 3 4 Brood Size 5 T 6 F i g . 6: Average asymptotic v;eights of the d i f f e r e n t brood s i z e s on ? 1andarte and C l e l a n d . The values and standard e r r o r s are i n appendix 2. 24 were s i g n i f i c a n t (one-way a n a l y s i s of v a r i a n c e ; P < 0.01) i n 1969 but not i n 1971. On Mandarte i n 1969 one and p o s s i b l y two chick broods appeared to approach a maximum weight of 1000 g. This was not the case i n 1971 when the average asymptotic weights of a l l brood s i z e s were w e l l below the average a d u l t weight. Chicks appeared to reach an asymptotic weight (Table 2) at about 37 days of age. A one-way a n a l y s i s of variance of these ages between i s l a n d - y e a r s showed a s i g n i f i c a n t d i f f e r e n c e ( P < 0.01 ) between Mandarte i n 1969 and the other two years. , There were a l s o some d i f f e r e n c e s between brood s i z e s f o r each of the i s l a n d - y e a r s . Generally the d i f f e r e n c e s were no more than three or f o u r days. The growth r a t e s given i n f i g . 5 do not show the p a t t e r n of growth r a t e s f o r i n d i v i d u a l s i n a brood. T h i s p a t t e r n w i l l give some i n d i c a t i o n of how a shortage of food during the chick stage w i l l i n f l u e n c e c h i c k growth and most l i k e l y s u r v i v a l w i t h i n a brood. The growth r a t e s f o r i n d i v i d u a l s w i t h i n a brood were ranked from f a s t e s t to slowest. The d i f f e r e n t ranks were then averaged f o r each brood s i z e to obtain the data shown i n Table 3. The data show that i n the l a r g e r broods there was a considerable d i f f e r e n c e i n growth r a t e s between the c h i c k ranked f i r s t and the one ranked l a s t . On C l e l a n d i n 1970 the growth r a t e of the f a s t e s t growing c h i c k s i n brood s i z e s two to s i x appeared to be somewhat 25 Table 2 Average Age (days) (± SE) a t Which the Asymptotic Weight Was A t t a i n e d by Chicks i n the D i f f e r e n t S i z e d Broods. Brood S i z e I s l a n d 8 Average year 1 2 3 4 5 6 Age Mandarte 36.1 36.9 36.5 35.2 30.7 33.0 35.0 1969 ±0.74 ±0.62 ±0.84 ±1.38 ±0.79 ±1.13 ±0.40 l (19) (29) (26) (14) (24) ( 8 ) (120) Mandarte 38.5 36.4 36.5 38.2 40.7 37.6 1971 ±0.72 ±0.71 ±0.52 ±1.23 ±1.39 ±0.37 (22) (27) (24) (11) (9) (93) C l e l a n d 36.7 35.8 36.5 37.7 37.5 37.6 36.8 1970 ±0.38 ±0.36 ±0.39 ±0.43 ±0.55 ±0.52 ±0.24 (42) (53) (72) (65) (40) (58) (330) i (n) Number i n bracket i s sample s i z e , Table 3 Average Growth Rates (g/day) (±SE) and Sample Size (n) For Chicks Ranked According to Growth Rates in the Individual Broods. Brood Size Rank in « J brood 1 2 3 4 5 6 Cleland 1970 n = (63) (37) (29) (19) (14) (10) 1 33.-4 ±0.64 36.5 ±0.70 36.9 ±0.59 35.6 ±0.82 34:9 ±1.38 35.2 ±2.24 2 31.9 ±0.66 33.7 ±0.70 31.7 ±1.00 32.4 ±1.42 32.2 ±1.83 3 29.5 ±0.90 29.0 ±0.90 31.0 ±1. 30 29. 1 ±1.85 4 24.9 ±1.33 27.7 ±1.01 26.8 ±2.87 5 25.7 ±1.01 25.3 ±1.72 6 Mandarte 1969 22.5 ±1.98 n = (33) (27) (15) (14) (10) («»> 1 32.2 ±0.99 33.6 ±0.77 33.5 ±2.65 32.3 ±0.99 3 0.5 ±1.96 32. 1 ±2. 13 2 28.2 ±0.84 27.3 ±1.49 28.9 ±0.82 27.5 ±1.94 29.3 ±1.31 3 24.0 ±1.47 26.4 ±0.92 25. 1 ±1.64 27.2 ±0.91 4 23.4 ±1.04 23.3 ±1.80 25.7 ±1.27 5 20. 1 ±2.07 21.0 ±2. 11 6 17.5 ±1.47 27 f a s t e r than that f o r a s i n g l e c h i c k . Except f o r brood s i z e s two and three ( P < 0.01 ) , the d i f f e r e n c e s were not s i g n i f i c a n t s t a t i s t i c a l l y ( t - T e s t ) . on Mandarte i n 1969 t h i s trend d i d not appear. On both Mandarte and Cleland those i n d i v i d u a l s ranked f i r s t f o r growth rate a l s o had the highest average asymptotic weight (Table 4 ) . This was s i m i l a r f o r the other ranks i n growth r a t e s . Some of the d i f f e r e n c e s i n i n d i v i d u a l growth r a t e s w i t h i n a brood w i l l be due to competition f o r the a v a i l a b l e food. However, a part of t h i s was most l i k e l y due to the d i f f e r e n c e s i n sex of i n d i v i d u a l s w i t h i n a brood. In t h i s species the males are heavier than the females and could be expected to have a f a s t e r growth r a t e . Snow (1960) showed t h i s to be the case i n the Shag Phalacrocorax a r i s t o t e l i s . S u f f i c i e n t data were a l s o a v a i l a b l e f o r analyzing growth ra t e s on Mandarte i n 1969 and Cle l a n d i n 1970 according to whether broods were e a r l y or l a t e . Broods were placed i n t o one of the f o l l o w i n g three c a t e g o r i e s . The f i r s t , c a l l e d e a r l y broods, included those broods with parents which were among the f i r s t ten percent of a l l p a i r s to l a y eggs i n the study area. The second, c a l l e d mid-early broods, i n c l u d e d those broods with parents among the f i r s t 10 to 50 percent. The t h i r d , c a l l e d mid-late broods, had parents w i t h i n the 50 to 90 percent range. Ho data were c o l l e c t e d f o r broods with parents f a l l - l a g i n the l a s t 10 percent category. 28 Table 4 Average Asymptotic Weight (grams) (±SE) and Sample S i z e (n) f o r Chicks Banked by Growth Bate As Shown i n Table 3. Brood S i z e Bank i n • brood 1 2 3 4 5 6 Clel a n d 1970 n = (34) (19) (23) (15) (5) CO 1 977 1037 1053 1092 1105 1149 ±19,4 ±24.0 ±21.7 ±29.6 ±22.8 ±37.6 2 969 989 1040 1033 1168 ±25.7 ±21.1 ±30.0 ±55.3 ±26.6 3 932 927 1019 1070 ±24.9 ±19.8 ±22.2 ±60. 3 4 890 1020 990 ±24.0 ±35.6 ±49. 7 5 899 945 ±15.7 ±37.8 6 895 ±40. 7 Mandarte 1969 n = (18) (15) (7) * CO 1 958 988 936 837 ±34.2 ±26. 1 ±27.3 ±71.5 2 887 882 820 ±27.9 ±28.0 ±95. 1 3 865 749 ±38.8 ±63.9 4 703 ±39.0 5 668 ±67.8 I n s u f f i c i e n t data were a v a i l a b l e f o r t h i s brcod s i z e • 29 Growth r a t e s f o r the d i f f e r e n t brood s i z e s with respect to the time of the breeding season are shown i n Table 5 f o r Mandarte i n 1969 and Cleland i n 1970- An a n a l y s i s of covariance, t e s t i n g f o r d i f f e r e n c e s i n slopes with respect to time of season f o r each of the d i f f e r e n t brood s i z e s , showed no s i g n i f i c a n t d i f f e r e n c e s . On Mandarte i n 1969 the trend was f o r growth r a t e s to change as the season progressed, but the trend was not c o n s i s t e n t f o r a l l brood s i z e s . D i s c u s s i o n : I n i t i a l l y I had expected that the c h i c k s on Mandarte would have f a s t e r growth r a t e s than those on C l e l a n d , s i n c e the colony was c l o s e to sources of refu s e . The growth r e s u l t s i n t h i s study did not support t h i s hypothesis. The growth r a t e s and asymptotic weights on Mandarte were l e s s than those found on Cleland i n a l l years studied. Thus garbage, i f used, d i d not appear to be a good food source f o r the Mandarte b i r d s . This was contrary to what Spaans (1971) found f o r the h e r r i n g g u l l i n the Netherlands. Studies by Fordham (1964) i n New Zealand and by Hunt (1972) on the east coast of the U.S.A. al s o i n d i c a t e d b e t t e r breeding success on those c o l o n i e s located c l o s e r to refuse dumps. The growth of c h i c k s on Mandarte i n 1962 (Vermeer's study) may have been more l i k e that of c h i c k s on Cleland and QCI as suggested by Vermeer's p o s t - f l e d g i n g d a t a ; however, no weight data were c o l l e c t e d to bear t h i s out. The decrease i n asymptotic weight with i n c r e a s i n g brood s i z e on Mandarte suggests that these b i r d s 30 Table 5 average Growth Hates (g/day) (±SE) f o r The D i f f e r e n t Brood S i z e s with Respect To the Breeding Season. ** L_ ; J Brood S i z e E a r l y * * Mid-early** M i d - l a t e * * Mandarte 1969 1 29.6 ± 0.98 32.9 ± 1. 13 33. 1 ± 1.52 2 30.9 + 0.84 32.5 ± 0.59 22.8 ± 2. 64 3 30. 1 ± 0.88 26.7 ± 0.94 24.9 ± 3.66 4 25.0 ± 1.45 29.4 ± 0.79 28.8 ± 0.98 5 29. 4 ± 0.93 26.8 ± 0.74 26.6 ± 1.27 6 28.2 ± 3.32 27.3 ± 0.86 * C l e l a n d 1970 1 29.7 1.91 33.3 ± 0.60 33.8 ± 0.86 2 34.9 ± 1.30 34.7 ± 0.53 33.9 ± 0.64 3 33.7 ± 1. 10 33. 3 ± 0. 47 32.7 ± 0.89 4 32.5 ± 0.97 30.6 ± 0.74 32.6 ± 1. 10 5 30.9 ± 1. 18 30.3 ± 0.70 35.8 ± 1.47 6 32.4 ± 1. 03 30.5 ± 0.68 34.8 ± 1.45 * No data a v a i l a b l e f o r t h i s p o i n t . ** Explanation of e a r l y , mid-early, and mid-late on page 27. 31 even have t r o u b l e adequately feeding three c h i c k s . H a r r i s (1964) obtained r e s u l t s f o r the h e r r i n g g u l l s i m i l a r to those on Mandarte. The h e r r i n g g u l l c h i c k s showed a decreasing asymptotic weight with i n c r e a s i n g brood s i z e f o r broods of one to three c h i c k s . However, H a r r i s concluded that food was not l i m i t i n g f o r three reasons. (1) S i n g l e c h i c k s should have grown f a s t e r than c h i c k s i n a brood of three i f food was l i m i t i n g . H a r r i s d i d not t h i n k that t h i s was o c c u r r i n g . (2) Larns fuscus (average a d u l t weight 830 g) c h i c k s being r a i s e d by Larus marinus a d u l t s - (average weight 1800 q) should have received more food than normal and developed f a s t e r . This was not the case i n H a r r i s * study. (3) Lar us marinus c h i c k s , f o s t e r e d by L._ Arqentatus and L.-Fuscus parents, should not r e c e i v e as much food as normal and grow slower. H a r r i s * data i n d i c a t e d that they d i d not grow slower, supporting h i s argument. In t h i s study I found that on Mandarte s i n g l e c h i c k s d i d grow f a s t e r than c h i c k s i n a brood of three although I could not show a s t a t i s t i c a l d i f f e r e n c e . In terms of H a r r i s * f i r s t reason given above, food appeared to be l i m i t i n g on Mandatte i n 1969. This was most l i k e l y the case i n 1971 as w e l l . On Cleland the data show that the a d u l t s are able tp adequately feed broods up to double the normal s i z e . A l l the brood s i z e s reached the same asymptotic weight at about the same age. In a d d i t i o n , the a d u l t s accomplished t h i s r e s u l t 32 using only n a t u r a l food (as i s shown i n the next c h a p t e r ) . The good c o n d i t i o n s on Cleland a p p l i e d f o r two successive years (1969 and 1970) i n t h i s study. Information f o r Cleland i n 1971 (Henderson, personal communication) i n d i c a t e d that growth r a t e s f o r a t h i r d successive year were a l s o e x c e l l e n t . Thus w i t h i n the l i m i t s of t h i s study, i t appears that on the average growth r a t e s are very good on Cleland f o r both normal and supernormal broods. 0r» Handarte they are not even adequate f o r a brood of three. The growth r e s u l t s f o r QCI i n 1972, some 300 miles north of C l e l a n d I s l a n d , suggest that the c o n d i t i o n s found on Cleland may be common along much of the outer coast of B r i t i s h Columbia. In three d i f f e r e n t years (Cleland i n 1969 and 1970, and QCI i n 1972) two chick broods appeared to grow f a s t e r than one, although t h e i r asymptotic weights were not d i f f e r e n t . H a r r i s (1964) obtained s i m i l a r r e s u l t s f o r the Lesser Black-backed G u l l as did Fordham (1964) f o r the Dominican G u l l . Henderson (1972) showed that as brood s i z e increased, the t o t a l e f f o r t reguired by c h i c k s w i t h i n a brood i n order to e l i c i t feeding by the parent remained the same. Thus as brood s i z e i n c r e a s e s , the e f f o r t r e q u i r e d by any one chick i n the brood i n order to be f e d decreases (assuming a l l c h i c k s begging at the same time). Since the e f f o r t expended by an i n d i v i d u a l c hick i n order f o r i t to be fed was dependent on i t s hunger l e v e l , then c h i c k s i n a brood of two would be fed at a lower hunger l e v e l than s i n g l e c h i c k s . Thus c h i c k s i n a 33 two-brood can t h e o r e t i c a l l y be fed more o f t e n . The net r e s u l t of t h i s would be a f a s t e r growth r a t e . C Chick M o r t a l i t y and Fledging Success Methods: The ra t e of n e s t l i n g m o r t a l i t y f o r d i f f e r e n t aged c h i c k s i n normal and supernormal broods was c a l c u l a t e d using those broods f o r which complete data on the age of ch i c k deaths were obtained. This excluded a number of broods f o r which only data on number of c h i c k s f l e d g i n g were a v a i l a b l e . Fledging success f o r the d i f f e r e n t brood s i z e s was c a l c u l a t e d by expressing the t o t a l number f l e d g i n g from each brcod s i z e as a percentage of the t o t a l number of c h i c k s o r i g i n a l l y i n that sizie. The log l i k e l i h o o d r a t i o t e s t (G-test described by Sokal and Rohslf, 1969) was used to t e s t f o r d i f f e r e n c e s i n the data f o r both iprefledging m o r t a l i t y and f l e d g i n g success. R e s u l t s : D e t a i l e d n e s t l i n g m o r t a l i t y data were only a v a i l a b l e f o r Handarte i n 1969 and Cleland i n 1970. N e s t l i n g m o r t a l i t y r a t e s with respect to age and brood s i z e are shown i n t a b l e 6 f o r these two years. There were no d i f f e r e n c e s s t a t i s t i c a l l y i n the m o r t a l i t y r a t e s between normal and supernormal broods. However, on both i s l a n d s the m o r t a l i t y appeared to be higher i n the supernormal than i n the normal broods. This i s r e f l e c t e d i n the f l e d g i n g success given below f o r a l l i s l a n d - y e a r s . On both i s l a n d s , the i n i t i a l l y high r a t e of m o r t a l i t y i n both normal and supernormal brcods Table 6 Nestling Mortality Bates for Mandarte in 1969 and Cleland in 1970. The Number of Chicks Dying during Each Age Interval i s Expressed as a Percent of the Total Number of Chicks at the Beginning of the Interval. Mandarte 1969 Cleland 1970 Chick Age (days) Normal Broods Supernormal Broods Normal Broods Supernormal Broods n = 263 % 475 % 445 % 473 % 0 - 5 6.5 8.0 5.2 4.0 6 -10 6.9 9.2 1.4 4.2 11-15 5.7 6.5 3.8 3.4 16-20 4.6 4.3 2.5 5.0 21-25 6.8 5.1 2.0 3.3 26-30 2. 1 7.4 1.3 1.0 31-35 1.6 1.9 0.8 0.5 36-40 1.1 2.6 0.3 0.5 41 + 2.2 5.0 0.3 0.0 Total loss 84 192 73 95 In Percent 31.9 40.4 16.4 20.1 n - Number of chicks at age 0. 35 appeared to continue longer i n the supernormal broods. The m o r t a l i t y r a t e s on both i s l a n d s decreased s i g n i f i c a n t l y with i n c r e a s i n g c h i c k age (G-test; P < 0.05). O v e r a l l the m o r t a l i t y r a t e s on Mandarte were s i g n i f i c a n t l y higher (P ;< 0.01) than those on C l e l a n d . Table 7 shows that a l a r g e percentage of the chick deaths oc c u r r i n g on Mandarte and Cleland were due to pecking by other g u l l s . Rain appeared to cause r e l a t i v e l y more deaths on Cleland than on Mandarte. Fledging success, which i s an i n d i r e c t measure of n e s t l i n g m o r t a l i t y , appeared to d e c l i n e with i n c r e a s i n g brood s i z e i n a l l years recorded ( f i g . 