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Aspects of foraging in black oystercatchers (Aves: Haematopodidae) Groves, Sarah 1982

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ASPECTS OF FORAGING IN BLACK OYSTERCATCHERS (AVES: HAEMATOPODIDAE) by SARAH GROVES B. A. B i o l o g y , H a r v a r d C o l l e g e , 1973 THESIS SUBMITTED IN PARTIAL FULFILMENT THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s t h e s i s as co n f o r m i n g t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1982 © Sarah Groves, 1982 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a gree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f 7?:OOL.O Gf)/  The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e V ancouver, Canada V6T 1W5 DE-6 (2/79) i i ABSTRACT I s t u d i e d f o r a g i n g e c o l o g y of b l a c k o y s t e r c a t c h e r s (Haematopus bachmani) i n the r o c k y i n t e r t i d a l . The aims of t h i s study were: 1) t o a n a l y z e prey c h o i c e and p a t c h c h o i c e by a d u l t b l a c k o y s t e r c a t c h e r s and e v a l u a t e how w e l l t h e i r f o r a g i n g performance was p r e d i c t e d by f o r a g i n g t h e o r y ; 2) t o study development of f o r a g i n g i n young o y s t e r c a t c h e r s ; 3) to i n d i r e c t l y examine the r e l a t i o n s h i p between p a r e n t a l f o r a g i n g performance and f i t n e s s by measuring c h i c k growth and c h i c k p r o d u c t i o n . The f o l l o w i n g c o n c l u s i o n s were reached: 1) Prey s e l e c t i o n by o y s t e r c a t c h e r s was g e n e r a l l y as p r e d i c t e d by t h e o r y , but b i r d s showed p a r t i a l p r e f e r e n c e s f o r p r e y . P a t c h c h o i c e f o l l o w e d g e n e r a l t h e o r e t i c a l p r e d i c t i o n s , but p r o f i t a b i l i t i e s a c h i e v e d w i t h i n p a r t i c u l a r p a t c h t y p e s were h i g h l y v a r i a b l e . Reasons f o r t h i s are d i s c u s s e d . 2) Growth and p h y s i c a l m a t u r a t i o n are i m p o r t a n t components i n development of f o r a g i n g . D u r i n g the p e r i o d of t h i s s t u d y , c h i c k s were h e a v i l y dependent on p a r e n t a l f e e d i n g , and the a b i l i t y of c h i c k s t o f o r a g e i n d e p e n d e n t l y d e v e l o p e d a f t e r c h i c k s l e f t t h e i r n a t a l a r e a a t about 50 days of age. 3) C h i c k growth v a r i e d between o n e - c h i c k and t w o - c h i c k broods, and t h i s may be r e l a t e d t o p a r e n t a l f o r a g i n g performance. However, d u r i n g t h i s study c h i c k p r o d u c t i o n was c o n s t r a i n e d by weather and p r e d a t i o n , and no r e l a t i o n s h i p between p a r e n t a l f o r a g i n g performance and f i t n e s s c o u l d "be def i n e d . TABLE OF CONTENTS A b s t r a c t i i L i s t of T a b l e s v i L i s t of F i g u r e s v i i Acknowledgements i x G e n e r a l I n t r o d u c t i o n 1 L i t e r a t u r e C i t e d 5 E x p l o i t i n g a Patchy Environment: a F i e l d Study of B l a c k O y s t e r c a t c h e r s F o r a g i n g i n the Rocky I n t e r t i d a l ..6 I n t r o d u c t i o n 6 Study Area . 8 Study Animal 9 Methods 9 D i s t r i b u t i o n and Abundance of I n t e r t i d a l Organisms ..9 B l a c k O y s t e r c a t c h e r F o r a g i n g B e h a v i o u r 12 R e s u l t s 16 Prey D i s t r i b u t i o n 16 D u r a t i o n s of F o r a g i n g B e h a v i o u r s 21 S e a r c h i n g B e h a v i o u r 23 Prey S e l e c t i o n 25 Prey P r o f i t a b i l i t y 32 C u m u l a t i v e Prey Consumption 36 A l l o c a t i o n of F o r a g i n g Time between Zones 39 D i s c u s s i o n 43 Prey Choice 43 P a t c h C h o i c e and Time A l l o c a t i o n between Zones 46 i v C o n c l u s i o n s 49 L i t e r a t u r e C i t e d 51 Development of F o r a g i n g S k i l l s i n Young B l a c k O y s t e r c a t c h e r s 55 I n t r o d u c t i o n 55 Study Areas • 55 Study Animal 56 Methods . . 57 I n t e r t i d a l Prey Organisms 57 C h i c k Growth -. 57 F o r a g i n g B e h a v i o u r 58 R e s u l t s .' 59 Prey D i s t r i b u t i o n and Abundance 59 C h i c k Growth 59 A l l o c a t i o n of F o r a g i n g Time between Zones 62 Prey S e l e c t i o n by C h i c k s and t h e i r P a r e n t s 63 S e a r c h i n g B e h a v i o u r 72 W i n t e r F o r a g i n g 74 ' ' Prey S t e a l i n g ..." 75 D i s c u s s i o n 77 C h i c k Growth 78 A l l o c a t i o n of F o r a g i n g Time between Zones 78 Prey C h o i c e 79 S e a r c h i n g B e h a v i o u r and Prey H a n d l i n g 79 Prey S t e a l i n g 81 Important P r o c e s s e s i n Development of F o r a g i n g S k i l l s 81 L i t e r a t u r e C i t e d • 83 V Growth, S i b l i n g R i v a l r y , and C h i c k P r o d u c t i o n i n B l a c k O y s t e r c a t c h e r s 86 I n t r o d u c t i o n 86 Study Area 86 Study Animal 87 Methods 88 B l a c k O y s t e r c a t c h e r T e r r i t o r i e s 88 C h i c k Growth and S u r v i v a l 88 C h i c k F e e d i n g 89 R e s u l t s ,.89 T e r r i t o r y F i d e l i t y , C l u t c h S i z e , and B r e e d i n g Success 90 Brood S i z e and C h i c k Growth ..93 Weight D i f f e r e n c e s i n Two-chick Broods ...98 C h i c k S u r v i v a l 101 C l u t c h S i z e and C h i c k P r o d u c t i o n 107 D i s c u s s i o n . . 109 C h i c k Growth and S i b l i n g R i v a l r y ..109 C h i c k S u r v i v a l 111 C h i c k P r o d u c t i o n and P a r e n t a l Investment 112 L i t e r a t u r e C i t e d 114 G e n e r a l C o n c l u s i o n 116 L i t e r a t u r e C i t e d 120 Appendix A . 121 L i t e r a t u r e C i t e d 122 Appendix B ' 123 v i LIST OF TABLES Table I : C o r r e l a t i o n s between Event D u r a t i o n and Time S i n c e S t a r t of a F o r a g i n g Bout 23 Table I I : D u r a t i o n s of Search i n Three I n t e r t i d a l Zones ...25 Table I I I : Rate of E f f e c t i v e Search as C a l c u l a t e d by M u l t i -s p e c i e s D i s c E q u a t i o n f o r 5 Prey Types i n 3 Zones 30 Table IV: Average Bout D u r a t i o n s and P r o f i t a b i l i t i e s i n 3 I n t e r t i d a l Zones 40 Table V: A l l o c a t i o n of F o r a g i n g Time between Zones 63 Table V I : Numbers of D i f f e r e n t Prey Taken by C h i c k s of V a r i o u s Ages 72 Table V I I : I n i t i a t i o n of Chases i n W i n t e r F o r a g i n g F l o c k s .77 T a b l e V I I I : B l a c k O y s t e r c a t c h e r C l u t c h S i z e 91 Table IX: C h i c k P r o d u c t i o n on 11 B l a c k O y s t e r c a t c h e r T e r r i t o r i e s , 1975-1978 93 Table X: R e s i g h t i n g s of B i r d s C o l o r - b a n d e d as C h i c k s 107 T a b l e X I : C h i c k P r o d u c t i o n from One and