7) except 1962 (Vermeer's stud y ) . The d i f f e r e n c e s i n f l e d g i n g success among the brood s i z e s f o r each i s l a n d - y e a r were s i g n i f i c a n t (G-test) f o r Mandarte i n 1969 and 1971 (P<0.01), and Cleland i n 1970 (P<0.025). Broods on Cleland i n 1970 and QCI i n 1972 had a higher o v e r a l l f l e d g i n g success (P < 0.01) than d i d those on Mandarte i n 1969, 1971, and 1962. D i s c u s s i o n : In t h i s study the pattern of m o r t a l i t y of c h i c k s with respect to age does not d i f f e r g r e a t l y from that of other g u l l s t u d i e s (Paynter, ; 1949; Paludan, 1951; Fordham, 1964; Vermeer, 1967; Parsons, 1971). These s t u d i e s a l s o showed that the highest m o r t a l i t y r a t e s occurred i n the c h i c k s ' f i r s t two weeks of l i f e . Most l i k e l y the c h i c k s ' s m a l l s i z e plays a l a r g e part i n t h e i r v u l n e r a b i l i t y to both 36 Table 7 Percentage of Deaths Due to D i f f e r e n t Factors on Mandarte i n 1969 and Cleland i n 1970. Mandarte 1969 Cleland 1970 I I L Cause of Normal Supernormal Normal Supernormal Death Broods Broods Broods Broods n = 84 192 73 95 % % % % K i l l e d by 59.5 66.7 42.5 40.0 G u l l s Heavy 1.2 3.0 20.5 25.3 Bain Disappeared* 15.5 8.3 1.4 10.5 Unknown* 23.8 22.0 35.6 24.2 •disappeared: The chick disappeared and was assumed to be dead. No carcass was found, •unknown: Carcass of dead c h i c k found, but cause of death was not known. 37 G i— 1 1 r~ 1 — r 1 2 3 4 5 6 Brood Size F i g . 7 : Percentage fledging success for the d i f f e r e n t brood s i z e s on Mandarte, Cleland, and QCI. 38 a t t a c k s by other g a l l s and the weather. H a r r i s and Plumb (1965) found t h a t very young h e r r i n g g u l l c h i c k s i n the supernormal broods s u f f e r e d a high m o r t a l i t y r e l a t i v e to / normal broods i n a year of inclement weather. The a d u l t b i r d s were unable to adequately cover more than three c h i c k s at one time because only one b i r d brooded the c h i c k s at any one time. This study a l s o showed that inclement weather can a f f e c t the m o r t a l i t y of young g u l l s . In t h i s study the longer period of high m o r t a l i t y r a t e s i n supernormal broods could be a consequence of t h e i r slower growth. This would mean that the c h i c k s are i n a vulnerable s i z e category f o r a longer period of time. The reason f o r the lower m o r t a l i t y on Cleland and QCI than on Mandarte i s not known, although i t could be due to s e v e r a l f a c t o r s . (1) The lower nest d e n s i t i e s on C l e l a n d and QCI may lower the number of a d u l t - c h i c k encounters. (2) Faster growth r a t e s on these i s l a n d s i n general may r e s u l t i n h e a l t h i e r c h i c k s b e t t e r able to withstand inclement weather and/or a t t a c k s by other a d u l t s . (3) Perhaps there i s b e t t e r p a r e n t a l attendance on Cleland and QCI because food i s more a v a i l a b l e and thus r e q u i r e s l e s s time to o b t a i n , as i s shown i n the next chapter. Fordham (1964) a l s o noted that chick m o r t a l i t y due to a t t a c k s by a d u l t s increased when the food supply f o r a colony decreased. The important c o n s i d e r a t i o n i n t h i s s e c t i o n i s the 39 f l e d g i n g success and i t s r e l a t i o n to brood s i z e . Contrary to Vermeer's f i n d i n g s , the percent f l e d g i n g success decreased with i n c r e a s i n g brood s i z e on both Cleland and Mandarte. This type of trend i s the one most commonly found i n the l i t e r a t u r e . However, i t should be kept i n mind t h a t although the percent f l e d g i n g success decreased with i n c r e a s i n g brood s i z e , the absolute numbers of c h i c k s fledged per brood s i z e a c t u a l l y increased i n a l l years ( f i g . 8 ) . Pa r t of the decrease i n f l e d g i n g success with respect to brood s i z e i n t h i s study may have been due to the disturbance during weighing. However, i f t h i s disturbance did have a d e l e t e r i o u s a f f e c t , i t operated only on the l a r g e r brood s i z e s (4 to 6). There was no d i f f e r e n c e between t h i s study and Vermeer's f o r normal s i z e d broods on Mandarte. Even i n 1971 on Mandarte when the c h i c k s were not weighed u n t i l 30 days of age, the f l e d g i n g success decreased with i n c r e a s i n g brood s i z e . In the l a r g e r broods the number of h i d i n g spots w i t h i n a t e r r i t o r y may become c r i t i c a l r e s u l t i n g i n c h i c k s l e a v i n g the t e r r i t o r y to hide when there i s a disturbance. This i n turn would increase t h e i r chances of being attacked by neighbouring ' g u l l s . I n t e r e s t i n g l y , the percent f l e d g i n g success f o r a brood of s i x c h i c k s on Cleland i s s t i l l as high or higher than that reported i n the l i t e r a t u r e f o r normal s i z e d broods. 4 0 5 -H Mandarte 1969 • Mandarte 1971 • Cleland 1970 4-Q LU CD 3 Q LxJ _ J cr 2 LU DO 1 " i i 2 3 4 BROOD S I Z E Fig. 8: Absolute numbers of c h i c k s fledged f o r the d i f f e r e n t brood s i z e s on Mandarte and C l e l a n d . 41 D P o s t - f l e d g i n g Success Methods: As many c h i c k s as p o s s i b l e from d i f f e r e n t brood s i z e s were banded i n 1969 and 1971 on Mandarte and i n 1970 on Cleland. Estimates of p o s t - f l e d g i n g s u r v i v a l were derived from returns f o r these banded b i r d s . I received some band returns f o r dead b i r d s , but by f a r the most p r o f i t a b l e returns were the l i v e s i g h t i n g s (bands read with a t e l e s c o p e ) . The majority of these s i g h t i n g s were made by Ian MacGregor at refuse s i t e s l o c a t e d between Vancouver and S e a t t l e . Although the number of l i v e r e t urns f o r Mandarte was reasonably la r g e ( > 50 per cohort banded), the r e t u r n f o r Cleland b i r d s was sm a l l (16 out of 600 bands). Apparently the j u v e n i l e s from C l e l a n d I s l a n d do not come i n t o the Puget Sound-Georgia S t r a i t area i n any number- Most of the 16 returns were obtained by MacGregor along the outer coast of the s t a t e s of Washington and Oregon. The a n a l y s i s of the s i g h t i n g s were done with respect to the brood s i z e the c h i c k s were i n at the time the brood was i n i t i a l l y set up. The f i r s t year s u r v i v a l r a t e i s assumed to be p r o p o r t i o n a l to the percentage of b i r d s seen a l i v e away from the breeding colony. In a d d i t i o n , Vermeer's data were reanalyzed i n c o r p o r a t i n g the s i g h t i n g s of b i r d s obtained i n subsequent years. Results!: The r e s u l t s are shown i n Table 8. On Mandarte i n 1969 and 1971 the p o s t - f l e d g i n g s u r v i v a l f o r c h i c k s i n normal broods was higher than f o r those i n supernormal broods. 42 Table 8 Percentage S u r v i v a l Rates of F i r s t Year Birds as Related to Brood S i z e . These S u r v i v a l Values Are Based on the Number of Live S i g h t i n g s of B i r d s Away from the Colony. I s l a n d S Year 1 Brood Size Mandarte ** 27.7 30.8 28.6 12.5 16.7 1962 (47) (146) (171) (120) (90) Mandarte * 31.0 40.4 21.0 13.5 15.0 19.7 1969 (29) (47) (57) (52) (40) (71) Mandarte 1971 20.0 (100) 21.0 (109) 20.0 (93) 13.0 (82) 8.0 (72) 10.0 (81) Cleland # 1970 2.6 (267) 3.0 (300) * The d i f f e r e n c e s i n p o s t - f l e d g i n g s u r v i v a l r a t e s between broods s i g n i f i c a n t at P < 0.025 (G-test). ** The d i f f e r e n c e s i n p o s t - f l e d g i n g s u r v i v a l rates between broods s i g n i f i c a n t at P < 0.01 (G- t e s t ) . (n) Number i n bracket i s the t o t a l number banded f o r that brood s i z e . # Data combined f o r both normal and supernormal. 1 In 1969 the d i f f e r e n c e s were s i g n i f i c a n t ( G-test; P < 0.025). There was no d i f f e r e n c e i n p o s t - f l e d g i n g s u r v i v a l between normal and supernormal broods on Cleland i n 1970. The a n a l y s i s of r e t u r n s f o r the c h i c k s produced on Mandarte i n 1962 showed that the s u r v i v a l f o r normal broods was s i g n i f i c a n t l y b e t t e r ( P < 0.01) than that f o r supernormal broods. Table 9 shows a comparison of the average asymptotic weight (given i n Appendix 2) f o r the d i f f e r e n t brood s i z e s on Mandarte (1969 and 1971) and the average weight of those b i r d s seen a l i v e away from the i s l a n d . Although the sample s i z e was s m a l l , the data suggest that the heavier b i r d s had the best chance of s u r v i v i n g a f t e r f l e d g i n g . Table 10 shows the r e l a t i v e number of young per brood s i z e that should s u r v i v e to breeding. I have assumed that the recovery r a t e s are p r o p o r t i o n a l to the a c t u a l s u r v i v a l r a t e s of the b i r d s i n t h e i r f i r s t year of l i f e and t h a t m o r t a l i t y i n subsequent years i s independent of brood s i z e . In 1962 and 1971 on Mandarte, the broods of three c h i c k s would c o n t r i b u t e the greatest number of i n d i v i d u a l s to fu t u r e breeding populations. In 1969 on Handarte and 1970 on C l e l a n d , a brood of s i x would c o n t r i b u t e the most young. D i s c u s s i o n : The data i n t h i s study suggest that the p o s t - f l e d g i n g s u r v i v a l of g u l l c h i c k s was dependent on the asymptotic weight they a t t a i n e d p r i o r to f l e d g i n g . On Cleland 44 Table 9 Average Asymptotic Weight (grams) (±SE) of a l l Individuals for Different Brood Sizes Compared With the Averaged Asymptotic Weight of Those Individuals Seen Alive Away from the Colony For Mandarte in 1969 and 1971. Mandarte 1969 Mandarte 1971 Brood All. Fledged A l l Fledged Size Chick's Chicks Chicks Chicks 1 958 ± 32.9 988 ± 37.8 877 + 28.3 957 ± 88.8 (19) (7) (22) (5) 2 941 ± 21.5 943 ± 36.3 824 ± 30.5 867 + 100.8 (29) (14) (27) (4) 3 894 ± 16.3 944 ± 17.2 825 ± 31.8 965 ± 54.6 (26) (8) (24) (3) 4 882 ± 38.5 810 ± 35.9 (14) (11) 823 ± 65.2 * 5 756 ± 27.3 894 ± 55.7 * 797 ± 35.9 (4) (24) (8) (9) 6 809 ± 34.3 (8) * A l l supernormal chicks sighted were averaged to obtain this value. (n) Number of individuals sighted for which asymptotic weights were available. 45 Table 10 Theoretical Contribution to Future Breeding Populations for Broods one to six on Mandarte in 1962, 1969, and 1971, flnd Cleland in 1970. Brood Size No. Of Chicks Fledged Index of Chicks Surviving to Breeding * 0.73 0.19 Mandarte 1962 1.40 2.10 2.70 3.40 4.80 0.43 0.60 0.34 0.57 Mandarte 1969 No. Of Chicks Fledged 0.70 1.40 2.07 2.44 2.85 3.24 Index of Chicks 0.22 0.57 0.44 0.33 0.43 0.64 Surviving to Breeding * Mandarte 1971 0.75 1.56 1.95 2. 36 2.43 2.81 0.31 0.29 0.28 Cleland 1970 No. Of Chicks Fledged Index of Chicks 0.15 0.3.3 p. 39 Surviving to Breeding * No. Of Chicks 0.88 1.72 2.41 3.38 3.84 4.45 Fledged Index of Chicks 0.25 0.45 0.67 0.90 1.05 r.26 Surviving to Breeding * * Index i s based on the number of sightings of birds away from the colony. It i s assumed that there were no differences in the mortality after the f i r s t year. 46 where a l l brood s i z e s a t t a i n e d s i m i l a r asymptotic weights, the p o s t - f l e d g i n g success d i d not d i f f e r between normal and supernormal broods. On Mandarte those broods with heavier asymptotic weights a l s o had higher p o s t - f l e d g i n g s u r v i v a l r a t e s associated with them. The d e t a i l e d study on t h i s aspect i n the Great T i t by P e r r i n s (1965) showed that p o s t - f l e d g i n g s u r v i v a l was i n f l u e n c e d by the f l e d g i n g weight. The heavier n e s t l i n g s had the highest chances of s u r v i v a l i n a l l years with the l i g h t e r i n d i v i d u a l s having a v a r i a b l e s u r v i v a l depending on the year. In both 1969 and 1971 on Mandarte, s i n g l e brooded c h i c k s had the heaviest asymptotic weight suggesting that they should a l s o have the highest p o s t - f l e d g i n g success. This was not the case f o r e i t h e r year. Perhaps the c h i c k s r a i s e d by themselves are at a disadvantage when competing f o r food a f t e r f l e d g i n g , s i n c e they never had t o compete with nest-mates as c h i c k s . The r e s u l t s Vermeer obtained i n 1962 on p o s t - f l e d g i n g success (supernormal broods b e t t e r than normal ones) were changed by l a t e r r e c o v e r i e s . However, I do not think the trends i n p o s t - f l e d g i n g s u r v i v a l f o r Mandarte b i r d s i n t h i s study w i l l fce changed by l a t e r r e t u r n s . An explanation f o r Vermeer^s r e s u l t s may l i e i n the nature of h i s s i g h t i n g s . He only had a s m a l l number of r e t u r n s (<15$ as opposed to up to 40% f o r t h i s study) and a l l were from the Vancouver refuse dump or nearby parks i n Vancouver. By chance there may have 47 been more b i r d s from supernormal broods than from normal ones at these s i t e s . In t h i s study, returns were obtained from many d i f f e r e n t places along the west coast. The l a t e r r e c o v e r i e s of Vermeer's b i r d s i ncluded s i g h t i n g s at other places i n c l u d i n g s i g h t i n g s of these b i r d s as a d u l t s on the colony. Vermeer had no weight data on h i s b i r d s and t h i s would i n d i c a t e whether the r e s u l t he got was l o g i c a l . In t h i s study the s u r v i v a l r a t e s corresponded with the asymptotic weights. In a d d i t i o n , I reanalyzed my data omitting the r e t u r n s during September and October; the months when the l a r g e s t amount of f i r s t year m o r t a l i t y occurs (van Tets, 1968). The trends i n the data were not changed by t h i s procedure. In a c t u a l f a c t then, Vermeer's experiment d i d support Lack's hypothesis, as d i d the data from Mandarte i n 1971. In both these years a brood of three was the optimal s i z e . In 1969 on Mandarte, a brood of s i x c h i c k s would have c o n t r i b u t e d the l a r g e s t number of progeny, although the broods of two were cl o s e behind. However, I think i t i s l i k e l y t h a t a d u l t g u l l s could not evolve a brood patch capable of s u c c e s s f u l l y i n c u b a t i n g s i x large eggs. I f the g u l l evolved smaller eggs, i n order to accommodate them a l l , the p r e f l e d g i n g m o r t a l i t y could p o s s i b l y i n c r e a s e as suggested i n a study by Parsons (1971). He found that the s u r v i v a l of c h i c k s from the 'C egg which i s the s m a l l e s t egg was a l s o the lowest. This would then remove the advantage gained by having a c l u t c h of s i x H8 eggs. I f a c l u t c h of s i x l a r g e eggs i s dismissed as being i m p o s s i b l e , then a two egg c l u t c h was optimal on Mandarte i n 1969. A l l the r e s u l t s from Mandarte, then, support Lack's hypothesis concerning the a b i l i t y of parents t o s a t i s f a c t o r i l y feed l a r g e r than normal numbers of o f f s p r i n g . This was not the case f o r g u l l s on Cleland and perhaps f o r much of the west coast of B r i t i s h Columbia, i f QCI can be considered t y p i c a l of other c o l o n i e s . In these places the c o n t r i b u t i o n t o future breeding populations by d i f f e r e n t brood s i z e s increased with each a d d i t i o n a l young added to the brood s i z e . Although I discounted the p o s s i b i l i t y of a s i x brood orj Mandarte and t h i s would a l s o apply f o r C l e l a n d , i t i s q u i t e conceivable f o r a g u l l to evolve brood patches capable of accommodating four eggs. This i s perhaps demonstrated best by the shorebirds which have a brood patch arrangement such that they can incubate f o u r l a r g e eggs (Lack, 1968). I t would appear then that the r e s u l t s from Cleland do not support Lack's hypothesis. , There are a number of reasons put f o r t h i n the l i t e r a t u r e which might account f o r t h i s discrepancy. Because some of these i n v o l v e the food supply, t h i s part of the d i s c u s s i o n w i l l be d e a l t with at the end of the next chapter which describes the food s i t u a t i o n on the d i f f e r e n t c o l o n i e s . 49 E Summary Experiments u s i n g the Glaucous-winged G u l l were c a r r i e d out i n o r d e r t o t e s t L a c k ' s h y p o t h e s i s on the e v o l u t i o n of c l u t c h - s i z e . P r e v i o u s e x p e r i m e n t s had shown t h a t t h i s s p e c i e s c o u l d r a i s e e x t r a c h i c k s but t h a t the b i r d s may have used r e f u s e s i t e s as a food s o u r c e . I n t h i s s t u d y , e x p e r i m e n t s were c a r r i e d out on both an i s l a n d (Mandarte) where r e f u s e i s a v a i l a b l e t o t h e b i r d s and on i s l a n d s ( C l e l a n d and QCI) where g e n e r a l l y o n l y n a t u r a l f o o d i s a v a i l a b l e t o the b i r d s . Eroods of one t o s i x c h i c k s were s e t up i n the experiment i n which growth r a t e s , a s y m p t o t i c w e i g h t s , n e s t l i n g m o r t a l i t y r a t e s , f l e d g i n g s u c c e s s , and p o s t - f l e d g i n g s u c c e s s were measured. Growth r a t e s were b e s t on C l e l a n d and QCI i n a l l y e a r s , a s y m p t o t i c w e i g h t s o f c h i c k s i n a l l brood s i z e s on C l e l a n d and p o s s i b l y QCI were e q u a l to the average a d u l t weight (1000 g ) . On Mandarte t h e c h i c k s had s i g n i f i c a n t l y l i g h t e r a s y m p t o t i c w e i g h t s than t h o s e c h i c k s on C l e l a n d . F l e d g i n g s u c c e s s , which appeared t o d e c r e a s e w i t h brood s i z e , was s i g n i f i c a n t l y b e t t e r on C l e l a n d and QCI t h a n on Mandarte. The a b s o l u t e numbers of c h i c k s f l e d g e d per brood s i z e i n c r e a s e d w i t h brood s i z e f o r a l l i s l a n d s and y e a r s . P o s t - f l e d g i n g s u r v i v a l was the same f o r normal and s u p e r n o r m a l broods on C l e l a n d . On Mandarte p o s t - f l e d g i n g s u r v i v a l was h i g h e r i n normal broods than i n sup e r n o r m a l broods. On C l e l a n d the su p e r n o r m a l broods c o n t r i b u t e d t h e g r e a t e s t number o f i n d i v i d u a l s to f u t u r e b r e e d i n g p o p u l a t i o n s , on Mandarte, normal broods c o n t r i b u t e d 50 the greatest number. Thus the r e s u l t s from Handarte did support Lack's hypopothesis; but t h i s was not the case f o r Cleland where the most productive brood si2e was l a r g e r than the most common. 51 CHAPTER I I I Food U t i l i z e d By Breeding G u l l s A I n t r o d u c t i o n The Glaucous-winged G u l l ; l i k e the other g u l l s p e c i e s , i s able to swallow food at the time of catching i t and then r e g u r g i t a t e i t f o r the young at the nest. Limited observation using s c a l e s placed i n the t e r r i t o r y of i n d i v i d u a l p a i r s showed that the b i r d s could b r i n g back a g u a n t i t i y of food weighing up to 20 percent of t h e i r own body weight. The number of such t r i p s that the b i r d can make i n a day w i l l be determined by both the ease at which they can f i n d the food and the dis t a n c e the food i s lo c a t e d from the colony. Lack (1966) objected to Vermeer's chick a d d i t i o n experiment because i t was conceivable that the adu l t g u l l s obtained large q u a n t i t i e s of food a t nearby refuse sources. As shown i n the l a s t chapter, on r e a n a l y s i s , the r e s u l t s of Vermeer's experiment were not contrary to Lack's hypothesis on the e v o l u t i o n of c l u t c h - s i z e . As w i l l be shown i n t h i s chapter, LackJs c r i t i c i s m concerning the use of refuse a l s o turns out to be i n c o r r e c t . This chapter describes the food s i t u a t i o n on the d i f f e r e n t i s l a n d s s t u d i e d , showing that the i n i t i a l assumptions (see page 8) were c o r r e c t concerning the type of food fed to the c h i c k s on the d i f f e r e n t i s l a n d s . Information 52 i s a l s o presented on the r e l a t i v e a v a i l a b i l i t y of foods used oi| the d i f f e r e n t i s l a n d s . B Diet of Adult G u l l s P r i o r to Chick Hatching Methods: The f a c t that g u l l s r e g u r g i t a t e any undigested m a t e r i a l i n the form of a p e l l e t lends i t s e l f as a means of t e s t i n g , although not c r i t i c a l l y , f o r the presence or absence of garbage i n t h e i r d i e t . An a n a l y s i s of p e l l e t s c o l l e c t e d on Mandarte i n 1969 and Cleland i n 1970 was c a r r i e d out i n order to determine the presence or absence of refuse i n t h e i r d i e t . These p e l l e t s were c o l l e c t e d on p l o t s between May 11 and June 19 on Mandarte and between May 18 and June 22 on C l e l a n d . Results,: P e l l e t s of a d u l t g u l l s on Mandarte i n 1969 consis±ed of about 60 percent garbage (Table 11). The garbage found i n the p e l l e t s included chicken, beef, and pork bones, paper* b o t t l e caps, b i t s of g l a s s , and s t r i n g , on Cleland i n 1970 there was l e s s than one percent refuse i n the p e l l e t s c o l l e c t e d . Here the refuse was paper although the o c c a s i o n a l beef or pork bone was seen outside the p l o t s . D i s c u s s i o n : This technique does not give absolute values f o r the use made of d i f f e r e n t food types. However the r e s u l t s d i d i n d i c a t e t h a t ; p r i o r to chick hatching, the g u l l s used f a r more refuse on Mandarte than they d i d on Cleland.; F i s h o f f a l , i f used by the C l e l a n d b i r d s , would not show up i n the p e l l e t s and thus r e s u l t i n a b i a s towards n a t u r a l foods.' 53 Table 11 P e l l e t s C o l l e c t e d on Mandarte i n 1969 and Cleland i n 1970 C l a s s i f i e d as to Whether Garbage or Non-garbage Items. A More Complete L i s t i n g of Species of Natural Food I s Found i n Appendix 4. Mandarte 1969 Food Type May 11^ 17 May 18-24 May 25-31 June 1-8 June 9-19 % % % H Garbage 65.9 67.5 68.2 56.9 60.0 Natural 34. 1 32.5 31.8 43.1 40.0 n = 473 1054 1110 1087 473 Cleland 1970 Food May May May June June June Type 18 24 31 8 15 22 % % % % % % Garbage 0.7 0.5 0.0 0.0 0.0 0.0 Natural 99. 3 99.5 100.0 100.0 100.0 100.0 n - 240 182 197 217 78 53 54 Observations by Henderson (1972) on the stomach contents of i n c u b a t i n g g u l l s showed t h a t the Cleland b i r d s contained only P a c i f i c Sandlance ftmmodytes hexapterus whereas the Mandarte b i r d s contained 29 percent garbage and the remainder n a t u r a l food. This supports the b a s i c c o n c l u s i o n drawn fUom the p e l l e t counts t h a t , p r i o r to chick hatching, the b i r d s on Mandarte use garbage i n f a i r l y s u b s t a n t i a l amounts whereas on C l e l a n d they use very l i t t l e . C Chick R e g u r g i t a t i o n s Methods: G u l l c h i c k s , when being handled, w i l l o c c a s i o n a l l y r e g u r g i t a t e the food they have i n t h e i r p r o v e n t r i c u l u s . This provides an opportunity to determine the composition of food fed to the c h i c k s by the a d u l t s . One o b j e c t i o n to t h i s method i s the guestion of whether a l l food types are r e g u r g i t a t e d by the c h i c k s with equal ease. The work of Spaans (1971) i n d i c a t e d that they probably are. During the r e g u l a r weighing of c h i c k s on Mandarte i n 1969, Cleland i n 1970, and QCI i n 1972, a l l chick r e g u r g i t a t i o n s were i d e n t i f i e d and recorded. R e s u l t s : On Mandarte i n 1969 the majority of r e g u r g i t a t i o n s were P a c i f i c Herring Clu^ea j a ^ l a s i j i (61.3 percent) ( f i g . 9 ) . Refuse was only r e g u r g i t a t e d 5.4 percent of the time. On Cleland i n 1970 the majority of the r e g u r g i t a t i o n s were P a c i f i c Sandlance (72.3 percent). F i s h 55 Figure 9: Percentage composition of food types found in chick regurgitations on Mandarte, Cleland, and QCI. Mandar te 1969 Natural fo & Refuse C l e l a n d 1970 QCI 1972 56 o f f a l was found only once (0.9 percent). On QCI i n 1972 sandlance and h e r r i n g made up 86.6 percent of the r e g u r g i t a t i o n s . The remainder were e i t h e r i n t e r t i d a l organisms or s m a l l euphausids. D i s c u s s i o n ; These data a l s o support the assumptions made at the s t a r t of the study concerning the usage of garbage on the d i f f e r e n t i s l a n d s . Henderson (1972) s y s t e m a t i c a l l y sampled c h i c k s i n 1970 on Mandarte I s l a n d , removing the food from t h e i r g u l l e t . He obtained r e s u l t s s i m i l a r to the chick r e g u r g i t a t i o n s on Mandarte i n 1969. These data a l s o support the contention of Spaans that the use of ch i c k r e g u r g i t a t i o n s as a method of determining the d i e t of g u l l c h i c k s does not d i f f e r g r e a t l y from the r e s u l t s obtained by d i r e c t l y sampling the gut contents of c h i c k s . Qn C l e l a n d i n 1970, f i s h o f f a l was very r a r e l y fed to the c h i c k s . Observations by Henderson i n 1971 on Cle l a n d a l s o supported t h i s c o n c l u s i o n . During h i s observations, he saw no f i s h o f f a l being fed (personal communication). Thus the b i r d s on Cleland and QCI d i d not need mans* refuse i n order to be able to adeguately feed up to twice the normal number of c h i c k s . 57 D Feeding observations on Mandarte i n 1971 I n t r o d u c t i o n : The growth r e s u l t s from Mandarte i n 1969 i n d i c a t e d that the a d u l t s there were having d i f f i c u l t i e s f eeding more than three c h i c k s . According to Lack»s c r i t i c i s m mentioned e a r l i e r , t h i s should not have been the case. In 1971 ox\ Mandarte a more d e t a i l e d study was c a r r i e d out on the food fed to the c h i c k s i n order to l e a r n why. I wanted to know i f garbage was fed more often to supernormal broods than to normal broods, and whether l e s s time was r e q u i r e d f o r the a d u l t s to o b t a i n refuse than to obtain n a t u r a l food. In a d d i t i o n , information was obtained both on the amount of time spent by a d u l t s i n o b t a i n i n g food f o r t h e i r c h i c k s with respect to the brood s i z e , and on the frequency of food types found i n the c h i c k s * d i e t . Methods: D i r e c t observations were made on p a i r s of g u l l s and t h e i r c h i c k s i n order to answer the above guestions. There was a good opportunity to i d e n t i f y the food fed to the c h i c k s by the a d u l t s because i t was r e g u r g i t a t e d before i t had been digested t o any extent. However,, t h i s method did have two disadvantages. F i r s t I could not see what was r e g u r g i t a t e d i n a l l the feedings because the view was p a r t i a l l y obstructed e i t h e r by grass or by the c h i c k s themselves. In the a n a l y s i s I assumed that the frequency of the unseen food types was s i m i l a r to that seen by myself. Secondly, the p r o p o r t i o n of ^food fed by frequency and by 58 weight may d i f f e r . I have a sm a l l amount of data suggesting! that l e s s garbage by weight than f i s h was brought back from a forag i n g t r i p . The weight of f i s h brought back from a for a g i n g t r i p averaged 150 g (n = 10) whereas garbage averaged only 107 g (n - 7) . A s u b j e c t i v e impression of the weight of these two food types brought back per t r i p conformed with t h i s as w e l l . Thus the evidence suggests that perhaps garbage was overestimated by the frequency occurrence method and not underestimated. Observations were made on 42 d i f f e r e n t p a i r s of g u l l s s i t u a t e d around three d i f f e r e n t b l i n d s . Appproximately equal numbers of broods of one to s i x c h i c k s were set up around each b l i n d , although subsequent c h i c k m o r t a l i t y changed these f i g u r e s . Observation periods extended continuously from 0400 to 2200 hours and were made at each b l i n d at s i x to eight day i n t e r v a l s . A t o t a l of 11 days of observation was obtained between June 23 and J u l y 25. I recorded the a r r i v a l s and departures of the adul t g u l l s being observed and the food they fed to t h e i r c h i c k s . Sixteen hours of s i m i l a r observations (two e i g h t hour periods) were c a r r i e d out both on Handarte i n 1969 and cn QCI i n 1972. The absence of an a d u l t was c l a s s i f i e d as a f o r a g i n g absence only i f the b i r d returned and fed the c h i c k s with r e l a t i v e l y undigested food. 59 R e s u l t s : (i) Foraging absences: The average d u r a t i o n of a fo r a g i n g absence or t r i p by a d u l t s with normal or supernormal broods on Mandarte, C l e l a n d , and QCI are presented