Two-egg C l u t c h e s ..108 v i i LIST OF FIGURES F i g u r e 1: Map of the Study Area 4 F i g u r e 2: S t r u c t u r e of B e h a v i o u r Data 15 F i g u r e 3: Abundance and Biomass of I n v e r t e b r a t e s i n Quadrats 18 F i g u r e 4: Abundance and Biomass of I n v e r t e b r a t e s i n M u s s e l s Beds 20 F i g u r e 5: R e l a t i v e Prey A v a i l a b i l i t y and Prey C h oice 28 F i g u r e 6: Average Weight of Prey Taken i n Each Zone 32 F i g u r e 7: P r o f i t a b i l i t y of Prey w i t h r e s p e c t t o Prey Weight .35 F i g u r e 8: C u m u l a t i v e Prey I n t a k e d u r i n g F o r a g i n g Bouts ....38 F i g u r e 9: Bout D u r a t i o n i n Each Zone v e r s u s P r o f i t a b i l i t y .42 F i g u r e 10: C h i c k B i l l Lengths and Weights v e r s u s C h i c k Age 62 F i g u r e 11: P r o p o r t i o n of C h i c k F e e d i n g s from P a r e n t s and C h i c k s 66 F i g u r e 12: Average Weights of Prey Taken i n Each Zone v e r s u s C h i c k Age 68 F i g u r e 13: Prey Types S e l e c t e d under 3 C o n d i t o n s ..71 F i g u r e 14: I n t e r - p e c k I n t e r v a l s f o r C h i c k s F o r a g i n g i n each Zone .74 F i g u r e 15: C h i c k Growth 95 F i g u r e 16: S i b l i n g Growth i n the Two Broods w i t h Minimum and Maximum Weight D i f f e r e n c e s 98 F i g u r e 17: S i b l i n g Chases and P a r e n t a l F e e d i n g s 101 •Figure 18: C h i c k S u r v i v a l t o F l y and Weight at 20 Days ....103 v i i i F i g u r e 19: C h i c k S u r v i v a l from H a t c h i n g 105 i x ACKNOWLEDGEMENTS I t i s a p l e a s u r e t o acknowledge the many people who c o n t r i b u t e d t i m e , i d e a s , and energy t o t h i s p r o j e c t . Jamie Smith s u p e r v i s e d the p r o j e c t , and I am g r a t e f u l f o r h i s p a t i e n c e , h e l p i n the f i e l d , and c r i t i c a l comments from s t a r t t o f i n i s h of t h i s work. Steve Borden was the p r i n c i p a l d e s i g n e r of hardware and s o f t w a r e f o r the event r e c o r d e r , but more i m p o r t a n t l y I a p p r e c i a t e S t e v e ' s p h i l o s o p h i c i n s i g h t s i n t o the w o r l d . In the f i e l d , Mary T a i t t and P a t r i c k M i c h i e l gave g e n e r o u s l y of t h e i r time and energy on many o c c a s i o n s d u r i n g t h i s p r o j e c t . B r i s t o l F o s t e r of the B r i t i s h Columbia E c o l o g i c a l Reserves U n i t p r o v i d e d f i n a n c i a l s u p port i n 1977, and Wayne Campbell of the B r i t i s h Columbia P r o v i n c i a l Museum suggested C l e l a n d I s l a n d as a stu d y s i t e i n the f i r s t p l a c e . Members of my r e s e a r c h committee - L a r r y D i l l , Lee Gass, Ray H i l b o r n , C h a r l e y Krebs - and Yoram Yom-Tov p r o v i d e d h e l p f u l s u g g e s t i o n s a t v a r i o u s s t a g e s of t h i s r e s e a r c h and c r i t i c a l l y r ead d r a f t s of t h i s t h e s i s . Don Brandys p r o v i d e d the e l e c t r o n i c e x p e r t i s e t o debug and m a i n t a i n the event r e c o r d e r . Fergus O'Har.a d e s i g n e d a weather-proof h o u s i n g f o r the event r e c o r d e r and f r e q u e n t l y s e r v i c e d .a c h r o n i c a l l y a i l i n g o u t b o a r d motor. Ter e s a T e n i s c i and B i l l Webb h e l p e d s o l v e s e v e r a l major computing problems. D u r i n g the l a s t months of t h i s p r o j e c t M a r i a Weston a c t e d as my agent a t U. B. C , and E. E. Cudby and L. C. Zerr made time ' a v a i l a b l e f o r me to complete • t h i s work. Many i n d i v i d u a l s came on f i e l d t r i p s , h e l p e d w i t h boat X h a n d l i n g , endured l i f e - t h r e a t e n i n g a t t a c k s by g u l l s , and s h a r e d the unique p l e a s u r e s and p a i n s of l i f e on C l e l a n d I s l a n d . I thank my f i e l d a s s i s t a n t s L. D i c k , R. J a r e m o v i c , and L. P a u l l , and I a l s o a p p r e c i a t e the h e l p at v a r i o u s times by A. J . Baker, D. Dog, P. Groves, B. H u t c h i n s , S. Krepp, P. Lee, K. L i n d s a y , S. McCormack, S. McCoy, . J . Myers, R. O l e n i c k , A. Peacock, C. R e d s e l l , L. R i c h a r d s , B. S t i l i n g , C. Whitney.. F i n a l l y , my p a r e n t s Mary Groves and Laurence Groves c o n t r i b u t e d t o t h i s p r o j e c t i n many ways. I am g r a t e f u l f o r t h e i r u n d e r s t a n d i n g and c o n t i n u i n g i n t e r e s t i n b l a c k o y s t e r c a t c h e r s . 1 GENERAL INTRODUCTION How a n i m a l s fo r a g e i n p a t c h y environments i s of c o n s i d e r a b l e i n t e r e s t t o e c o l o g i s t s . A l a r g e and growing body of work i n t h i s f i e l d (Krebs 1978, Pyke e t a l . 1977) a d d r e s s e s t h r e e major i s s u e s i n f o r a g i n g e c o l o g y : 1) f u n c t i o n - what a n i m a l s do i n t h e i r s e a r c h f o r fo o d , 2) mechanism - how a n i m a l s make d e c i s i o n s about f o r a g i n g and how they l o c a t e , h a n d l e , and d i g e s t p r e y , and 3) consequences - the e f f e c t s of f o r a g i n g performance on an a n i m a l ' s f i t n e s s . F u n c t i o n a l d e s c r i p t i o n i s the g o a l of most d a t a and t h e o r e t i c a l s t u d i e s of f o r a g i n g , but such s t u d i e s are not concerned w i t h mechanisms a n i m a l s use i n f o r a g i n g (Krebs e_t a l . 1981). Mechanisms and p r o c e s s e s a n i m a l s use i n f o r a g i n g have, w i t h few e x c e p t i o n s (e. g. O l l a s o n 1980, Waage 1979), been i g n o r e d by e c o l o g i s t s . However, the importance of u n d e r s t a n d i n g f o r a g i n g mechanisms, e s p e c i a l l y t h o s e i n v o l v e d i n d e c i s i o n making - l e a r n i n g , memory, and p e r c e p t i o n - has been s t r e s s e d by O r i a n s (1981). F i t n e s s consequences of f o r a g i n g a re imp o r t a n t because f o r a g i n g performance a f f e c t s how much energy an a n i m a l can i n v e s t i n r e p r o d u c t i o n (Schoener 1971). However, the use of f o r a g i n g performance t o e v a l u a t e f i t n e s s remains 'an - e l u s i v e g o a l . R esearch I d i d on f o r a g i n g e c o l o g y of b l a c k o y s t e r c a t c h e r s (Haematopus bachmani) a d d r e s s e s each of these i s s u e s i n f o r a g i n g e c o l o g y - f u n c t i o n , mechanism, and consequences. .Black o y s t e r c a t c h e r s on C l e l a n d I s l a n d ( F i g u r e 1) near T o f i n o , B r i t i s h 2 Columbia, were the s u b j e c t s of my r e s e a r c h . O y s t e r c a t c h e r s are l a r g e s h o r e b i r d s w i t h p o w e r f u l , l a t e r a l l y compressed b i l l s t h a t enable them t o e