i n t a b l e 12. I have i n c l u d e d HendersonAs (1972) data here f o r ease of comparison. O v e r a l l the Mandarte g u l l s spent more time on an i n d i v i d u a l f o r a g i n g t r i p than d i d those on Cleland or QCI. The du r a t i o n of a t r i p on Mandarte i n 1971 was the longest ( t - T e s t ; P < 0i01) f o r a l l the years and places s t u d i e d . The data a l s o show a d i f f e r e n c e between parents of normal and supernormal broods, but i t i s not c o n s i s t e n t from i s l a n d to i s l a n d and from year to year. With the exception of QCI none of the d i f f e r e n c e s are s i g n i f i c a n t s t a t i s t i c a l l y . D i s c u s s i o n : Henderson (1972) suggested that a d u l t s with supernormal broods spend l e s s time per for a g i n g t r i p than do a d u l t s with normal s i z e d broods. He t h e o r i z e d that the increased s t i m u l a t i o n r e s u l t i n g from the e x t r a c h i c k s caused the a d u l t s t o r e t u r n sooner regard l e s s of the amount of food they had obtained. The r e s u l t s of t h i s study do not support h i s hypothesis. The data on for a g i n g absences f o r Mandarte i n 1971 showed no d i f f e r e n c e between parents with normal and supernormal broods. The data on QCI showed a s i g n i f i c a n t d i f f e r e n c e between a d u l t s with normal and supernormal broods, but the a d u l t s with supernormal broods spent more time per fora g i n g t r i p r a t h e r than l e s s time as suggested by Henderson. The a d u l t s with supernormal broods on Mandarte i n 1969 a l s o I 60 Table 12 Average Duration of Foraging Absences (±SE) for Adults with both Normal and Supernormal Broods. Island 8 Normal Broods Supernormal Eroods Year (1 - 3) ( 4 - 6 ) (minutes) (minutes) Mandarte 123 ± 8.0 140 ± 3.6 1969 (24) (13) Mandarte 156 ± 34.4 91 ± 5.1 1970 # (22) (35) Mandarte 194 ± 6.6 194 ± 8.0 1971 ** (335) (275) Cleland 122 ± 13.2 93 ± 6.9 1971 # (42) (86) QCI 69 ± 5.7 * 101 ± 12.6 1972 (20) (13) # Data from Henderson, 1972. (n) Number i n bracket i s sample s i z e . * D i f f e r e n c e between normal and supernormal broods s i g n i f i c a n t (P<0.01) using t-Test. ** Mandarte 1971 s i g n i f i c a n t l y d i f f e r e n t from any of the other i s l a n d - y e a r s (P<0.01) using t-Test. 61 appealed to be spending more time per t r i p than a d u l t s with normal broods. The s i t u a t i o n does not appear to be a simple one and may be complicated by the a v a i l a b i l i t y of food. On Handarte i n 1971 the time spent on a f o r a g i n g t r i p was r e l a t i v e l y long. On QCI i n 1972 i t was r e l a t i v e l y s h o r t . Henderson*s conclusions were based on s i t u a t i o n s where the foraging time was intermediate between these two extremes. The greater amount of time taken per foraging t r i p by a d u l t s on Mandarte suggest as do the growth data that food was harder to o b t a i n there than i t was on Cleland or QCI. More informati o n on t h i s aspect i s given below. ( i i ) Foraging T r i p s on Mandarte i n 1971: In t h i s p a r t i c u l a r year there were s u f f i c i e n t data f o r an a n a l y s i s of the duration of the f o r a g i n g t r i p s with respect to the age of the c h i c k s . Data i n Table 13 show an increase i n the average time taken with i n c r e a s i n g c h i c k age f o r both normal and supernormal broods. A l i n e a r r e g r e s s i o n of the duration of f o r a g i n g t r i p s on c h i c k age showed a s i g n i f i c a n t p o s i t i v e slope f o r both types of broods (P < 0.01). However, there were no d i f f e r e n c e s i n slopes between a d u l t s with normal broods and a d u l t s with supernormal broods. The data c o l l e c t e d i n 1971 were a l s o analyzed to see i f the t o t a l amount of time spent foraging each day by a d u l t s changed as the brood s i z e increased or as the c h i c k s got I o l d e r . Each p a i r of b i r d s had a p o s s i b l e t o t a l of 36 hours 62 Table 13 Change i n Length of Average Foraging T r i p s (minutes) (±SE) with Chick Age f o r Both normal And Supernormal Broods On Mandarte i n 1971, Brood S i z e 0-5 Chick Age (days) 6-12 13-19 20-26 27 + Normal 145 177 188 208 224 (1-3 chicks) ±16.2 ±12.9 ±15.5 ±16.8 ±29.7 (32) ( 6 1 ) (50) (58) (20) Supernormal 158 195 208 221 217 (4-6 chicks) ±27.3 ±15.7 ±30.1 ±74.3 ±47.2 (46) (59) (62) (19) (13) (n) Number i n bracket i n d i c a t e s the sample s i z e . Regression equations: normal - ¥=142+3.4X supernormal - Y=153+3.4X 63 each day i n which to forage f o r food. A b i r d ' s f o r a g i n g day was considered to extend from 0400 to 2200 hours (18 hours). I f both members of a p a i r foraged continuously f o r the whole day, t h e i r t o t a l time spent f o r a g i n g would be 36 hours. This never happened. Generally only one b i r d would forage at any p a r t i c u l a r time, although of t e n i n l a r g e r broods and the broods with odder c h i c k s , both members of a p a i r would be away fo r a g i n g at the same time. The t o t a l amount of time spent fo r a g i n g each day by a p a i r ( f i g . 10), on average, increased with i n c r e a s i n g c h i c k age and a l s o with i n c r e a s i n g brood s i z e . A l i n e a r r e g r e s s i o n of these times on i n c r e a s i n g c h i c k age f o r each brood s i z e (5 and 6 combined) was s i g n i f i c a n t (P < 0.01) f o r broods four and f i v e and j u s t over the f i v e percent l e v e l f o r broods of three (P = 0.052). There was not a s i g n i f i c a n t slope f o r broods of one and two . c h i c k s . The slope of a reg r e s s i o n of these times on brood s i z e was a l s o s i g n i f i c a n t (P < 0.01). The average number of fo r a g i n g t r i p s made d a i l y by a d u l t s f o r the d i f f e r e n t brood s i z e s d i d not appear t o change with c h i c k age (Table 14 ). However, the number of t r i p s made by a d u l t s d i d increase with i n c r e a s i n g brood s i z e . A two-way a n a l y s i s of ivariance showed t h a t there were no d i f f e r e n c e s i n the number of t r i p s made d a i l y with respect to the chick age but that there were d i f f e r e n c e s a s s o c i a t e d with the brood s i z e (P < Q.01). 64 Brood s ize + ---+ 1 o—o 2 •---© 3 £ — A 4 •s /v 5 & 6 W a y s ; Fig. 10: Change in average total tine spent foraging each day by a pair with respect to the age of the chicks and the various brood sizes. Table 14 Average Number (±SE) of Foraging Trips Made per Day By Adults with Respect to the Age of the Chicks And the Brood Size. Chick Age (days) 0-5 2.4 ±.50 Brood Size 4.5 ±.50 4.6 ±.64 4.0 ±.89 5 S 6 4.4 ±.85 (5) (2) (7) (5) (9) 6-12 3.2 ±.46 2.9 ±.34 5.0 ±.69 3.9 ±.34 4.8 ±.72 (10) (7) (7) (10) (9) 13-19 2.8 ±.31 4.0 ±.57 3.9 ±.40 4.4 ±.50 5.1 ±.58 (8) (6) (7) (9) (8) 20-26 2.8 ±.32 3.3 ±.42 4.8 ±.36 5.0 ±.00 4.3 ±.85 (10) (6) (8) (2) (4) 27 + 1.8 ±.37 3.7 ±.88 3.6 ±.88 4.5 ±. 50 4.5 ±1.50 (5) (3) (3) (2) (2) Average no. 2.7 3.5 4.5 4.2 4.7 of Trips/day ±.18 ±.24 ±.25 ±.25 ±.35 Over Season (n) sample size 66 Henderson (197 2) a l s o found that the number of f o r a g i n g t r i p s increased with i n c r e a s i n g brood s i z e . However, the trend he found was much more marked than the one i n t h i s study. As i s shown i n the next s e c t i o n , the increases with c h i c k age i n the t o t a l amount of time spent f o r a g i n g d a i l y by a d u l t s i s l i n k e d with the change i n the d i e t s i n c e the number of t r i p s made per day d i d not change. ( i i i ) ; Food U t i l i z a t i o n and A v a i l a b i l i t y : The frequency at which garbage was brought back to the c h i c k s ( f i g . 1.1) increased with i n c r e a s i n g c h i c k age f o r both normal and supernormal broods. There were no d i f f e r e n c e s ( G - s t a t i s t i c , Sokal and Rohlf, 1969) between normal and supernormal broods regarding the frequency of t r i p s i n which garbage was brought back to the c h i c k s . However, there was a trend f o r parents with supernormal broods to b r i n g back more garbage at an e a r l i e r c h i c k age than the a d u l t s with normal broods. The m a j o r i t y of the category " f i s h " was made up of P a c i f i c H e r r i n g . A more d e t a i l e d l i s t i n g of the food fed the c h i c k s i s given i n appendix 5. In t h i s study the amount of time taken to obtain a food type (foraging t r i p ) was used as a measure of the a v a i l a b i l i t y of that food type. As shown i n Table 15 the f o r a g i n g absence was s i g n i f i c a n t l y longer ( t - T e s t ; P < 0.01) f o r garbage than f o r h e r r i n g , sandlance, or i n t e r t i d a l organisms. There were no d i f f e r e n c e s i n the length of the foraging absence among the 67 10 8-6 -UJ o cr UJ Q_ 8 N O R M A L B R O O D S n = 38 S 3 n = 67 zzz n=50 n=62 n=21 intert idal refuse f i sh S U P E R N O R M A L B R O O D S Z Z Z ] 0-5 6-12 13-19 2 0-26 CHICK AGE (days) F i g . 11: Change i n frecmency of food types i n the di e t of the chicks with respect to the i r age and brood s i z e (?•• and arte, 1971) t o t a l on which percentages based. Percentage values i n a n n p n d i x 1. 'n' i s Table 15 Average Duration (minutes) (±SE) of Foraging T r i p s For D i f f e r e n t Food Types with Respect To' Brood S i z e on Mandarte i n 1971. Food Type Normal Broods Supernormal Broods Herring Sandlance I n t e r t i d a l Refuse * (minutes) 172 ± 8.7 (144) 199 ± 21.1 (38) 163 ± 26.8 (14) 271 ± 22.7 (25) (minutes) 175 ± 11.5 (103) 191 ± 23.4 (43) 177 ± 35.1 (18) 265 ± 24.9 (35) * Using a t-Test> the d i f f e r e n c e between refuse and each of the other types was h i g h l y s i g n i f i c a n t (P < 0.01). 69 n a t u r a l food items. In a d d i t i o n , there were no d i f f e r e n c e s between a d u l t s with normal and supernormal broods f o r the d i f f e r e n t food types. As mentioned above, the average duration of a fo r a g i n g t r i p f o r parents with e i t h e r normal or supernormal broods increased with i n c r e a s i n g c h i c k age. a three-way a n a l y s i s of variance of fo r a g i n g absence with age, food type and brood s i z e i n d i c a t e d that t h i s i n crease was due to the change i n the food type (P < 0.01) and not the ch i c k age or brood s i z e The p o s s i b i l i t y of a change i n the foraging absence with respect to date f o r each of the d i f f e r e n t food types was a l s o checked. The l i n e a r r e g r e s s i o n of foraging absence on date r e s u l t e d i n a p o s i t i v e slope f o r both h e r r i n g (0.7 min./day): and sandlance (0.9 min./day). The slope was not s i g n i f i c a n t i n e i t h e r case but f o r h e r r i n g i t was j u s t over the s i g n i f i c a n c e l e v e l (P = 0.075). There was no r e l a t i o n s h i p f o r e i t h e r garbage or i n t e r t i d a l foods. D i s c u s s i o n : The food s i t u a t i o n on Handarte i n 1971 was not q u i t e what Lack (1966) suggested when he r e j e c t e d Vermeer's experiments as not being a good t e s t of h i s hypothesis. The b i r d s d i d use garbage as he suggested but i t probably formed l e s s than 25 percent o i the t o t a l d i e t . However, the f a c t remains that without garbage the b i r d s may not have been able t o r a i s e as many young as they d i d . The most s u r p r i s i n g r e s u l t was the time required by the a d u l t s to 7 0 o b t a i n garbage r e l a t i v e to the n a t u r a l foods used. The reason why the b i r d s used as much garbage as they d i d i s unknown. The d u r a t i o n of the f o r a g i n g t r i p f o r h e r r i n g and sandlance did increase as the season progressed but the r a t e of increase was not s u f f i c i e n t l y marked to make these two food types harder to o b t a i n than garbage. I t i s p o s s i b l e that f o r some b i r d s the d i f f i c u l t y i n o b t a i n i n g h e r r i n g and sandlance increased to the point where garbage was a more "profitable*?, food type. The i n t e r t i d a l food was not a good a l t e r n a t i v e because i t s a v a i l a b i l i t y i s l i m i t e d to the s i n g l e period of low t i d e each day. There i s another-possible reason as to why the b i r d s used more garbage l a t e r i n the season. I t was noted that when the a d u l t b i r d s r e g u r g i t a t e d garbage, the c h i c k s o f t e n would only peck at the food and t u r n away. I f they did eat i t , i t was not with the same gusto with which they would eat n a t u r a l food. P o s s i b l y as the chicks*; food demands increased with age, the a d u l t s found i t i n c r e a s i n g l y d i f f i c u l t to s a t i s f y them with n a t u r a l foods. When feeding the older c h i c k s with r e f u s e , the a d u l t s received s i g n a l s from the c h i c k s (e.g. not e a t i n g the food r e g u r g i t a t e d ) i n d i c a t i n g that they were s a t i a t e d . In a c t u a l f a c t the c h i c k s would not be g e t t i n g as much food as they needed. Sjaans (1971) found that c h i c k s which were fed both n a t u r a l food and garbage grew much b e t t e r than those c h i c k s 71 fed only n a t u r a l food. No data were a v a i l a b l e i n t h i s study i n order t o check t h i s p o s s i b i l i t y . However, s e v e r a l reasons lead me to b e l i e v e that c h i c k s fed garbage i n a d d i t i o n to n a t u r a l food i n t h i s study would not grow b e t t e r . (1) The very long time reguired to obtain garbage r e l a t i v e to n a t u r a l foods should reduce the number of feeding t r i p s which could be made by the a d u l t s per day. This would reduce the t o t a l amount of food that could be brought i n any p a r t i c u l a r day. (2) The l i m i t e d data (presented on page 58) a l s o i n d i c a t e d that l e s s refuse was brought back per foraging t r i p than h e r r i n g . (3) L a s t , I do not think that the q u a l i t y of the ref u s e was comparable to that of f i s h . However, I have no data on t h i s aspect. In Spaahs» study, part of the refuse fed the c h i c k s was f i s h and f i s h o f f a l discarded by the commercial f i s h i n g i n d u s t r y . I f t h i s i s e a s i l y obtained, then i t i s not s u r p r i s i n g that c h i c k s fed t h i s , along with other refuse and n a t u r a l food, grew b e t t e r than those fed n a t u r a l food alone. In a d d i t i o n the refuse dumps were much c l o s e r to the breeding colony than was the case f o r t h i s study. Some of the g u l l s i n t h i s study were probably f o r a g i n g f o r refuse i n Sidney (only f i v e miles away). However t h i s place could not provide food f o r very many g u l l s . Also some b i r d s may forage i n V i c t o r i a or F r i d a y Harbour, both of which are about 15 miles d i s t a n c e from Mandarte/. The nearest l a r g e source of a v a i l a b l e refuse i s the Vancouver refuse dump some 30 miles away.. The long fo r a g i n g absences suggested t h a t they were i n f a c t using t h i s 72 source. Also; s i g h t i n g s have been made at t h i s s i t e cf a d u l t s which had young on Mandarte. Spaans a l s o found that the supernormal broods were fed more garbage than normal broods. This trend a l s o occurred i n t h i s study but cannot be explained on the b a s i s that i t i s a b e t t e r food source. On C l e l a n d and QCI only n a t u r a l foods were used. Thus the foraging t,imes given i n Table 12 f o r those i s l a n d s were the times r e q u i r e d by the a d u l t s to obtain n a t u r a l food. A comparison of the f o r a g i n g absences f o r n a t u r a l foods between Cl e l a n d S QCI and Mandarte shows that on Mandarte the b i r d s a l s o took much longer to o b t a i n n a t u r a l food than did b i r d s on C l e l a n d or QCI. The d i f f e r e n c e may be due to a d i f f e r e n c e i n the a c t u a l amount of food a v a i l a b l e and/or a d i f f e r e n c e i n the numbers of g u l l s competing f o r the a v a i l a b l e food. For Mandarte, besides the 4000 g u l l s breeding there, there were an a d d i t i o n a l 3000 to 3500 g u l l s breeding on other i s l a n d s i n the general area (Drent, personal communication).. This means that f o r Mandarte there were between seven and e i g h t thousand g u l l s hunting f o r food w i t h i n the same general area. On Cleland t h i s number was probably l e s s than four thousand. (iv) D i u r n a l Changes i n Use of Food: The foraging day (0400 to 2200 hours) was d i v i d e d i n t o nine two hour i n t e r v a l s . The time at which an a d u l t returned with a p a r t i c u l a r food type during the day was assigned to one of the nine i n t e r v a l s 73 on the ba s i s of the food type- This was done f o r a l l t r i p s i n v o l v i n g h e r r i n g , sandlance, r e f u s e , and i n t e r t i d a l organisms. The number of returns per i n t e r v a l f o r the d i f f e r e n t food types i s shown i n f i g . 12. More he r r i n g were brought back i n the 0600 - 0800 i n t e r v a l than f o r subseguent periods during the day. The peak i n returns f o r garbage did not s t a r t u n t i l the 1000 to 1200 hour i n t e r v a l . Sandlance and i n t e r t i d a l organisms were brought back i n more or l e s s equal q u a n t i t i e s throughout the day. Di s c u s s i o n : I t would appear t h a t there were d i f f e r e n c e s i n the frequency at which d i f f e r e n t foods ( e s p e c i a l l y h e r r i n g and refuse) were used throughout the day. The high frequency f o r her r i n g i n the e a r l y morning, r e l a t i v e to the r e s t of the day, may be r e l a t e d to i t s a v a i l a b i l i t y . Outram (1965) i n d i c a t e d that these f i s h , which feed on plankton, are s t i l l near the surface at daybreak but, l i k e the plankton, move to deeper depths as the l i g h t i n t e n s i t y i n c r e a s e s . T h is could i n part e x p l a i n why more h e r r i n g are caught i n the e a r l y morning. I f h e r r i n g a v a i l a b i l i t y decreases l a t e r i n the day, then perhaps t h i s e x p l a i n s why more of the subsequent f o r a g i n g t r i p s made by the g u l l s are to refuse s i t e s . The i n t e r t i d a l r e s u l t s do not mean that i n t e r t i d a l food was a v a i l a b l e a l l day. This e f f e c t could be produced by the f a c t t h a t the time of low t i d e advances each day, and over a period of s e v e r a l weeks, a low t i d e w i l l have occurred at each of the i n t e r v a l s during one of the days. 74 Figure 12: Numbers of foraging trips for different food types classified according to the time of day the adults returned (Mandarte, 1971) . 74a 70-1 60 50 Herr ing 40 -30 cn 20-Q_ • — 4 cr io-- O 20- Sandlance CT U 10-10-Intert idal J 1 Ref use 10-I I I 00 I O I C N J 00 O C M « - C N < M I I I ID OO O «- CM T I M E O F D A Y (hours xWO) 75 E D i s c u s s i o n : E v o l u t i o n of C l u t c h - S i z e The data presented i n chapter two f o r Cleland and QCI d i d not support L a c k , s hypothesis. l a c k predicted that a d u l t s should should not be able to adequately feed young i n broods l a r g e r than normal. In t h i s study, the a d u l t s were able to adequately feed both normal and supernormal broods using only n a t u r a l food. In a d d i t i o n the l i m i t e d number of s i g h t i n g s of j u v e n i l e s from Cleland suggested that c h i c k s from supernormal broods survived as w e l l as those from normal broods. A p o s s i b l e explanation f o r the above r e s u l t s concerns the nature of the food supply. The P a c i f i c Sandlance was the major food fed t o the c h i c k s on C l e l a n d . At present t h i s s pecies appears to be very abundant around both Cleland and QCI, as i t was common to see g u l l s feeding on schools of these f i s h w i t h i n a mile of the colony. Casual communication with fishermen suggest t h a t t h i s i s the case along much of the B r i t i s h Columbia coast. However, to my knowledge no s p e c i f i c data . are a v a i l a b l e on whether the sandlance has been as abundant i n the past as i t i s today. P r i t c h a r d and Tester (1944) found that i n the period 1939-41 the sandlance formed up to 40 percent of the summer d i e t of s p r i n g and cohoe salmon Oncorh^nchus along the west coast of B.C.. Their study included both the west coast of Vancouver I s l a n d and the Queen Ch a r l o t t e I s l a n d s . Ho data were c o l l e c t e d by P r i t c h a r d and 76 Tester to i n d i c a t e whether the sandlance was used by salmon i n the winter as w e l l . & paper by Macer (1966) on the sandlance i n the North Sea i n d i c a t e d t h a t these species may only be a v a i l a b l e i n dense schools during the spawning season i n the summer. I f sandlance i s only a seasonal food item f o r the salmon, then they might only be expected to use them i f they were abundant. A major d e c l i n e i n those salmon species preying on sandlance due to human f i s h i n g pressure could r e s u l t i n an increase i n the numbers of sandlance. However, an examination of catch s t a t i s t i c s f o r B r i t i s h Columbia d i d not i n d i c a t e any major d e c l i n e s i n the numbers of salmon caught since 1920. In a d d i t i o n , other species of f i s h are known to prey on sandlance, although t h e i r impact on the population i s unknown. These predators i n c l u d e the Dogfish Shark Sgualus s u c k l e y ! (Chatwin and F o r e s t e r , 1953), and the Cod and Whiting (Gadidae) (Hart, 1949). P r i t c h a r d and Tester a l s o showed that the h e r r i n g formed up to 50 percent of the d i e t of these salmon. In recent years the h e r r i n g population on the west coast has d e c l i n e d (Taylor 5, 1964; Outram and Haegele, 1969). This could have two e f f e c t s . F i r s t , t h i s could r e s u l t i n present salmon stocks feeding even more h e a v i l y on sandlance. Second,* the g u l l s which now r e l y h e a v i l y on sandlance on C l e l a n d , may have used more herring i n the past when t h i s f i s h was more abundant. I n t e r e s t i n g l y , on 77 QCI h e r r i n g formed a l a r g e part of the g u l l s d i e t and Taylor (1964) s t a t e d that h e r r i n g populations i n that area were one of the l e a s t e x p l o i t e d . A change i n the a v a i l a b i l i t y of n a t u r a l food and s p e c i f i c a l l y the sandlance, cannot be discounted completely i n e x p l a i n i n g the r e s u l t s obtained f o r Clel a n d and QCI. However, c i r c u m s t a n t i a l evidence suggests that an abundant food supply i n the form of sandlance aijd/or h e r r i n g has a l s o been a v a i l a b l e to the g u l l s i n the past. Cody (1971) c r i t i c i z e d the type of experiment performed i n t h i s study because the food supply was not measured. In t h i s study only a s m a l l f r a c t i o n of the t o t a l numbers of p a i r s on the colony were given e x t r a c h i c k s . I t i s p o s s i b l e that i f a l l the p a i r s on the colony had four or more c h i c k s , the ad u l t s would not be able to f i n d s u f f i c i e n t food to r a i s e the extra c h i c k s as occurred on Mandarte. Considering the s i z e of many of the c o l o n i e s along the west coast (generally s m a l l ) , the distances s e p a r a t i n g them,; and the ease at which a d u l t s obtained food on Cleland and QCI, I suspect that i f every p a i r s t a r t e d out with a four egg c l u t c h (assuming that they could incubate them) they would s t i l l be able to adeguately feed the r e s u l t i n g c h i c k s . However/ no data are a v a i l a b l e to t e s t t h i s at the moment. Wynne-Edwards (1962) and Skutch (1967) have argued that reproductive r a t e s have evolved v i a group s e l e c t i o n to correspond to the low m o r t a l i t y r a t e s . This i n turn has 78 prevented the b i r d s from o v e r e x p l o i t i n g t h e i r food resources. However, I f i n d i t d i f f i c u l t to v i s u a l i z e how t h i s type of s e l e c t i o n could e l i m i n a t e the i n d i v i d u a l producing a l a r g e r number of e q u a l l y v i a b l e o f f s p r i n g . Mountfordfs (1968) suggestion concerning the coincidence of the most productive and most common c l u t c h - s i z e does not apply i n t h i s study. When the frequency d i s t r i b u t i o n of c l u t c h - s i z e i s s t r o n g l y skewed to the r i g h t as i n the Glaucous-winged G u l l (Drent et a l . , 1964), the most productive and most common c l u t c h - s i z e have to c o i n c i d e . In t h i s study the two d i d not c o i n c i d e f o r C l e l a n d where a s i x brood (never found n a t u r a l l y ) was the most prod u c t i v e . another explanation of the r e s u l t s concerns the p o s s i b l e r e l a t i o n s h i p between a d u l t m o r t a l i t y and reproductive e f f o r t . Adult m o r t a l i t y , as suggested by Williams (1966) and Charnov and Krebs ( i n prep), could have a major a f f e c t on the t o t a l number of o f f s p r i n g produced by an i n d i v i d u a l during i t s l i f e s p a n . This would apply e s p e c i a l l y to long l i v e d b i r d s such as the Glaucous^«inged G u l l which has an annual m o r t a l i t y of 10 percent (Vermeer, 1963). I f a l a r g e r brood has a higher a d u l t m o r t a l i t y a s s o c i a t e d with i t , then f i g . 13 (a f t e r Charnov and Krebs) i l l u s t r a t e s the r e l a t i o n s h i p that would have to e x i s t between brood s i z e and ad u l t m o r t a l i t y i n order to account f o r an optimal brood s i z e of three young on Clel a n d . The numbers of c h i c k s s u r v i v i n g to breeding f o r the 79 d i f f e r e n t brood s i z e s are p l o t t e d a g a i n s t ^ r c o d s i z e . This curve represents the r e l a t i v e c o n t r i b u t i o n to future breeding populations by each brood s i z e . The values are those given i n chapter two. The other curve ( l a b e l l e d as g i n f i g . 13) i s a h y p o t h e t i c a l p l o t of a d u l t m o r t a l i t y against brood s i z e i n order to produce the maximum f i t n e s s f o r a brood of three young. Here f i t n e s s i s the d i f f e r e n c e between chick s u r v i v a l and a d u l t m o r t a l i t y . In t h i s case the r a t e of a d u l t m o r t a l i t y has to increase with i n c r e a s i n g brood s i z e . The assumption i s made t h a t annual adult m o r t a l i t y does not change during the b i r d s ' l i f e , but does change with brood s i z e . Paludan (1951) and Ludwig (1967) have data which suggest that a d u l t m o r t a l i t y does not change with age f o r a d u l t g u l l s . There i s some reason to suspect that the adult m o r t a l i t y curve as r e l a t e d to brood s i z e may take on some such form. Several people (Bichhdale, 1947; B e l o p o l s k i i , 1958; F i s h e r , 1967; I n g o l f s s o n , 1967; Mercerj 1968; Hussel, 1972) have shown, th a t a d u l t weight does change during the breeding season. Hussel showed that i n the Snow Bunting the weight l o s s f o r parents with supernormal broods was s i g n i f i c a n t l y l a r g e r than that f o r parents with normal broods. Both B e l o p o l s k i i and Ingolfsson showed a weight l o s s i n g u l l s during the breeding season. I n the Glaucous-winged G u l l the increased amount of energy expended -by a d u l t s ( r e f l e c t e d i n increased time spent foraging) i n r a i s i n g e x t r a c h i c k s could take the form of a higher weight l o s s than normal during breeding. The major 80 CT C 1 2 3 4 5 6 Brood Size F i g . 