x p l o i t a v a r i e t y of w e l l - a r m o r e d marine i n v e r t e b r a t e s . F o r a g i n g p r o f i c i e n c y i s a c q u i r e d g r a d u a l l y w i t h p h y s i c a l m a t u r a t i o n and e x p e r i e n c e . C o n s e q u e n t l y , young c h i c k s are unable t o handle prey n o r m a l l y taken by a d u l t s and depend on p a r e n t a l f e e d i n g u n t i l they d e v e l o p f o r a g i n g p r o f i c i e n c y . T h i s t h e s i s i s d i v i d e d i n t o t h r e e s e c t i o n s concerned w i t h how a n i m a l s f o r a g e i n patchy e n v i r o n m e n t s . The f i r s t s e c t i o n , " E x p l o i t i n g a patchy environment: a f i e l d study of b l a c k o y s t e r c a t c h e r s f o r a g i n g i n the r o c k y i n t e r t i d a l " , e v a l u a t e s f o r a g i n g performance (prey c h o i c e and p a t c h c h o i c e ) of a d u l t b l a c k o y s t e r c a t c h e r s i n terms.of f o r a g i n g t h e o r y . D i s c r e p a n c i e s between t h e o r y and performance of a n i m a l s i n t h e i r n a t u r a l environment a r e d i s c u s s e d . The second s e c t i o n , "Development of f o r a g i n g s k i l l s i n young b l a c k o y s t e r c a t c h e r s " . , a n a l y z e s the development of f o r a g i n g b e h a v i o u r i n young o y s t e r c a t c h e r s and d i s c u s s e s p r o c e s s e s t h a t p l a y i m p o r t a n t r o l e s i n b e h a v i o u r a l development of young b i r d s . The importance of t h e s e p r o c e s s e s i n a d u l t f o r a g i n g b e h a v i o u r i s a l s o d i s c u s s e d . The t h i r d s e c t i o n , "Growth, S i b l i n g R i v a l r y , and C h i c k P r o d u c t i o n i n B l a c k O y s t e r c a t c h e r s " , a t t e m p t s t o r e l a t e p a r e n t a l f o r a g i n g performance t o f i t n e s s i n d i r e c t l y by measuring c h i c k growth and c h i c k p r o d u c t i o n . 3 F i g u r e 1: Map of C l e l a n d I s l a n d , B r i t i s h C olumbia, L a t . 49°10'N; Long. 126°05'W. C L E L A N D I S L A N D H G R A S S S H R U B H I B E A C H • B A R E R O C K METERS 50 100 150 200 5 LITERATURE CITED Krebs, J . R. 1978. Op t i m a l f o r a g i n g : d e c i s i o n r u l e s f o r p r e d a t o r s . In . J . R. Krebs and N. B. Da v i e s ( e d s . ) . B e h a v i o u r a l e c o l o g y , an e v o l u t i o n a r y approach. B l a c k w e l l S c i e n t i f i c P u b l i c a t i o n s . O x f o r d , pp. 23-63. Krebs, J . R., Houston, A. I . , Charnov, E. L. 1981. Some r e c e n t developments i n o p t i m a l f o r a g i n g . In . A. C. K a m i l and T. D. Sargent ( e d s . ) . F o r a g i n g b e h a v i o r : e c o l o g i c a l , e t h o l o g i c a l , and p s y c h o l o g i c a l approaches. G a r l a n d STPM P r e s s , New York. pp.3-18. O l l a s o n , J . G. 1980. L e a r n i n g t o f o r a g e - o p t i m a l l y ? Theor. Pop. B i o l . 18:44-56. O r i a n s , G. H. 198.1. F o r a g i n g b e h a v i o r and the e v o l u t i o n of d i s c r i m i n a t o r y a b i l i t i e s . I_n . A. C. K a m i l and T. D. Sargent ( e d s . ) . F o r a g i n g b e h a v i o r : e c o l o g i c a l , e t h o l o g i c a l , and p s y c h o l o g i c a l approaches. G a r l a n d STPM P r e s s , New York. pp. 389-405. Pyke, G. H., P u l l i a m , H. R., Charnov, E. L. 1977. O p t i m a l f o r a g i n g : a s e l e c t i v e review of t h e o r y and t e s t s . Q. Rev. Bi o l . 5 2 : 1 3 7 - 1 5 4 . Schoener, T. W. 1971. Theory of f e e d i n g s t r a t e g i e s . Ann. Rev. E c o l . S y s t . 2:369-404. Waage, J . K. 1979. F o r a g i n g f o r p a t c h i l y - d i s t r i b u t e d h o s t s by the p a r a s i t o i d , Nemer i t i s canescens . J . Anim. E c o l . 48:353-371. 6 EXPLOITING A PATCHY ENVIRONMENT: A FIELD STUDY OF BLACK OYSTERCATCHERS FORAGING IN THE ROCKY INTERTIDAL INTRODUCTION E c o l o g i s t s view the w o r l d as a c r a z y - q u i l t of r e s o u r c e p a t c h e s . E m p i r i c a l and t h e o r e t i c a l e c o l o g i c a l i n v e s t i g a t i o n s of f o r a g i n g a n i m a l s have been p a r t i c u l a r l y c oncerned w i t h how a n i m a l s e x p l o i t prey i n p a t c h y e n v i r o n m e n t s . Three i s s u e s , prey c h o i c e , p a t c h c h o i c e , and a l l o c a t i o n of time between p a t c h e s , have r e c e i v e d c o n s i d e r a b l e a t t e n t i o n from t h e o r e t i c i a n s and e m p i r i c i s t s . Pyke e_t a_l. (1977) and Krebs (1978) h i g h l i g h t some of t h e s e i n v e s t i g a t i o n s . T h e o r e t i c a l c o n s i d e r a t i o n s (e. g. Schoener 1971; Charnov 1976a) have been u s e f u l i n c r y s t a l l i z i n g key i s s u e s i n f o r a g i n g e c o l o g y and i d e n t i f y i n g i m p o r t a n t v a r i a b l e s t o be measured i n the l a b o r a t o r y and f i e l d . However, t h e o r y i s o n l y as s t r o n g as the assumptions on which i t i s based ( O l l a s o n 1980), and r e a l a n i m a l s must be s u b s t i t u t e d f o r e q u a t i o n s t o t e s t t h e o r y -and i t s u n d e r l y i n g a s s u m p t i o n s . L a b o r a t o r i e s and c a r e f u l l y s e l e c t e d f i e l d s i t u a t i o n s are the t e s t i n g grounds f o r f o r a g i n g t h e o r y . L a b o r a t o r i e s are d i s t i l l a t i o n s of the r e a l w o r l d . They p r o v i d e c o n t r o l l e d environments f o r p r e s e n t i n g a n i m a l s w i t h f o r a g i n g problems and e v a l u a t i n g t h e i r s o l u t i o n s t o these problems. For i n s t a n c e , Smith -and Sweatman (1974) used a l a b o r a t o r y study t o ' t e s t p r e d i c t i o n s about how b i r d s s h o u l d 7 fo r a g e i n pat c h y environments. T h e i r r e s u l t s showed t h a t b i r d s l e a r n e d which p a t c h was most p r o f i t a b l e , but they c o n t i n u e d t o sample o t h e r l e s s p r o f i t a b l e p a t c h e s . Smith and Sweatman's study i s a good example of the u t i l i t y of l a b o r a t o r y environments as t o o l s f o r s t u d y i n g f o r a g i n g , but l a b environments impose severe l i m i t a t i o n s on the s c a l e , c o m p l e x i t y , and, p o s s i b l y , r e a l i t y of f e a s i b l e e x p e r i m e n t s . For example, i n a c a r e f u l l y d e s i g n e d and exec u t e d l a b o r a t o r y study of o p t i m a l f o r a g i n g , Cowie (1977) had to s u b s t i t u t e i n c r e a s e d h a n d l i n g time (prey c o n t a i n e r s t h a t took b i r d s a l o n g time t o open) f o r i n c r e a s e d t r a v e l time between p a t c h e s . F i e l d s t u d i e s a re the r i c h e s t source of i n f o r m a t i o n about how a n i m a l s a c t u a l l y f o r