13: Change i n c h i c k s u r v i v a l to breeding (b) w i t h respect to brood s i z e . The curve f o r adul t m o r t a l i t y (q) w i t h respect to brood s i z e i s a h y p o t h e t i c a l one which would be required i n order f o r a brood of three c h i c k s to be optimum, ( a f t e r Charnov and Krebs, i n prep) 81 moult during and immediately a f t e r breeding could a l s o accentuate t h i s weight l o s s . Hard (1969) found that moult represents a major p r o t e i n demand i n b i r d s . Drent (1967) c a l c u l a t e d that the h e r r i n g g u l l uses 20 to 33 percent of i t s d a i l y net energy i n in c u b a t i n g three eggs. The a d d i t i o n cf a fo u r t h large egg and the accompanying brood patch would inc r e a s e the energy demand even more. Di f f e r e n c e s i n weight between a d u l t s could r e s u l t i n d i f f e r e n c e s i n m o r t a l i t y . Van Tets (1968) noted that the Glaucous-winged G u l l had i t s highest m o r t a l i t y immediately a f t e r the breeding season which i s the time when the b i r d s should be at t h e i r lowest weight. Petfrins (1965) showed t h a t , at l e a s t i n j u v e n i l e great t i t s , the weight of the b i r d s had an i n f l u e n c e on subsequent m o r t a l i t y r a t e s . I f a d u l t weight l o s s e s a s s o c i a t e d with broods l a r g e r than three i n the Glaucous-winged G u l l were c r i t i c a l , then t h i s could be r e f l e c t e d i n a sharp r i s e i n a d u l t m o r t a l i t y . Higher m o r t a l i t y f o r a d u l t s with l a r g e r Broods d i d not appear to occur during the breeding season. Only one instance of m o r t a l i t y was noted among 40 p a i r on Mandarte i n 1971 and that concerned a b i r d with only one c h i c k . Van Tets (1968) a l s o noted that the Glaucous-winged G u l l had a low summer m o r t a l i t y , whereas other species of g u l l s had a r e l a t i v e l y higher one. Coulson (1960) noted that there was a m o r t a l i t y associated with breeding i n the s t a r l i n g . P e r r i n s (1965) found that parents r a i s i n g supernormal broods had a higher 82 m o r t a l i t y i n one year but not i n another. This study i s not the only one i n which a d u l t m o r t a l i t y could be operating as a f a c t o r i n the e v o l u t i o n of c l u t c h - s i z e . Nelson (1964), who found that the Gannet could r a i s e two c h i c k s as w e l l as they could r a i s e one, a l s o noted that the parents with supernormal broods were spending f a r more time f o r a g i n g i n the l a t t e r stages of r a i s i n g t h e i r young than were parents with normal broods. In the Kitt i w a k e (Coulson and White/ 1958), only some of the ol d e r experienced b i r d s r a i s e d the thre e - c h i c k broods. H a r r i s and Plumb (1965) and Pearson (1968) presented data which suggested that the Lesser Black-backed G u l l could r a i s e more than three young much l i k e the Glaucous-winged G u l l . The p o s s i b l i l i t y a l s o e x i s t s that adult m o r t a l i t y does not increase with i n c r e a s i n g brood s i z e . Kluyver (1970) found no r e l a t i o n s h i p between adu l t m o r t a l i t y and brood s i z e i n the Great T i t although he d i d note a decrease i n weight of a d u l t s during breeding (1952). H a r r i s (1966) found that Sooty Shearwaters d i d not los e weight when t r y i n g to feed an e x t r a young. This supports a suggestion by Hussel (1972) that a d u l t feedin-g r a t e s could have evolved along with the brood s i z e . P o s s i b l y a feeding r a t e has evolved which prevents the a d u l t s from overworking themselves i n t r y i n g to feed young, and thus lowering i t s chances of f u t u r e s u r v i v a l . , Bergman (1971) found that the Black Guillemot Cepphus g r y l l e d i d not s a c r i f i c e 8 3 i t s e l f f o r i t s young when the food supply f a i l e d . Hussel (1972) noted that Snow Buntings with supernormal broods (undernourished) s t i l l had a r e s t period even though there was a f u l l 24 hours of d a y l i g h t . Too l i t t l e i s known about adult m o r t a l i t y and i t s r e l a t i o n s h i p to reproductive e f f o r t i n order to come to any conc l u s i o n concerning the Glaucous-winged G u l l i n t h i s study. P r e s e n t l y the importance of a d u l t m o r t a l i t y needs f a r more inve s t i g a t i o n ' . The r e s u l t s from Cleland and QCI do not support Lack's hypothesis as i t pre s e n t l y stands. fts discussed above, a recent change i n the food supply does not appear to be the reason. The i n f l u e n c e of reproductive e f f o r t on a d u l t m o r t a l i t y may be an explanation of why t h i s species has a brood of three and not f o u r . B a s i c a l l y Lack's hypothesis i s s t i l l c o r r e c t but needs to be extended tp i n c l u d e the p o s s i b l e i n f l u e n c e of reproductive e f f o r t on a d u l t m o r t a l i t y i n long l i v e d b i r d s . F Summary This chapter described the food s i t u a t i o n on the d i f f e r e n t i s l a n d s , g i v i n g information on the foods used, i t s a v a i l a b i l i t y , and the time spent foraging f o r i t by the a d u l t s . P r i o r t o chick hatching, a d u l t s were found to be using refuse on Handarte but not on C l e l a n d . Chick r e g u r g i t a t i o n s ; c o l l e c t e d while weighing c h i c k s , showed that 84 refuse was fed to c h i c k s on Mandarte but only r a r e l y on Cleland and QCI. Observations were a l s o made on p a i r s of g u l l s i n t h e i r t e r r i t o r i e s on the colony. These i n d i c a t e d that g u l l s on Mandarte i n 1971 had the longest foraging t r i p (in time) when compared t o g u l l s f o r the other i s l a n d - y e a r s . G u l l s on QCI had the s h o r t e s t f o r a g i n g t r i p s . In 1971 on Mandarte i t was found that the time f o r a fo r a g i n g t r i p increased with the age of c h i c k s f o r both normal and supernormal broods. A three-way a n a l y s i s of variance of for a g i n g t r i p w i t h c h i c k age, food type, and brood s i z e showed that the d i f f e r e n c e s were r e l a t e d to changes i n d i e t of the b i r d s and not age of the c h i c k s . I t was found that more garbage was brought to the ol d e r c h i c k s and that t h i s food type took s i g n i f i c a n t l y longer to obtai n than d i d n a t u r a l foods. I t was noted that the t o t a l amount of time spent fora g i n g each day by p a i r s of b i r d s increased with chick age and with brood s i z e . The number of t r i p s made each day d i d not change with c h i c k age but d i d change with brood s i z e . The r e s u l t s on Cleland and QCI did not support Lack's hypothesis. P o s s i b l e reasons f o r t h i s discrepancy are discussed i n the l a s t s e c t i o n of t h i s chapter. 85 CHAPTER IV Use Of Refuse Dumps By The Glaucous-Winged G u l l A I n t r o d u c t i o n Many g u l l s p e c i e s c o n g r e g a t e i n a r e a s o f human h a b i t a t i o n d u r i n g the w i n t e r months. S e v e r a l s t u d i e s i n d i c a t e t h a t t h i s i s l a r g e l y due t o the presence of open-face garbage dumps. The a t t r a c t i v e f e a t u r e of these dumps i s the tons of e d i b l e r e f u s e (meat! brea d , e t c . ) d i s c a r d e d d a i l y by man and l e f t l y i n g exposed a t these s i t e s . The use of t h e s e food s o u r c e s by the g u l l s i s thought t o be one of t h e major f a c t o r s r e s p o n s i b l e f o r the r e c e n t i n c r e a s e s i n the g u l l p o p u l a t i o n s . T h i s l a r g e f o o d s u p p l y has p o s s i b l y a l l o w e d a l a r g e r number of b i r d s t o s u r v i v e the w i n t e r than was p o s s i b l e b e f o r e . The Glaucous-winged G u l l , which a l s o u t i l i z e s r e f u s e s i t e s as a f o o d s o u r c e , w i n t e r s a l o n g most of the west c o a s t of N o r t h America w i t h l a r g e c o n c e n t r a t i o n s o c c u r r i n g wherever l a r g e urban a r e a s o c c u r . One of the l a r g e r c o n c e n t r a t i o n s of t h i s s p e c i e s o c c u r s i n the lower mainland a r e a of B r i t i s h Columbia. A p p r o x i m a t e l y one m i l l i o n p e o p l e r e s i d e i n t h i s a r e a . Perhaps the l a r g e s t c o n c e n t r a t i o n of w i n t e r i n g g u l l s o c c u r s i n t h e San F r a n c i s c o Bay a r e a where g u l l numbers exceed 200 thousand; however/ t h i s number i n c l u d e s s e v e r a l s p e c i e s of g u l l s o f which t h e Glaucousi-winged G u l l a c c o u n t s f o r r o u g h l y 30 thousand o f the t o t a l ( C o g s w e l l , p e r s o n a l c o m m u n i c a t i o n j . 86 The lower mainland area (described i n chapter one) was s e l e c t e d to serve as the study area because of the la r g e numbers of glaucous-winged g u l l s wintering here and i t s easy a c c e s s a b i l i t y during the winter. The main purpose of t h i s part of the -study was to assess the use made of garbage dumps by these g u l l s i n the study area during the winter, and to r e l a t e t h i s to the t o t a l numbers of g u l l s i n the area. B D a i l y A c t i v i t y During the Winter The two major f o c a l p o i n t s f o r the g u l l s * a c t i v i t y i n t h i s area during the winter were the roosts and the refuse s i t e s . The ro o s t s were located at various points throughout the lower mainland (fig'. 14) and were d i v i d e d i n t o two types. The f i r s t type c o n s i s t e d of l o g booms anchored i n a body of water deep enough such t h a t even at low t i d e i t was s t i l l surrounded by water. Two of the r o o s t s f a l l i n t o t h i s category and are l a b e l l e d as A and 0 on f i g u r e 14. The other type of roost was lo c a t e d where extensive sand and mud f l a t s form as the t i d e recedes. There were three r o o s t s of t h i s type and are l a b e l l e d B, D, and E. Both types of r o o s t s provide good p r o t e c t i o n from t e r r e s t r i a l predators. The second type of ro o s t (mud f l a t s ) makes i t p o s s i b l e f o r the g u l l s to stand cn the shore without having to swim against an outgoing current and s t i l l be able to detect p o t e n t i a l predators from a f a r . The exodus of the g u l l s from these r o o s t s s t a r t e d at 87 F i g u r e 14: Map of winter study area showing l o c a t i o n of r e f u s e dumps, roost s i t e s , and f l i g h t l i n e s between refuse s i t e s and r o o s t s . Refuse s i t e s : 1 - West Vancouver (closed Oct 31/69); 2 - North Vancouver; 3 - Barnet; 4 - Terra Nova; 5 - Leeder; 6 - Port Mann; 7 - Burnaby (closed Oct. 31/69); 8 - Richmond; 9 - Vancouver. 88 f i r s t l i g h t and continued f o r about a two hour period ( f i g . 15). The most r a p i d r a t e of roost departure occurred i n the period of f i r s t l i g h t o c c u r r i n g p r i o r to s u n r i s e . From the roost the g u l l s proceeded to the various daytime s i t e s which i n c l u d e d r e f u s e s i t e s , l o g booms on nearby bodies of water, c i t y parks, f i e l d s , f i s h docks, e t c . . . A sm a l l proportion of the b i r d s as w i l l be shown l a t e r , d id remain on the i n t e r t i d a l areas to feed. A l l the dumps are located e i t h e r along the Fraser River or near Burrard I n l e t ( f i g . 14). A l l are near some source of f r e s h water which i s p r e f e r r e d by the g u l l s over s a l t water. Once the machines on the dumps s t a r t e d o p e r a t i n g , the numbers of g u l l s feeding a t the dump increased r a p i d l y . During the day there was a constant turnover of g u l l s between the refuse s i t e s and the nearby l o a f i n g s i t e s which included logbooms or any large open area. About tiwo hours before dark, the g u l l s s t a r t e d to leave the refuse and l o a f i n g s i t e s and move towards the r o o s t s i t e s . The f l i g h t l i n e s of the g u l l s are shown i n f i g . 14. The peak movement of g u l l s unto the roo s t occurred between sunset and t o t a l darkness ( f i g . 15). Schreiber (1968) noted the same type of behaviour f o r the h e r r i n g g u l l on the east coast of the D.S.A.. The roost counts which w i l l be described i n the next s e c t i o n were always made i n the evening. I t was found that i n the morning the b i r d s began l e a v i n g the r o o s t before 89 30 24 -% 18 12-1 6 MORNING n = 7700 710 , 725 740 755 810 825 840 Tsunrise E V E N I N G 30 24 -i % 18 12-6 -n=9136 1500 1515 153 0 1545 1500 1615 1630 1645 1700 tsunset TIME OF DAY (PDT ) ' Fig. 15: Rate of departure from roosts in morning and rate of arrival at roosts in evening for Dec, 20/68. 90 i t was l i g h t enough to a c c u r a t e l y count them. In the evening a l l the b i r d s coming to the roost could be counted before i t was too dark. C numbers of G u l l s and Their Use of Befuse Dumps For t h i s study, the numbers of g u l l s wintering i n the study area were recorded f o r the months of October through to March over a period of four years (1968-72). The f i r s t year (1968-69) was spent l e a r n i n g the h a b i t s and movements of these b i r d s . As a r e s u l t only incomplete counts were obtained during that season. Boost counts were c a r r i e d out at a l l r o o s t s i n order to obtain estimates of the numbers of g u l l s i n t h i s area. In 1968-69 some r o o s t counts were made i n the morning but because some b i r d s l e a v i n g the ro o s t were missed, a l l subseguent counts were made i n the evening. The counts at roosts C, D, and E were c a r r i e d out at the same time because of p o s s b i l e interchange between these r o o s t s . This was e s p e c i a l l y evident f o r roosts D and E where g u l l s would switch from one roost to another depending on weather c o n d i t i o n s . The numbers of g u l l s at roost C were more constant from count to count. The counts at r o o s t s on E n g l i s h Bay and Burrard I n l e t (roost a and B) were often not done or done on another day. I t was not always p o s s i b l e to o b t a i n s u f f i c i e n t observors to roan a l l the r o o s t s at one time. This d e f i c i e n c y was not p a r t i c u l a r i l y s e r i o u s as 91 there appeared to be l i t t l e interchange between t h i s area and the Fraser River d e l t a . The r e s u l t s of the counts are shown i n Table 16. The majority of the g u l l s i n the study area used roosts C, D, and E l o c a t e d on the Fraser River d e l t a . Only about 12 percent of the b i r d s were found to be using r o o s t s A and B on Burrard I n l e t and E n g l i s h Bay r e s p e c t i v e l y . The winter population of g u l l s i n the study area i n 1969-70 numbered i n the order of 45 to 50 thousand. In 1970-71 and 1971-72 the numbers were probably i n the order of 55 to 60 thousand. During the same time period (1969-72), monthly censuses were made at a l l the known refuse s i t e s i n the study area. The s i z e of the area made i t impossible to cover a l l the s i t e s i n one day. As a r e s u l t the area was di v i d e d i n t o two p a r t s and done on t»o consecutive days. Refuse s i t e s around Burrard I n l e t were done on one day and the remainder were done on the other. Again there appealed to be l i t t l e interchange between these two areas. The r e s u l t s are shown i n Table 17. The numbers of g u l l s using the refuse s i t e s i n the study area increased r a p i d l y i n October and decreased r a p i d l y i n the f i r s t part of March. No counts were made of g u l l numbers i n the study area during the summer months; however, t h e i r numbers were probably s i m i l a r to those found i n March and September. . The majority of the b i r d s were found i n the general v i c i n i t y of the three l a r g e s t roosts 92 Table 16 Boost Counts for the Winter Seasons between 1969 and 1972 For the Lower Mainland Area of B.C. Boost Site Year and Dates of Count 1969 - 1970 Nov. 7 Nov.. 2J[ _ec_ 5 Dec.. 23 Jan. 9 Jan. 23 D 24,000 19,000 25,000 25,500 11,500 19,000 E 9,000 8,000 6,000 14,000 14,500 9, 200 C 4,500 10,000 7,000 - 12,000 12,000 A S B ** ** 8,000 11,000 9,400 6,400 Total 377500 377000 467000 507500 477400 467600 (45,000) (45,000) 1970 - 1971 Oct. 30 Nov. 26 Dec. 2 1 Jan. 28 D 17,000 29,000 11,000 17,000 E 4,000* 11,700 26,000 13,700 C 11,700 17,000 15,700 15,000 A 6 B 8,500 7,400 6,500 4,200 Total 41,600 657100 59,200 49,900 (cont'd) 93 Table 16 (cont'd) D E C ft 8 B Nov. 9 30,500 16,400 T o t a l 46,900 (54,000) 1971 - 1972 Dec,. 