a g e i n pat c h y e n v i r o n m e n t s . For i n s t a n c e , s t u d i e s of redshank, T r i n g a t o t a n u s ( G o s s - C u s t a r d 1977a), p r o v i d e a good p i c t u r e of how the s e b i r d s change t h e i r f o r a g i n g b e h a v i o u r and prey s e l e c t i o n i n response t o v a r i a t i o n s i n prey d e n s i t i e s and a v a i l a b i l i t i e s . Major o b s t a c l e s i n most f i e l d s t u d i e s of f o r a g i n g a re the immensity and c o m p l e x i t y of n a t u r a l e n v i r o n m e n t s . In r e a l l i f e , p r e d a t o r s such as the g r e a t t i t (Parus m a j o r ) , the l a b o r a t o r y r a t of f o r a g i n g s t u d i e s , move through l a r g e t h r e e - d i m e n s i o n a l volumes of space w h i l e f o r a g i n g , d i s t r i b u t i o n s of prey and prey p a t c h e s a re o f t e n d i f f i c u l t t o a s s e s s (Myers e_t a_l. 1980), and many p r e d a t o r s a re d i f f i c u l t t o see and f o l l o w . Zach and F a l l s (1976a) s o l v e d some of the l o g i s t i c problems of f i e l d s t u d i e s by c o n d u c t i n g f o r a g i n g e x p e r i m e n t s w i t h o v e n b i r d s ( S e i u r u s a u r o c a p i l l u s ) i n an outdoor l a b o r a t o r y , a s e t of w a l l s e n c l o s i n g a p i e c e of f o r e s t f l o o r where o v e n b i r d s f o r a g e d . • Zach-and F a l l s ran t h e i r e x p e r i m e n t s i n 8 the outdoor l a b o r a t o r y , but they used a r t i f i c i a l p a t c h e s of a r t i f i c i a l prey because of the d i f f i c u l t y of o b t a i n i n g and working w i t h r e a l p r e y . One way t o overcome the d i f f i c u l t i e s o f t e n e n c o u n t e r e d i n f i e l d s t u d i e s of f o r a g i n g i n patchy environments i s t o s e l e c t as s u b j e c t s h i g h l y v i s i b l e p r e d a t o r s f o r a g i n g i n environments where prey a r e r e l a t i v e l y immobile, prey d i s t r i b u t i o n s can be c h a r a c t e r i z e d by an o b s e r v e r , and p a t c h e s can be d i s t i n g u i s h e d i n the f i e l d . The study p r e s e n t e d here l o o k s a t how a c o n s p i c u o u s b i r d , the b l a c k o y s t e r c a t c h e r (Haematopus bachmani), e x p l o i t s m u l t i p l e t y p e s of p r e y , marine i n v e r t e b r a t e s , i n a r e l a t i v e l y s i m p l e p a t c h y environment, the rocky i n t e r t i d a l . I n t e r t i d a l z o n a t i o n p r e s e n t s a s i t u a t i o n where patc h e s (zones of the i n t e r t i d a l ) can be r e a d i l y i d e n t i f i e d i n the f i e l d . S p e c i f i c a l l y , t h i s s tudy a n a l y z e s p r e y c h o i c e and p a t c h c h o i c e and e v a l u a t e s how w e l l e c o l o g i c a l t h e o r y p r e d i c t s f o r a g i n g b e h a v i o u r of b l a c k o y s t e r c a t c h e r s i n the rocky i n t e r t i d a l . STUDY AREA C l e l a n d I s l a n d ( L a t . 49°10'N.; Long. 126°05'W.) i n Cl a y o q u o t Sound o f f the west c o a s t of Vancouver I s l a n d was the s i t e of- t h i s s t u d y . C l e l a n d i s a l o w - l y i n g , b a s a l t i s l a n d about 7.7 h e c t a r e s i n a r e a . The t i d e s a re mixed s e m i - d i u r n a l and range between extremes of 0.1 metre and 3.9 metres. Low t i d e exposes an e x t e n s i v e i n t e r t i d a l i n h a b i t e d by a d i v e r s e community of marine i n v e r t e b r a t e s (Campbell and S t i r l i n g 1968). A more g e n e r a l survey .o.f the i n t e r t i d a l .zones of the B r i t i s h Columbia c o a s t i s p r o v i d e d by C a r e f o o t (1977). 9 STUDY ANIMAL Bl a c k o y s t e r c a t c h e r s a re l a r g e s h o r e b i r d s ( c a . 600 g) t h a t i n h a b i t r o c k y i n t e r t i d a l a r e a s of the P a c i f i c c o a s t of N o r t h America from the A l e u t i a n I s l a n d s t o B a j a , C a l i f o r n i a . O y s t e r c a t c h e r s a re w e l l - a d a p t e d f o r f e e d i n g on a v a r i e t y of i n t e r t i d a l i n v e r t e b r a t e s . The b i r d s use t h e i r l a t e r a l l y compressed b i l l s (6-7 cm l o n g ) t o l e v e r some prey items ( c h i t o n s and l i m p e t s ) o f f the s u b s t r a t e and t o d i s a b l e o t h e r s (mussels, c r a b s , worms) by s t a b b i n g b e f o r e e a t i n g them. Each year about t h i r t y p a i r s of b l a c k o y s t e r c a t c h e r s breed on C l e l a n d I s l a n d . B i r d s m a i n t a i n b r e e d i n g t e r r i t o r i e s from e a r l y s p r i n g u n t i l a f t e r t h e i r c h i c k s a re f l y i n g i n l a t e summer. T e r r i t o r i e s i n c l u d e n e s t s i t e s and c o n t i g u o u s i n t e r t i d a l f o r a g i n g a r e a s used e x c l u s i v e l y by the t e r r i t o r y h o l d e r s . A few b i r d s f l y t o o f f s h o r e r e e f s t o f o r a g e a t low t i d e s , but most b i r d s f o r a g e o n l y on t h e i r t e r r i t o r i e s d u r i n g the b r e e d i n g season and can be r e a d i l y o b s e r v e d . H a r t w i c k (1976) a l s o s t u d i e d t h i s b l a c k o y s t e r c a t c h e r p o p u l a t i o n and took advantage of the p r o l o n g e d p e r i o d of c h i c k dependence on p a r e n t a l f e e d i n g t o t e s t Royama's (1970) p r o f i t a b i l i t y h y p o t h e s i s . METHODS D i s t r i b u t i o n and Abundance of I n t e r t i d a l Organisms The d i s t r i b u t i o n and abundance of organisms taken as prey by o y s t e r c a t c h e r s were a s s e s s e d by quadrat samples i n i n t e r t i d a l 10 a r e a s on 19 o y s t e r c a t c h e r t e r r i t o r i e s . On extreme low t i d e s between May and l a t e August 1977, q u a d r a t s were sampled a l o n g "randomly s e l e c t e d l i n e s r u n n i n g from the h i g h water to low water l i n e . On each l i n e a 50 x 50 cm q u a d r a t frame was l a i d down a t 1.5 metre i n t e r v a l s and each q u a d r a t was sampled non-d e s t r u c t i v e l y . P e r c e n t cover by Fucus spp., c o r a l l i n e and l a m i n a r i a n a l g a e , s u r f g r a s s ( P h y l l o s p a d i x ) , b a r n a c l e s (Balanus s p p . ) , gooseneck b a r n a c l e s ( P o l l i c i p e s p o l y m e r u s ) , mussels ( M y t i l u s c a l i f o r n i a n u s ) was e s t i m a t e d v i s u a l l y f o r each q u a d r a t b e f o r e i t was d i s t u r b e d . Each quadrat was s y s t e m i c a l l y s e a r c h e d f o r l i m p e t s ( C o l l i s e l l a d i g i t a l i s , C. p e l t a , Notoacmea  scutum), c h i t o n s ( K a t h a r i n a t u n i c a t a , T o n i c e l l a 1 i n e a t a ) , worms ( N e r e i s v e x i l l o s a ) , and amphipod c r u s t a c e a n s ( O r c h e s t o i d e a  c a l i f o r n i a n a , L i q i a p a l l a s i i ) . A l l i n d i v i d u a l s one c e n t i m e t r e or g r e a t e r i n l e n g t h were i d e n t i f i e d and measured t o the n e a r e s t m i l l i m e t r e . When c r a b s (Oedignathus i n e r m i s) were found, c a r a p a c e w i d t h a t the w i d e s t p o i n t was measured and r e c o r d e d . Specimens s h o r t e r than 1 cm were counted but not measured. A t o t a l of 176 q u a d r a t s was sampled. Data from i n t e r t i d a l q u a d r a t s were used t o e s t i m a t e the n u m e r i c a l abundance and t o t a l biomass of v a r i o u s i n v e r t e b r a t e prey of o y s t e r c a t c h e r s . Length-wet weight r e g r e s s i o n s (Appendix A) f o r l i m p e t s , c h i t o n s , worms, c r u s t a c e a n s , and c r a b s were used t o determine the wet weight biomass of prey items i n each q u a d r a t . Data on p e r c e n t cover were used t o a s s i g n q u a d r a t s t o f o u r i n t e r t i d a l zones ( p a t c h t y p e s ) between the h i g h water and low water l i n e s : 1 •) spray zone (SPZO), 2) fucus zone (FUCU) , 3) 11 m y t i l u s zone (MYTL), 4) l a m i n a r i a - p o s t e l s i a zone (LAPO). These zones c o u l d be r e a d i l y i d e n t i f i e d a t a d i s t a n c e by an o b s e r v e r . The f o l l o w i n g c r i t e r i a were used t o a s s i g n q u a d r a t s t o the f o u r zones: 1) spray zone ( a d j a c e n t t o h i g h water l i n e ) - o n l y s m a l l b a r n a c l e s ( Balanus g l a n d u l a ) p r e s e n t ; 2) fucus zone - cover g r e a t e r than 50% Fucus spp.; 3) m y t i l u s zone c o v e r g r e a t e r than 25% Myt i l u s c a l i f o r n i a n u s ; 4) l a m i n a r i a - p o s t e l s i a zone ( a d j a c e n t t o low water l i n e ) - g r e a t e r than 10% l a m i n a r i a n s or k e l p s ( P o s t e l s i a ) . ( T h i s c l a s s i f i c a t i o n scheme i s s i m i l a r t o t h a t of K o z l o f f (1973) and R i c k e t t s and C a l v i n (1968).) I n t e r t i d a l q u a d r a t s were a good way of s a m p l i n g s e s s i l e i n v e r t e b r a t e s and p l a n t s i n the r o c k y i n t e r t i d a l , but h i g h l y m o b i l e prey such as amphipod c r u s t a c e a n s and worms u s u a l l y moved q u i c k l y from the s a m p l i n g a r e a when they were d i s t u r b e d , so the abundance of t h e s e prey t y p e s was u n d e r e s t i m a t e d . T h i s method of s a m p l i n g was a l s o p o o r l y s u i t e d f o r e s t i m a t i n g the abundance of more m o b i l e i n v e r t e b r a t e s l i v i n g i n spaces between mussels i n dense mussel beds. To a s s e s s the abundance of i n v e r t e b r a t e s i n mussel beds, a l l of the mussels and a s s o c i a t e d i n v e r t e b r a t e s i n e l e v e n 25 x 25 cm q u a d r a t s were removed and measured. These data were then used t o e s t i m a t e the n u m e r i c a l abundance and t o t a l biomass of i n v e r t e b r a t e s i n mussel.beds. Some i n v e r t e b r a t e s , e s p e c i a l l y worms, escaped from quadrat s i t e s as c o l l e c t i o n s were b e i n g made, so d a t a from t h e s e q u a d r a t s r e p r e s e n t minimum e s t i m a t e s of numbers and biomass p r e s e n t . N e i t h e r s a m p l i n g method was s u i t a b l e f o r e s t i m a t i n g abundance of c r a b s , Oedignathus i n e r m i s . T h i s c r y p t i c s p e c i e s spends low t i d e p e r i o d s h i d i n g i n the h o l d f a s t s of k e l p 1 2 ( p e r s . obs.) and, i n s p i t e of p e r s i s t e n t e f f o r t s t o f i n d i t , was i n f r e q u e n t l y d e t e c t e d i n the q u a d r a t s . O b s e r v a t i o n of prey taken by o y s t e r c a t c h e r s i n d i c a t e s t h a t t h i s c r a b i s r e l a t i v e l y abundant, but sampling t e c h n i q u e s f a i l e d t o d e t e c t i t and p r o v i d e a r e a l i s t i c e s t i m a t e of i t s abundance. Myers e_t a l . (1980) have warned of the i n h e r e n t dangers i n u s i n g r e s u l t s of prey s a m p l i n g schemes t o e s t i m a t e prey a v a i l a b l e t o a p r e d a t o r . The p i t f a l l s they i d e n t i f y a r e , f o r the most p a r t , a v o i d e d i n t h i s study because 1) most prey are v i s i b l e and d i s t r i b u t e d a c r o s s a r o c k y s u b s t r a t e , 2) most prey are s e s s i l e d u r i n g low t i d e f o r a g i n g p e r i o d s , 3) a few prey types t h a t a r e d i f f i c u l t t o sample because they are v e r y c r y p t i c ( c r a b s ) or v e r y m o b i l e (worms and c r u s t a c e a n s ) have been e x p l i c i t l y i d e n t i f i e d and t h e i r abundances as d e t e r m i n e d by sampling r e c o g n i z e d as u n d e r e s t i m a t e s . B l a c k O y s t e r c a t c h e r F o r a g i n g B e h a v i o u r I n d i v i d u a l o y s t e r c a t c h e r s on seven t e r r i t o r i e s (a subset of the 19 t e r r i t o r i e s used f o r q u a d r a t sampling) were observed f o r a g i n g d u r i n g low t i d e p e r i o d s from 3 hours b e f o r e u n t i l 3 hours a f t e r low t i d e . C o n t i n u o u s r e c o r d s of t h e i r f o r a g i n g b e h a v i o u r were made and i n c l u d e d the f o l l o w i n g : b i r d i d e n t i f i c a t i o n and zone of the i n t e r t i d a l ( p a t c h t y p e ) ; d u r a t i o n of p r e y - h a n d l i n g e v e n t s , prey t y p e , and prey s i z e w i t h r e s p e c t t o the b i r d ' s b i l l l e n g t h ; d u r a t i o n s of s u c c e s s f u l and u n s u c c e s s f u l s e a r c h e s ( p e r i o d s of c o n t i n u o u s s e a r c h ending w i t h prey c a p t u r e s and w i t h o u t prey c a p t u r e s , r e s p e c t i v e l y ) ; d u r a t i o n s of n o n - f o r a g i n g b e h a v i o u r s such as s i t t i n g , s t a n d i n g , 13 and f l y i n g t h a t occur d u r i n g p e r i o d s of f o r a g i n g . Prey s i z e s were e s t i m a t e d as f r a c t i o n s of a b i r d ' s b i l l l e n g t h (7 s i z e c l a s s e s ) , and p r e y biomass was e s t i m a t e d u s i n g l e n g t h - w e i g h t r e g r e s s i o n s . Measurements of the remains of prey eaten by o y s t e r c a t c h e r s were v e r y s i m i l a r t o those r e c o r d e d d u r i n g f o r a g i n g o b s e r v a t i o n s , and gave c o n f i d e n c e i n the s i z e e s t i m a t e s . Data were encoded w i t h a d i g i t a l event r e c o r d e r and s t o r e d on magnetic tape (on a Uher tape r e c o r d e r ) f o r subsequent decoding by a PDP-11 computer. The event r e c o r d e r was one I b u i l t w i t h minor m o d i f i c a t i o n s t o the event r e c o r d e r d e s c r i b e d by Gass (1977). Data were r e c o r d e d as