7 16,320 19,456 17,000 3,500 *** 567300 (59,000) J a n . r8 11,790 11,339 12,840 ** 367000 (42,000) B i r d s did not use the roo s t . * Count low because of fog ** No count obtained f o r the roost *** Count low. (n) Estimate of numbers expected i f a l l r o o s t s counted. 94 Table 17 Numbers of Gulls Counted at or Near Refuse Sites. Refuse Sites * Date A B C Total 1969 - 1970 Oct. 6 - 7 6060 4730 3740 15530 Oct. 30 - 31 23100 5400 6610 35110 Dec. 1 - 2 17460 8130 7630 33220 Jan. 5 - 6 20470 4070 6280 30820 Feb. 5 - 6 26390 4680 4730 35800 Feb. 26 - 27 19670 7560 2500 29730 Mar. 19 - 20 8670 1095 925 10690 1970 - 1971 Sept . 2 9 - 3 0 9700 1710 2850 14260 Oct. 29 - 30 26500 4780 6 080 37360 Nov. 26 - 27 27380 8560 8030 44030 Dec. 30 - 31 28000 8450 5820 42170 Jan. 27 - 28 29840 5880 2695 38415 Feb. 25 - 26 19000 4620 2625 26245 Bar. 17 10020 4300 ** 14320 (cont'd) 95 Table 17 (cont'd) 1971 - 1972 Nov. 1 - 2 20500 Nov. 30 23700 Jan. 1 - 2 23500 Jan. 27 23500 7050 11880 5000 6390 6860 ** 5510 34410 35580 (42000) 33310 29890 (33000) * 'A* i n c l u d e s the Vancouver and Bichmond re f u s e dumps: »B' i n c l u d e s the Burnaby, T e r r a Nova, Port Mann, and Leeder r e f u s e dumps; ' C i n c l u d e s the West Vancouver, North Vancouver, and Barnet r e f u s e dumps and the Campbell Ave.' F i s h docks. ** no count was made a t t h i s s i t e . (n) Number i n brackets i s estimated numbers at r e f u s e s i t e s . 9 6 as might be expected. The Vancouver and Richmond refuse dumps, which are l o c a t e d very c l o s e together, had the l a r g e s t numbers of g u l l s (20 to 25 thousand) feeding there. Host of these b i r d s were at the Vancouver refuse s i t e which serves both Vancouver and the m u n i c i p a l i t y of Delta. approximately one m i l l i o n pounds of household garbage i s dumped there d a i l y during weekdays. Spaans (1971) showed a c o r r e l a t i o n between the number of g u l l s at a dump and the number of people using i t . In a d d i t i o n the number of g u l l s w i l l a l s o be determined by how q u i c k l y the garbage i s b u r i e d , the si2e of the area over which the garbage i s spread, and whether the l o c a l a u t h o r i t i e s attempt to keep the b i r d s away from the dumps. Table 18 shows a comparison of the numbers of g u l l s counted at refuse s i t e s and those counted at the r o o s t s . The data f o r Burrard I n l e t and E n g l i s h Bay were not included both because r o o s t counts were incomplete and because the roost counts may not have represented the t o t a l numbers of g u l l s i n the area. For t h i s area i t was noted that g u l l s would spend the night r o o s t i n g oi} p i l i n g s , barges, e t c . l o c a t e d throughout the i n l e t area. On the Fraser River d e l t a , approximately 70 percent of the winter g u l l population were using r e f u s e s i t e s f o r feeding. A s i m i l a r percentage was expected f o r b i r d s on Burrard I n l e t and E n g l i s h Bay. As mentioned before, some g u l l s do use the i n t e r t i d a l area f o r f e e d i n g , although only a small p o r t i o n of t h i s area Table 18 Comparison of Roost and Refuse Site Censuses. These Figures Do not Include the Numbers of Gulls for the Rcost and Refuse Sites on Burrard Inlet. Date 1969 - 1970 Dump census Roost census % gulls at refuse sites Nov. 7 28,900 37,500 Dec. 5 25,600 38,000 76.0 67.4 Average = 69.3 Jan. 9 24,500 38,000 64.5 1970 - 1971 , Dump census Roost census % gulls.at refuse sites Nov. 26 35,940 57,700 Dec. 31 36,450 52,700 62.3 69.2 Average = 69.8 % Jan. 28 35,700 45,700 78.0 1971 - 1972 Dump census Roost census % gulls at refuse sites Nov. 9 27,550 46,900 Dec. 7 35,580 52,776 58.7 67.4 Average = 69.7 % Jan. 18 29,890 35,969 83. 1 98 i s exposed during the d a y l i g h t hours. In order to obtain an estimate of these numbers, counts on two sample areas (shown i n f i g . 14) were c a r r i e d out i n 1970-71. One sample s i t e was a four mile length of shore on Boundary Bay. The other s i t e was a s i x mile length of beach around Stanley Park i n Vancouver. The l a t t e r area may not be e n t i r e l y r e p r e s e n t a t i v e , but the b i g g e s t problem i n s e l e c t i n g an area was to f i n d one that was e a s i l y a c c e s s i b l e f o r i t s whole length. The t o t a l length of s h o r e l i n e was c a l c u l a t e d foi? the area between White Bock and West Vancouver i n c l u d i n g Burrard I n l e t and Iaidian arm but excluding the waterfront area i n Burrard I n l e t . The numbers of g u l l s on the Boundary Bay sample area were used to compute the number of g u l l s feeding on the s h o r e l i n e between White Bock and Point Grey. The Stanley Park sample area was used to determine the number of g u l l s f o r the remaining length of s h o r e l i n e . The s h o r e l i n e census accounted f o r only 3300 g u l l s or about f i v e percent of the t o t a l numbers i n the area (Table 19). This number c c u l d have been higher (but s t i l l l e s s than 10 percent). The examination of p e l l e t s r e g u r g i t a t e d by g u l l s at the refuse s i t e s i n d i c a t e d that b i r d s feeding along the i n t e r t i d a l Zones were a l s o v i s i t i n g the refuse s i t e s . These b i r d s a l s o feed on the i n t e r t i d a l to a l i m i t e d extent at n i g h t . Observation i n d i c a t e d that at any one time during low t i d e s (occurring" between 2200 and 0200 hours), up to 300 g u l l s were u t i l i z i n g the mussel beds at the Second 99 Table 19 Numbers of B i r d s Along the I n t e r t i d a l Census Areas (shown i n f i g . 14 ) And the Age Composition of Those B i r d s . Date J u v e n i l e % Age C l a s s Subadult Adult T o t a l Numbers Boundary Bay Nov. 10/70 6.3 9.5 84.2 158 Dec. 9/70 11.4 9.2 79.4 141 Feb. 16/71 18. 1 8.7 73.2 127 S t a n l e y Park Seawall Nov. 17/70 20.5 18.0 61.5 122 Nov. 29/70 17.0 12.9 70. 1 147 Dec. 8/71 13.5 12.9 73.6 163 E x t r a p o l a t i o n of Sample Area to Whole of S t u d j Area,. Average Number on Boundary Bay S i t e : 142 Average number on E n g l i s h Bay S i t e : 144 S h o r e l i n e D i s t a n c e s : White Bock to P o i n t Grey Sample Area 4.0 miles 52.5 miles Number of G u l l s 1860 S h o r e l i n e From P o i n t Grey to West Vancouver Sample Area 6.0 miles 62.0 miles Number of G u l l s 1483 T o t a l Number of G u l l s Caculated to be on the I n t e r t i d a l Area 3340 100 Narrows i n Burrard i n l e t . S i m i l a r observations on Boundary Bay i n d i c a t e d that the b i r d s might a l s o be feeding on the mud f l a t s at n i g h t . However, i t was impossible to get c l o s e enough to the b i r d s to check on t h e i r success r a t e . On Boundary Bay, the b i r d s were g e n e r a l l y very c l o s e together and not a l l a c t i v e l y hunting, suggesting that success may have been very low. The numbers of g u l l s on refuse s i t e s and on the i n t e r t i d a l area together s t i l l leave some 25 percent of the t o t a l number unaccounted f o r . Some of these b i r d s w i l l be u t i l i z i n g r e f u s e sources i n places that were not counted, such as c i t y parks; e t c . . Others w i l l be r e s t i n g i n areas missed during counting but s t i l l f eeding on the dumps. Part of t h i s d i f f e r e n c e w i l l be accounted f o r by the d i f f i c u l t y i n v o lved i n counting l a r g e f l o c k s at a d i s t a n c e . Another source of e r r o r involved the C r o o s t where up to 5000 g u l l s would come down the rijver t o r o o s t from outside the study area. Thus f o r t h i s area at l e a s t 70 percent, of the b i r d s were using refuse sources. Considering the above sources of e r r o r , t h i s f i g u r e w i l l l i e somewhere between 70 and 90 percent. I f the garbage dumps represent an e a s i e r source of food than the i n t e r t i d a l , then one might a l s o expect to f i n d a greater percentage of j u v e n i l e s feeding on the dumps than on the i n t e r t i d a l area. J u v e n i l e s are considered to have more tr o u b l e o b t a i n i n g food than a d u l t s . , Counts were c a r r i e d out 101 i n these two areas to v e r i f y t h i s . The b i r d s were c l a s s i f i e d as j u v e n i l e s , subadults, and a d u l t s on the basi s of t h e i r plumage c h a r a c t e r i s t i c s . The r e s u l t s f o r the i n t e r t i d a l area, which i n c l u d e d a l l b i r d s seen on the census s t r i p , are shown i n Table 19. Only samples of b i r d s were counted a t the refuse s i t e s (Table 20). The r e s u l t s i n d i c a t e d that there were no large d i f f e r e n c e s i n age compostion of the b i r d s found on the i n t e r t i d a l and refus e s i t e s . However; i t i s p o s s i b l e that the j u v e n i l e s and subadults are underepresented i n both areas. Kadlec and Drury (1968) found t h a t , on the east coast of North America, the h e r r i n g g u l l population with a 4.5 percent rate of increase c o n s i s t e d of 15 percent j u v e n i l e s and 17 percent subadults. The Glaucous-winged G u l l has a l s o been shown to have a r a t e of increase of 5 percent (Drent et a l . , 1964). In t h i s study, the values f o r the percentage of j u v e n i l e s found were only s i m i l a r t o those of Kadlec and Drury f o r g u l l s at the North Vancouver refuse dump and at the other dumps when winter numbers were decreasing. Generally i t would appear that the j u v e n i l e s are under represented i n the study area. The reason f o r t h i s discrepancy may be due to a d i f f e r e n c e i n the numbers of j u v e n i l e s and a d u l t s which migrate. This d i f f e r e n t i a l type of migration does occur f o r the her r i n g g u l l on the east coast of North America (Kadlec and Drury, 1968). I t may also be that the Glaucous-winged G u l l population i s no longer i n c r e a s i n g . However, new c o l o n i e s are s t i l l being formed i n the Puget Sound area (MacGregor, personal 102 Table 20 Percentage Age Composition of Gulls at Three Different Refuse Sites. Age aclass L -Total number Date Juvenile Subaault Adult in Samp] % % Vancouver % Refuse Site Oct./69 9.8 8.3 81.9 ' 2813 Dec/69 7.1 8.9 84.0 2315. Jan./70 7.3 10.2 82.5 2190 Feb./70 13. 1 14. 1 72.8 2632 Har./70 i a . a 20.4 65.2 1330 Oct./70 4.7 4.9 90.4 887 Nov./70 5.4 8.9 85.7 3009 Dec/70 7.2 9.4 83.4 2316 Jan./71 6.3 9.2 84.5 2397 Feb./71 9.6 11.7 78.7 1403 Terra Nova Refuse Site Oct./69 11.4 8.4 80.2 2573 Dec/69 12.4 12.0 75.6 1976 Jan./70 18.5 10.8 70.7 1715 Feb./70 20.0 14.9 65. 1 999 Oct./70 11.2 8.3 80.5 881 Sov./70 6. 1 9. 1 84.8' 1417 Dec/70 9.2 7.5 83.3 1180 Feb./71 15.8 12.5 71.7 1277 (continued) 103 T a b l e 20 ( c o n t ' d ) N o r t h V a n c o u v e r R e f u s e S i t e J a n . / 7 0 14.5 15.8 69.7 1318 N O V . / 7 0 14.2 9.4 76.4 1469 D e c / 7 0 13.3 9.2 77.5 980 J a n . / 7 1 12.8 10.1 77.1 1162 Feb./71 20.2 13.1 66.7 1079 104 communication). As w i l l be shown below the c o l o n i e s i n the Georgia S t r a i t - Puget Sound area represent a small part of the t o t a l Glaucous-winged G u l l population. Along the coast cf Alaska i t i s not known whether the population i s i n c r e a s i n g , or how the migration of these b i r d s a f f e c t s the proportions of b i r d s i n d i f f e r e n t age c l a s s e s found i n t h i s area. D Discussion As was shown i n the l a s t s e c t i o n , l a r g e numbers of g u l l s were using refuse s i t e s i n t h i s area. However, Vancouver i s not the only urban area with w i n t e r i n g populations of t h i s s p e c i e s . San Francisco (30,000±), Tacoma (5000±), Por t l a n d (15-20*000), V i c t o r i a (6000±), and many other small urban areas together account f o r a l a r g e number of g u l l s presumably using r e f u s e . The t o t a l number using refuse sources along the coast could be i n excess of 150,000 i n d i v i d u a l s . Some of these g u l l s using the refuse s i t e s w i l l be from c o l o n i e s along the B r i t i s h Columbia coast. However, at l e a s t part and perhaps a la r g e part of these b i r d s have to be from c o l o n i e s along the coast of Alaska. Present breeding populations f o r B r i t i s h Columbia and Puget Sound, Washington l i e i n the 50 thousand p a i r range. These numbers, plus the non-breeding b i r d s associated with them, could only account f o r g u l l s using refuse sources but not those using n a t u r a l food. MacGregor (personal communication) has made s i g h t i n g s 105 on the Vancouver refuse dump of b i r d s banded i n the Kodiak area of Alaska. I s l e i b (personal communication) reported to me that there are large movements of glaucous-winged g u l l s through the Gulf of Alaska between the end of August and mid-November with the peak of movement i n the l a t t e r part of September and i n e a r l y October. This c o i n c i d e s with the l a r g e i n c r e a s e s i n g u l l numbers i n the Vancouver area i n October. S i m i l a r i l y the l a r g e exodus i n t h i s area during March matched the movement through the Gulf of Alaska i n the s p r i n g . Estimates of breeding populations f o r Alaska ( I s l e i b , personal communication) are probably i n excess of 150 thousand p a i r s . W i l l e t (1915) reported three thousand pairs on F o r r e s t e r I s l a n d , Alaska i n 1914. This number i s p o s s i b l y higher now. A breeding colony south of Cordova, Alaska i s estimated by I s l e i b at ten thousand p a i r s . These are but two of numerous other smaller c o l o n i e s s c a t t e r e d along the Alaska c o a s t l i n e . Some of these b i r d s winter i n the Alaska region* u t i l i z i n g both refuse and n a t u r a l food. Sowl and I s l e i b (personal communication) estimated the winter population i n P r i n c e W i l l i a m Sound, Alaska at 40 thousand. I s l e i b f u r t h e r estimates winter numbers i n excess of 100 thousand along the Gulf of Alaska coast. Many of the breeding b i r d s leave the Alaska area during the winter and move south. Some of these b i r d s as discussed above do use refuse s i t e s i n major urban areas. However, the Glaucous-winged G u l l 106 a l s o