a sequence of p o i n t e v e n t s (e. g. peck, pace, a t t a c k p r e y , s w a l l o w , s i t , f l y , e t c . ) and t h e i r . t i m e s of o c c u r r e n c e . L o g i c a l r u l e s t h a t d e f i n e d e v e n t s of b i o l o g i c a l i n t e r e s t w i t h b e g i n n i n g s , e n d i n g s , and d u r a t i o n s were a p p l i e d t o the sequence of p o i n t e v e n t s . The r e s u l t was a sequence of b e h a v i o u r a l e v e n t s (e. g. s u c c e s s f u l s e a r c h , u n s u c c e s s f u l s e a r c h , prey h a n d l i n g , e t c . ) , t h e i r b e g i n n i n g t i m e s , and d u r a t i o n s . F i g u r e 2 summarizes the r e l a t i o n s between p o i n t e v e n t s and b e h a v i o u r a l e v e n t s . B e h a v i o u r a l e v e n t s and t h e i r d u r a t i o n s were a n a l y z e d s t a t i s t i c a l l y w i t h the MIDAS ( M i c h i g a n I n t e r a c t i v e Data A n a l y s i s System) s t a t i s t i c a l package s u p p o r t e d by the Computing C e n t r e a t the U n i v e r s i t y of B r i t i s h Columbia and by F o r t r a n programs t h a t I wrote. The m u l t i - s p e c i e s d i s c e q u a t i o n (Murdoch and Oaten 1975) was used t o a n a l y z e prey c h o i c e . The f o r a g i n g d a t a p r e s e n t e d here are f o r a d u l t . b i r d s on .breeding t e r r i t o r i e s . H a r t w i c k ( 197:6) .showed t h a t compos i t i o n of 1 4 F i g u r e 2: An example of a b e h a v i o u r a l d a t a r e c o r d - sequence of p o i n t e v e n t s encoded by event r e c o r d e r and sequence of b e h a v i o u r a l e v e n t s d e f i n e d by l o g i c a l r u l e s (see t e x t f o r d e t a i l s ) . S T R U C T U R E of BEHAVIOUR DATA SEQUENCE OF SEQUENCE OF TIME OF BEHAVIOURAL POINT E V E N T S OCCURRENCE E V E N T S S T A N D V P A C E P A C E P E C K I P A C E ATTACK P R E Y V P E C K L I M P E T A 1 / 3 T9 P E C K S W A L L O W P A C E Tl2 P E C K T13 P A C E Tl4 P A C E T15 P E C K T16 S I T T17 P A C E % \ S T A N D S U C C E S S F U L " S E A R C H < - P R E Y - H A N D L I N G < U N S U C C E S S F U L " S E A R C H LOGICAL R A W DATA ENCODED BY E V E N T RECORDER R U L E S y—SIT STARTING TIME T6 T11 Tl7 DURATION P R E Y T Y P E T11-T6 L I M P E T T17- Tn Tift " T17 ! DATA F I L E OF BEHAVIOURAL E V E N T S , STARTING TIMES, AND DURATIONS 16 a d u l t o y s t e r c a t c h e r d i e t s depended on whether or not b i r d s had c h i c k s and t h a t the presence of c h i c k s a f f e c t e d the f r e q u e n c y and l e n g t h of f o r a g i n g t r i p s made by p a r e n t o y s t e r c a t c h e r s . The p resence of c h i c k s confounds the a n a l y s i s of a d u l t f o r a g i n g , so o n l y a d u l t s h o l d i n g t e r r i t o r i e s but w i t h o u t c h i c k s were i n c l u d e d i n t h i s a n a l y s i s . These b i r d s were c o m p l e t i n g c l u t c h e s of eggs, l a y i n g replacement c l u t c h e s , or i n c u b a t i n g eggs. RESULTS Prey D i s t r i b u t i o n The f o u r zones ( p a t c h t y p e s ) of the i n t e r t i d a l ( s p r a y zone, fuc u s zone, m y t i l u s zone, l a m i n a r i a - p o s t e l s l a zone) d i f f e r e d markedly i n numbers and t o t a l biomass (wet w e ight) of i n v e r t e b r a t e prey p r e s e n t . F i g u r e 3 summarizes the q u a d r a t sample d a t a a c c o r d i n g t o zone of the i n t e r t i d a l and prey type (except m u s s e l s ) . Numbers and biomass of prey i n c r e a s e d d r a m a t i c a l l y between the h i g h water ( s p r a y zone) and low water ( l a m i n a r i a - p o s t e l s i a zone) l i n e s . In a d d i t i o n , c e r t a i n t y p e s of prey such as c h i t o n s and c r a b s were found o n l y i n the lower zones of the i n t e r t i d a l . The abundance and t o t a l biomass of prey (except mussels) sampled i n 11 mussel bed q u a d r a t s are summarized i n F i g u r e 4. The number of p r e y organisms l i v i n g i n mussel beds was h i g h and q u i t e v a r i a b l e . Most of t h e s e i n v e r t e b r a t e s were s m a l l (1 cm or s m a l l e r ) , and .some of them were p r o b a b l y not a v a i l a b l e as p r e y t o o y s t e r c a t c h e r s as mussel beds may be 10-15 cm i n d e p t h , 1 7 F i g u r e 3: Average n u m e r i c a l abundance and biomass of i n v e r t e b r a t e s (except mussels) i n 176 50 x 50 . cm i n t e r t i d a l q u a d r a t s on 19 o y s t e r c a t c h e r t e r r i t o r i e s . ( V e r t i c a l l i n e s a r e upper 95% c o n f i d e n c e l i m i t s ) . II CP Z II t n I I cn n t n m > o c z p o z ! Z r m e oo -Or-O ^ -mr:> >> \ LIMPETS CRUSTAC. s s v w o i a L I M P E T S s s v w o i a L I M P E T S CHJTO NS WORMS \ L I M P E T S C R U S T A C . CHITONS C R A B S SSVW0I8 o -+-NUMBER o o o + ± CO 2 ( s w v a o ) o SSVNOIB o 19 F i g u r e 4: Average n u m e r i c a l abundance and biomass of i n v e r t e b r a t e s (except mussels) i n 11 25 x 25 cm mussel bed q u a d r a t s . ( V e r t i c a l l i n e s a re upper 95% c o n f i d e n c e l i m i t s ) . SEA C U C U M B E R S CRUSTAC. CHITONS L IMPETS WORMS SSVWOIB tn o N U M B E R o o t n o o o o CP * o o ( s w v y 9 ) S S V H 0 I 8 ho O O "I O 21 deeper than an o y s t e r c a t c h e r ' s b i l l . The number of mussels (not i n c l u d e d i n F i g u r e 3) i n each quadrat averaged 355.8 ( s . d . = 163.7) w i t h an average l e n g t h of 3.8 cm ( s . d . = 0.75). Because of the l a y e r i n g of mussels i n mussel beds, o n l y about o n e - t h i r d t o o n e - h a l f of the mussels were a c t u a l l y a c c e s s i b l e t o the b i r d s , but no q u a n t i t a t i v e e s t i m a t e of t h i s was o b t a i n e d . C o r r e l a t i o n s between p a r t i c u l a r t y p e s of c o v e r and the presence of prey may p r o v i d e b i r d s w i t h i n f o r m a t i o n about the average q u a l i t y of patches of the i n t e r t i d a l . Biomass of prey i n q u a d r a t s was c o r r e l a t e d w i t h the amount of some t y p e s of c o v e r . Amounts of c o r a l l i n e a l g a e and l a m i n a r i a n a l g a e were p o s i t i v e l y c o r r e l a t e d w i t h the biomass of prey i n q u a d r a t s ( c o r a l l i n e a l g a e , r = 0.55; l a m i n a r i a n a l g a e , r = 0.61; df = 175, p<0.0l f o r both t y p e s of a l g a e ) . Amounts of cover of Fucus spp., gooseneck b a r n a c l e s , and s u r f g r a s s , were not c o r r e l a t e d w i t h the biomass of prey i n q u a d r a t s . D u r a t i o n s of F o r a g i n g B e h a v i o u r s An a n i m a l ' s f o r a g i n g b e h a v i o u r may change a f t e r i t has f o r a g e d f o r a p e r i o d of t i m e . For i n s t a n c e , s a t i a t i o n may e f f e c t f o r a g i n g b e h a v i o u r by c a u s i n g an a n i m a l to be more s e l e c t i v e i n i t s c h o i c e of prey (Charnov 1976b) or l e s s r e s p o n s i v e t o food (Zach and F a l l s 1978). .If f o r a g i n g b e h a v i o u r changes w i t h the amount of time an a n i m a l has been f o r a g i n g , then the time of o c c u r r e n c e of v a r i o u s f o r a g i n g b e h a v i o u r s w i t h r e s p e c t t o the s t a r t of a f o r a g i n g bout w i l l be an i m p o r t a n t v a r i a b l e i n any a n a l y s i s of f o r a g i n g b e h a v i o u r . In b l a c k o y s t e r c a t c h e r s I examined the p o s s i b i l i t y t h a t d u r a t i o n s of f o r a g i n g b e h a v i o u r s 22 v a r y s y s t e m i c a l l y w i t h the amount of time a b i r d has been f o r a g i n g by t e s t i n g f o r c o r r e l a t i o n s between d u r a t i o n s of d i f f e r e n t f o r a g i n g b e h a v i o u r s and the amount of time" a b i r d had been f o r a g i n g i n an u n i n t e r r u p t e d bout w i t h i n one zone of the i n t e r t i d a l . C o r r e l a t i o n c o e f f i c i e n t s f o r b e h a v i o u r d u r a t i o n v e r s u s time of o c c u r r e n c e f o r s u c c e s s f u l s e a r c h and u n s u c c e s s f u l s e a r c h are shown f o r i n d i v i d u a l b i r d s i n Table I . No r e l a t i o n s between amount of time a b i r d had been f o r a g i n g and d u r a t i o n s of f o r a g i n g b e h a v i o u r s were d e t e c t e d s t a t i s t i c a l l y . In a l l subsequent a n a l y s e s d u r a t i o n s of events a r e t r e a t e d i n d e p e n d e n t l y of time of o c c u r r e n c e . S e a r c h i n g B e h a v i o u r Zonal d i f f e r e n c e s i n prey p r e s e n c e , abundance, and s u b s t r a t e might a f f e c t the way b i r d s s e a r c h f o r p r e y . Two components of s e a r c h i n g b e h a v i o u r were measured i n t h i s s t u d y : i n t e r v a l s between pecks and i n t e r v a l s between paces. I n t e r - p e c k i n t e r v a l s p r o v i d e a measure of the f r e q u e n c y w i t h which b i r d s i n v e s t i g a t e p o t e n t i a l prey items by t o u c h i n g t h e i r b i l l s t o the s u b s t r a t e and prey i t e m s . Average i n t e r - p e c k i n t e r v a l s v a r i e d s i g n i f i c a n t l y between zones ( f u c u s , 3.60 s e e s , s. d. = 3.21, N = 74; m y t i l u s , 4.72 sees, s. d. = 4.67, N = 779; l a m i n a r i a -p o s t e l s i a , 3.72 .sees, s. d. = 3.86, N = 373 ; F = 7.67, df = 2,1225, p < 0.001). I n t e r - p e c k i n t e r v a l s were s h o r t e s t i n the fucus and l a m i n a r i a n - p o s t e l s i a zones where b i r d s o f t e n searched f o r prey t a c t i l e l y by p r o b i n g through l a y e r s of a l g a e . I n t e r -pace i n t e r v a l s p r o v i d e a measure of how f a s t b i r d s moved w h i l e f o r a g i n g . I n t e r - p a c e i n t e r v a l s were s i m i l a r i n a l l zones ( f u c u s , TABLE I Correlations between durations of successful and unsuccessful search and amounts of time individual birds had been foraging* Succ. search Unsucc. search BIRD ZONE £ df r df 1 1 0 mytl -0.02 18 0.47 10 1 1 1 mytl -0.34 1 7 -0.30 3 1 1-1 lapo -0.12 54 -0.18 28 202 mytl -0.02 46 -0.12 48 202 lapo 0.07 28 -0.17 1 5 203 mytl 0.02 1 28 0.08 49 218 lapo 0.03 1 4 -0.45 5 219 mytl 0.58 1 0.91 1 220 - lapo 0.35 1 7 , 0.30 9 222 lapo 0.07 27 -0.26 18 223 lapo 0.09 2 too few cases 224 f ucu 0.05 1 6 - 0 . 6 9 1 224 mytl 0.09 1 6 0.24 17 225 fucu too few cases too few • cases 2.25 mytl 0.32 26 0.06 9 239 fucu 0.45 •3 too few cases 239 mytl 0.01 21 0.35 8 *None of these r-values are s i g n i f i c a n t at the 0.05 l e v e l . 24 1.05 sees, s. d. = 0.65, N = 74; m y t i l u s , 1.08 sees, s. d. 0.62, N = 779; l a m i n a r i a - p o s t e l s i a , 1.10 sees, s. d. = 0.83, N = 373; F = 0.239, df = 2,1225, NS). F o r a g i n g o y s t e r c a t c h e r s a d j u s t e d t h e i r s e a r c h i n g b e h a v i o u r t o the zone i n which they were f o r a g i n g by c h a n g i n g t h e i r i n t e r - p e c k i n t e r v a l s but not i n t e r - p a c e i n t e r v a l s which were r e l a t i v e l y c o n s t a n t between zones. Two measures of d u r a t i o n of s e a r c h were r e c o r d e d : d u r a t i o n of s u c c e s s f u l s e a r c h and d u r a t i o n of u n s u c c e s s f u l s e a r c h ( u n i n t e r r u p t e d p e r i o d s of a c t i v e s e a r c h e n d i n g w i t h a prey c a p t u r e and w i t h o u t a prey c a p t u r e , r e s p e c t i v e l y ) . In a l l zones of the i n t e r t i d a l , the d u r a t i o n of u n s u c c e s s f u l s e a r c h exceeded the d u r a t i o n of s u c c e s s f u l s e a r c h -(Table I I , F = 19.90, df = 728, p < 0.001). However, n e i t h e r the d u r a t i o n of s u c c e s s f u l s e a r c h (F = 0.30:, df ='466, P = 0.74) nor u n s u c c e s s f u l s e a r c h (F = 2.11, df •= 2,260, P = 0.12) v a r i e d s i g n i f i c a n t l y between zones. T h i s i n d i c a t e s t h a t b i r d s respond t o c o n d i t i o n s of prey abundance and s u b s t r a t e c h a r a c t e r i n p a r t i c u l a r zones by v a r y i n g the f r e q u e n c y of s p e c i f i c s e a r c h b e h a v i o u r s ( i n t e r - p e c k i n t e r v a l s ) r a t h e r than the d u r a t i o n of u n i n t e r r u p t e d s e a r c h . Prey S e l e c t i o n P r e d a t o r s s h o u l d become more s e l e c t i v e i n t h e i r c h o i c e of prey as p r e y abundance i n c r e a s e s (Schoener 1971). R e l a t i v e abundances of s i x d i f f e r e n t prey t y p e s and f r e q u e n c i e s w i t h which each type was taken from each zone by f o r a g i n g o y s t e r c a t c h e r s are shown i n F i g u r e 5. (These d a t a a r e c o r r e c t e d TABLE II D u r a t i o n (seconds) of s u c c e s s f u l and u n s u c c e s s f u l s e a r c h i n t h r e e i n t e r t i d a l zones* D u r a t i o n of D u r a t i o n of Zone S u c c e s s f u l Search U n s u c c e s s f u l Search FUCU 15.9 (N=24, s.d.=12.1) 19.4 (N=6, s.d.=21.9) MYTL 13.6 (N=289, s.d.=14.6) 18.1 (N=166, s.d.=19.6) LAPO 13.4 (N=154, s.d.=14.6) 24.4 (N=89, s.d.=29.8) *See t e x t f o r d e t a i l s . 