u t i l i z e s the n a t u r a l food resources along the west coast of B r i t i s h Columbia i n s u b s t a n t i a l numbers. Robertson (1973) reported numbers of glaucous-winged g u l l s of up to ten thousand i n the Gulf I s l a n d s i n B r i t i s h Columbia. These b i r d s were using both human refuse and n a t u r a l food. Robertson reported that numbers were gr e a t e s t i n t h i s area during h e r r i n g spawning i n February and March. Observations by myself and Bw Drent showed that up to 5000 b i r d s were u t i l i z i n g dead salmon on the Fraser and Harrison r i v e r i n November and December. Robertson (study i n progress)! has noted l a r g e numbers of g u l l s (>30 thousand) along the west coast of Vancouver I s l a n d and the east coast of the Queen C h a r l o t t e I s l a n d s . No f i g u r e s are a v a i l a b l e on the a c t u a l percentage of the glaucous-twinged g u l l population using n a t u r a l food during the winter;! however, at present i t appears to be as large as or l a r g e r than the number using r e f u s e . Recent i n c r e a s e s i n refuse sources may be l i n k e d to recent i n c r e a s e s i n glaucous-winged g u l l numbers. Large numbers of g u l l s appeared t o have migrated from the Alaska area i n the past as w e l l when refuse sources were l e s s abundant. Pearse (1923) reported large movements of g u l l s down the S t r a i t of Georgia. Before the advent of the l a r g e garbage dumps and other refuse sources, m o r t a l i t y r a t e s , e s p e c i a l l y those f o r j u v e n i l e s , may have been much higher s i n c e n a t u r a l food would be haEder to obtain than r e f u s e . The s t r a t e g y of a d u l t s feeding j u v e n i l e s away from the colony and 107 l e t t i n g them invade feeding t e r r i t o r i e s as described f o r h e r r i n g g u l l s by Drury and Smith (1968) would c e r t a i n l y have s u r v i v a l value under these c o n d i t i o n s . The occurrence of winter storms would a l s o heighten the e f f e c t of a hard to o b t a i n food supply. Both Drury (1963) and Spaans (1971) found that winter storms caused b i r d s which were using n a t u r a l food sources to switch to refuse s i t e s . In the past these would not be a v a i l a b l e . Perhaps with the increase i n refuse dumps, l e s s e f f i c i e n t feeders were able to obtain s u f f i c i e n t food to s u r v i v e the winter. There i s l i t t l e doubt t h a t refuse s i t e s along the coast a t t r a c t l a r g e numbers of g u l l s , as i s found i n the Vancouver area. For example, when S e a t t l e closed i t s refuse dump near Puget Sound and moved i t i n l a n d away from the water, and thus away from the g u l l s , the numbers of g u l l s i n the area decreased. Audubon b i r d counts i n d i c a t e d a drop i n the l o c a l g u l l population from over 12 thousand g u l l s to around 2 c r 3 thousand. However, i t i s not known whether populations of the Glaucous-winged G u l l could maintain t h e i r present s i z e without refuse s i t e s . The l a r g e numbers using refuse s i t e s suggest that t h i s may be the case; 1 but then i t may a l s o be that present n a t u r a l food sources could support the population. Answers to the above ( questions may be forthcoming as methods of refuse d i s p o s a l are changed. 108 E Summary This part of the study was c a r r i e d out i n the general v i c i n i t y of Vancouver, B r i t i s h Columbia i n order to assess the use made of refuse s i t e s :by the Glaucous-winged G u l l . Counts of nunrbers of g u l l s i n the study area were made at t h e i r r o o s t s . Between 15 and 65 thousand g u l l s were counted i n the various years of the study. Between 70 and 90 percent of these b i r d s were feeding on refuse s i t e s i n the area. Some b i r d s were feeding on the i n t e r t i d a l areas both during the day and at night. Counts of b i r d s with respect to t h e i r age c l a s s ( j u v e n i l e , subadult, adult) i n d i c a t e d that j u v e n i l e s were under-represented i n t h i s area. P o s s i b l e reasons f o r t h i s occurrence are discussed. 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Influence of food d i s t r i b u t i o n and human disturbance on the reprod u c t i v e success of herring g u l l s . 111 Ecology 53:1051-61. Hussel, D. J. T. 1972. Factors a f f e c t i n g clutch-size in a r c t i c passerines. Ecol. Moncgr. 42*317-64. Ingolfsson, A. 1967. The feeding habits of f i v e species of large g u l l s (Larus) i n Iceland. Ph.D. Dissertation, University of Michigan. Kadlec, J. A. and 8. H. Dfury. 1968. Structure of the Hew England Herring Gull population. Ecology 49:6=44-76. Klomp, H. 1970. The determination of clutch-size in birds. Ardea 58:1-124. Kluyver, H. N. 1952. Botes on body weight and time of breeding in the Great T i t Parus mw Major L. Ardea 40:123-41. _ _ _ J _ 1970% Regulation of numbers i n populations of Great T i t s ( Par us nj. Major ). In Dynamics of Populations; Proceedings of the Advanced Study Institute on Dynamics of Numbers in Populations," Oosterbeek, Netherlands. Edited bjj P. j . den Boer and Gi. R. Gradwell. Centre for Ag r i c u l t u r a l Publishing and Documentation, Wageningen. Koelink, A. 1972. Bioenergetics of growth in the Pigeon Guillemot, Cejgjshus dolumba. M.Sc. Thesis, University of B r i t i s h Columbia. Lack, D. 1947. The sig n i f i c a n c e of clutch s i z e . Ibis 89:302-52. . 1948. Natural selection and family size i n the " s t a r l i n g . Evolution 2:95-110. ) 1966'. Population studies of birds. Oxford University Press. 1968* Ecological Adaptations for Breeding in Birds. Methuen, London. Lack, D., J . Gibb, and D. F. Owen. 1957. Survival in r e l a t i o n to brood si z e i n t i t s . J . Zool. (Lond) 128:313-26. Lack, D., and E. Lack. 1952. The breeding behaviour of the Swift. B r i t i s h Birds 45:186-215. Lack, D., and D. F. Owen. 1955. The food of the Swift. J. Anim. Ecol. 24:120-36. Lohrl, H. 1957. Populationsokologische Untersuchungen beim Halsbandschnapper ( Ficedula a l b i c o l l i s ). Bonn. zool. 112 B e i t r . 2*130-77. Ludwig, J . P. 1967. 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E c o i . 35:443-470. N e t t l e s h i p , D. N. 1972. Breeding success of the Common P u f f i n ( F r a t e r c u l a a r c t i c a L.) on d i f f e r e n t h a b i t a t s at Great I s l a n d , Newfoundland. E c o l . Monogr. 42:239-68. Outram, D. N. and C. Haegele. 1969. The time and extent of h e r r i n g spawning along the B r i t i s h Columbia Coast i n 1969. F i s h . Bes. Board Can. Gen. Ser. C i r c . 88. Outram, D. N. 1965. Canada's P a c i f i c Herring. Department of F i s h e r i e s of Canada. Owen, D. F. 1960. The nesting success of the Heron Ardea ci n e r e a i n r e l a t i o n to the a v a i l a b i l i t y of food. J . Zool. (Lond7 133:597-617. Paludan, K. 1951. C o n t r i b u t i o n s to the breeding biology of Larus argentatus and Larus fuscus. Vidensk. Hedd. Dan. N a t u r h i s t . Foren. KBH. 114:1-128. Parsons, J . 1970. B e l a t i o n s h i p between egg s i z e and post-hatching m o r t a l i t y i n the Herring G u l l ( Larus argentatus ). Nature 228:1221-22 Parsons, J . 1971. The breeding biology of the Herring G u l l i-iX-SJi argentjatus. Ph.D. D i s s e r t a t i o n , U n i v e r s i t y of Durham. ~ 113 Paynter,' R. A. 1949. C l u t c h - s i z e and the egg and chick m o r t a l i t y of Kent I s l a n d Herring G u l l s . Ecology 30:146-669 Pearson, T. H'. 1968. The feeding biology of s e a - b i r d species breeding on the Fame I s l a n d s , Northumberland. J . Anim. E c o l . 37:521^53. Pearse, T. 1923. Banding Glaucous-winged G u l l s . With other notes on a colony i n southern B r i t i s h Columbia. Can. F i e l d Nat. 37:132-35. P e r r i n s , C M . 1964. Surviwal of young S w i f t s i n r e l a t i o n , to brood-size. Nature (lond) 201:1147-48. 1965. Population f l u c t u a t i o n s and c l u t c h - s i z e i n the Great T i t , Parus major I . . J . Anim. E c o l . 34:601-47. P r i t c h a r d , A. L. and A. JL. Tester. 1944. Food of s p r i n g and coho salmon i n B r i t i s h Columbia. F i s h . Res. Board Can. B u l l . 65:1-23. R i c e , D. W. and K. W. Kenyon. 1962. Breeding c y c l e s and behaviour of Laysan and Black-footed A l b a t r o s s e s . Auk: 79:517-67. Richdale, L. F. 1947. Seasonal f l u c t u a t i o n s i n weights of penguins and p e t r e l s . Wilson B u l l . 59:160-71. R i c k l e f s , R. E. 1967. A g r a p h i c a l method of f i t t i n g eguations to growth curves. Ecology 48:978-82. Robertson, I . 1971. The i n f l u e n c e of brood s i z e on r e p r o d u c t i v e success i n two species of cormorant, Phalacrofeorax a u r i t u s and P_ P e l a g i c u s , and i t s r e l a t i o n to the ^problem of c l u t c h - s i z e . M.Sc. Thesis, U n i v e r s i t y of B r i t i s h Columbia. Robertson, I . ( i n prep) Predation by f i s h - e a t i n g b i r d s on stocks of the P a c i f i c Herring i n the Gulf I s l a n d s , B.C.. Ms. S c h r e i b e r , R. W. 1967. Roosting behaviour of the Herring G u l l i n c e n t r a l Maine. Wilson B u l l . 79:421-431. So k a l , R. R. and F. J . Rohlf. 1969. Biometry. The p r i n c i p l e and p r a c t i c e of s t a t i s t i c s i n b i o l o g i c a l research. W. H. Freeman and Company, San Fr a n c i s c o . Skutch, A. F. 1967. Adaptive l i m i t a t i o n of the reproductive r a t e of b i r d s . I b i s 109:579-99. 114 Snow, B. K. 1960. The breeding biology of the Shag, PhalacrpGorax a r i s t o t g l i s on the i s l a n d of Lundy, B r i s t o l Channel. I b i s 102:554-759 Spaans, A. L. 1971. On the feeding ecology of the Herring G u l l Barus argentatus Pont, i n the northern part of the Netherlands. Ardea 59:73-188. Summers, K. 1970. Brood-rearing s t u d i e s on the Rhinoceros Au k l e t , Cerorhinca j o n o c e r a t a , at Cleland I s l a n d , B.C.. Unpublished ms. T a y l o r , F. H. C. 1964. L i f e h i s t o r y and present status of B r i t i s h Columbia h e r r i n g stock. F i s h . Res. Board Can. B u l l . 143:1-81. Tets, G. van. 1968. Seasonal f l u c t u a t i o n s i n the m o r t a l i t y r a t e s of three northern and three southern-hemishphere g u l l s . CSIRO W i l d l . Res. 13:1-9. Tinbergen, N. 1953. The Herring G u l l ' s World. C o l l i n s , London. Vermeer, K. 1963. The breeding ecology of the Glaucous-winged g u l l ( Larus glaucescens ) on Mandarte I s l a n d , B.C.. Occasional Papers of the B r i t i s h Columbia P r o v i n c i a l Museum. JNo. 13. 1967i Breeding biology of C a l i f o r n i a and R i n g - b i l l e d G u l l s . Can. W i l d l . Rep. Ser. No. 12. Ward, P. 1965. Feeding ecology of the Black-faced Dioch, fi__!iL§ guelea, i n N i g e r i a . I b i s 107:173-214. 1969* The annual c y c l e of the Yellow-vented Bulbul Pycnpngtus g g i a y i e r i n a humid e q u a t o r i a l environment. J . Z o o l . (Lond) 157:25-45. . W i l l e t , G. 1915. Summer b i r d s of F o r r e s t e r I s l a n d , Alaska. Auk 32:295-305. Wi l l i a m s , G. C. 1966. Natural s e l c t i o n , the c o s t s of rejproduction and a refinement of Lack's p r i n c i p l e . Am. Nat. 100:687-690. Wynne-Edwards, V. C. 1962. Animal Dispersion i n R e l a t i o n to S o c i a l Behaviour. Edinburgh. 115 Appendix 1 Average Growth Bates (g/day) (±SE) for Brood Sizes One to Six Chicks on the Different islands. Brood Size Year 1 2 3 4 5 6 Mandarte 29. 2 26.2 26.3 1961 * ±2. 54 ±2.09 ±2.01 (21) (21) (21) Mandarte 32. 3 31.3 28.5 28. 3 27.4 27.7 1969 ±0. 70 ±0.50 ±0.63 ±0. 58 ±0.62 ±0.70 (336) (506) (509) (54 3) (446) (285) Cleland 36. 5 37.6 33.8 37. 5 36.1** 1969 ±1. 62 1.06 ±0.59 ±0. 77 ±0.94 (45) (132) (230) (240) (143) Cleland 33. 3 34.4 33.2 31. 5 31.0 31.7 1970 ±0. 41 ±0.41 ±0.39 ±0. 56 ±0.59 ±0.59 (491) (584)' (667) (590) (4 86) (577) QCI 32. 9 36.3 36.5 36. 8 34.9 1972 ±1. 27 ±1.14 ±0.69 ±1. 63 ±0.81 (116) (120) (353) (74) (285) i_j ,_. _j «4 i — .„j„j-, ,.„,;,, ,, t i, _- - ,,-j : ; * Growth rates calculated using the average weights with age given i n Vermeer, 1963. ** Data for f i v e and six chick broods combined. (n) Number of chick weights used in c a l c u l a t i o n of growth rates. 116 Appendix 2 Average Asymptotic weight (grams) (±SE) f o r Brood S i z e s One to S i x Chicks on the D i f f e r e n t I s l a n d s . Btood Size Year » " 1 2 3 1 4 5 6 Mandarte 959 941 894 882 756 809 1969 ±32. 4 ±21.5 ±16. 5 ±38.5 ±27.3 ±34.3 H9) (29) (26) (14) (24) (8) Mandarte 877 824 825 810 797 1971 ±28.3 ±30.5 ±31. 8 ±35.9 ±35.9 (22) (27) (24) (11) (9) Cleland 1015* 1015* 1969 ±8. 1 ±21.8 (253) (29) Cl e l a n d 1009 1013 994 997 1011 1020 1970 ±17.8 ±14.8 ±13. 8 ±15.8 ±17.3 ±17.3 i _ : ; , (12) (53) (72) (65) <40) (58) * Normal broods and supernormal broods combined, (n) Number i n brackets i s the smaple s i z e . Appendix 3 Frequency o c c u r r e n c e o f Food Types ( i n p e r c e n t ) w i t h Respect to Age and Brood S i z e f o r Mandarte i n 1971. Food t y p e s f e d t o C h i c k s i~i i_ U ?, 1 , C h i c k Age F i s h * I n t e r t i d a l Refuse (n) Normal Broods (1-3) 0-5 84.2 10.5 5.3 38 6-12 89.5 *».5 4.5# 67 13-19 82.0 4.0 14.0 50 20-26 77.0 6.5 12.9# 62 27* 71.4 4.8 23.8 21 Supernormal Broods (4-6) 0-5 81.6 8.2 8.2# 49 6-12 74.2 12.1 7.9.# 66 13-19 63.2 11.8 23.5# 68 20-26 66.7 4.8 28.5 21 27 + 69.2 7.7 23. 1 13 L , , ,o_4 i .1. -. 1 • ?..;  * F i s h i n c l u d e s both h e r r i n g and s a n d l a n c e . # Remaining p e r c e n t a g e (<6%) a m i x t u r e o f r e f u s e and n a t u r a l food (n) Number o f f o r a g i n g t r i p s i n which the food f e d t o the c h i c k s was seen. 118 Appendix 4 D i f f e r e n t Food Types Recognized i n the P e l l e t s C o l l e c t e d on Mandarte (1969) and Cleland (1970). Food Mandarte Cleland L_ j-_ i i ! ; • - ,. ..- ..-I n t e r t i d a l Balanus sp. * * P o l l i c i p e s polynerus * M y t i l u s sp. * * Asteroidea * * ( s t a r f i s h ) Amph.ineura * * (chitons) Clinocardium sp. * * P o l i n i c e s sp. * * Strongylocentrotus * drobachiensis H a l i o t u s kamtschatkana * P a t e l l a c e a * (limpets) Brachyura * * (crabs) Insecta Hymenoptera * (ants) Refuse * * F i s h bones * * 119 Appendix 5 Food Type Found i n Chick R e g u r g i t a t i o n s and In Observed Feedings of Chicks by Adults. Food Mandarte Cleland QCI •s • • H l- ! • »-•• - "• P e l a g i c Clupea p a l l a s i i * Ammodytes hexapterus * * * Pholidae * Cephalopoda * (squid) Intertidal C l i n o c a r d i u n sp. * * Polychaeta * Euphausiacea * * Brachyura * Refuse Bread 6 meat scraps * O f f a l * 

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