26 f o r d i f f e r e n t p r o p o r t i o n s of f o r a g i n g time spent i n each z o n e ) . Prey s e l e c t i o n r e f l e c t s prey a v a i l a b i l i t y i n each zone and may a l s o r e f l e c t b i r d s ' prey p r e f e r e n c e s and prey d e t e c t i o n a b i l i t i e s i n each zone. For i n s t a n c e , the abundance of c r a b s and c h i t o n s i n c r e a s e d from h i g h t o low water l i n e as d i d the frequ e n c y w i t h which t h e s e prey t y p e s were taken by o y s t e r c a t c h e r s . By c o n t r a s t , the abundance of l i m p e t s was s i m i l a r i n the m y t i l u s and l a m i n a r i a - p o s t e l s i a zones, but 37% of l i m p e t s eaten were taken from the m y t i l u s zone w h i l e o n l y 14% were taken from the l a m i n a r i a - p o s t e l s i a zone. The h i g h e s t p e r c e n t a g e of l i m p e t s e a t e n , 45%, was taken from the fucus zone where l i m p e t s had a very low a b s o l u t e abundance. A n a l y s i s of prey s e l e c t i o n by b l a c k o y s t e r c a t c h e r s i s c o m p l i c a t e d by changes i n r e l a t i v e d e n s i t i e s of d i f f e r e n t prey t y p e s between zones of the i n t e r t i d a l . To overcome t h i s problem the m u l t i - s p e c i e s d i s c e q u a t i o n (Murdoch and Oaten 1975) was used t o a n a l y z e prey s e l e c t i o n i n more d e t a i l . The e q u a t i o n i s Nj = a; TP; 1 + £ahD Where Ni = number of i t h prey eaten a; = r a t e of e f f e c t i v e s e a r c h f o r i t h prey T = t o t a l f o r a g i n g time D; = d e n s i t y of i t h prey h = average h a n d l i n g time 27 F i g u r e 5: R e l a t i v e prey a v a i l a b i l i t y and prey s e l e c t i o n by f o r a g i n g o y s t e r c a t c h e r s i n 4 zones of the i n t e r t i d a l . Each row of the graph r e p r e s e n t s one prey t y p e . For each prey type ( r o w s ) , p e r c e n t a v a i l a b l e and p e r c e n t taken each sum t o 1 0 0 % . Data a r e c o r r e c t e d f o r d i f f e r e n t p r o p o r t i o n s of f o r a g i n g time spent i n each zone. ( R e l a t i v e mussel a v a i l a b i l i t y was e s t i m a t e d from p e r c e n t c o v e r d a t a . I n t e r t i d a l q u a d r a t data were used t o determine r e l a t i v e a v a i l a b i l i t y of o t h e r prey t y p e s ) . S P R A Y F U C U S M Y T I L U S L A M . - P O S T . Z O N E Z O N E Z O N E Z O N E 29 T h i s e q u a t i o n can be s o l v e d f o r the r a t e of e f f e c t i v e s e a r c h , a measure of the p r o b a b i l i t y t h a t a prey type w i l l be t a k e n . Rate of e f f e c t i v e s e a r c h , a; , i s a p r o d u c t of a r e a s e a r c h e d , p r o b a b i l i t y of d e t e c t i n g a p r e y , and p r o b a b i 1 i t y • of a t t a c k i n g a p r e y . The m u l t i - s p e c i e s d i s c e q u a t i o n was s o l v e d f o r a; f o r each of f i v e prey t y p e s i n t h r e e i n t e r t i d a l zones (see Appendix B ) . V a l u e s of r a t e of e f f e c t i v e s e a r c h a r e shown i n Table I I I . These r e s u l t s show t h a t r a t e of e f f e c t i v e s e a r c h f o r each prey type and, c o n s e q u e n t l y , .the p r o b a b i l i t y t h a t a prey type would be taken v a r i e d between zones. For i n s t a n c e , a c r o s s t h r e e i n t e r t i d a l zones, l i m p e t s were more l i k e l y t o be taken by f o r a g i n g o y s t e r c a t c h e r s i n the f u c u s zone than i n the m y t i l u s or l a m i n a r i a - p o s t e l s i a zone. W i t h i n the l a m i n a r i a - p o s t e l s i a zone, where prey abundances and d i v e r s i t i e s were g r e a t e s t , r a t e of e f f e c t i v e s e a r c h was h i g h e s t f o r c r a b s . T h i s s u g g e s t s t h a t o y s t e r c a t c h e r s were s e l e c t i n g c r a b s over o t h e r prey t y p e s . Prey s e l e c t i o n i s a f f e c t e d by both p r e y d i s t r i b u t i o n and prey p r e f e r e n c e s of o y s t e r c a t c h e r s . As a consequence of prey d i s t r i b u t i o n , prey abundance, and s e l e c t i v i t y by o y s t e r c a t c h e r s , the l a r g e s t prey items were taken i n the l o w e s t zones of the i n t e r t i d a l . F i g u r e 6 shows average w e i g h t s of prey taken i n each zone of the i n t e r t i d a l . Average prey w e i g h t s v a r i e d s i g n i f i c a n t l y between zones (F = 5.18, df = 3,395, p < 0.01). Prey P r o f i t a b i l i t y Prey p r o f i t a b i l i t y (grams of wet weight i n g e s t e d / s e c o n d of h a n d l i n g time) v a r i e d w i t h prey type and prey s i z e . Even though TABLE I I I Rate of e f f e c t i v e s e a r c h , a; , as c a l c u l a t e d by m u l t i - s p e c i e s d i s c e q u a t i o n f o r 5 prey t y p e s i n 3 zones of the i n t e r t i d a l * Zone Laminar i a -Prey type Fucus M y t i l u s p o s t e l s i a l i m p e t 0.15 0.02 0.006 mussel 0.00025 0.000014 0.000031 c h i t o n 0.0043 0.00013 c r a b 0.108 worm 0.032 *See t e x t f o r d e t a i l s . 31 F i g u r e 6: Average weight of prey taken by f o r a g i n g o y s t e r c a t c h e r s i n each zone of the i n t e r t i d a l . ( V e r t i c a l l i n e s a r e upper 95% c o n f i d e n c e l i m i t s ) . 32 10 .0T < g 1.0 o Hi >. 0.1 U J cr T I F= 5.1ft df= 3,395 p<0.01 SPRAY ZONE N = 1 FUCUS ZONE N=21 MYTILUS LAM.- POST. ZONE ZONE N= 260 N = 117 33 m o s t p r e y t a k e n b y b i r d s f e l l i n t o a v e r y l i m i t e d r a n g e o f p r e y s i z e s a s i n d i c a t e d b y s a m p l e s i z e s p r e s e n t e d i n F i g u r e 7, t h e r e a r e e n o u g h d a t a t o e x a m i n e t h e r e l a t i o n b e t w e e n p r o f i t a b i l i t y a n d p r e y s i z e . P r e y t a k e n b y b l a c k o y s t e r c a t c h e r s f a l l i n t o t w o g r o u p s w i t h r e s p e c t t o p r o f i t a b i l i t i e s . T h e p r o f i t a b i l i t i e s o f some p r e y ( l i m p e t s , m u s s e l s , c r a b s ) i n c r e a s e w i t h p r e y s i z e o v e r t h e r a n g e o f p r e y s i z e s t a k e n b y o y s t e r c a t c h e r s i n t h e s t u d y a r e a ( F i g u r e 7a, 8b, 8c), a n d t h e p r o f i t a b i l i t i e s o f o t h e r p r e y ( c h i t o n s , a n d p e r h a p s a l s o w o r m s ) r e m a i n r e l a t i v e l y c o n s t a n t o v e r a r a n g e o f p r e y s i z e s ( F i g u r e 7c, 8d). D i f f e r e n c e s i n t h e r e l a t i o n s h i p s b e t w e e n p r e y p r o f i t a b i l i t i e s a n d p r e y s i z e r e f l e c t d i f f i c u l t i e s b i r d s h a v e i n h a n d l i n g some p r e y a n d t h e e f f e c t t h e s e d i f f i c u l t i e s h a v e on h a n d l i n g t i m e . L i m p e t s , m u s s e l s , a n d c r a b s a r e r e l a t i v e l y s t a t i o n a r y p r e y , a n d n o n e o f t h e m c a n f l e e r a p i d l y f r o m a p r e d a t o r . When a n o y s t e r c a t c h e r a t t a c k s , a l i m p e t p u l l s i t s v a l v e down f l u s h w i t h t h e s u r f a c e a n d c l i n g s t i g h t l y t o t h e s u b s t r a t e , a m u s s e l p u l l s i t s v a l v e s f i r m l y t o g e t h e r b y m u s c l e c o n t r a c t i o n , a n d a c r a b r e m a i n s h i d d e n i n a c u l - d e - s a c a t t h e b a s e o f a k e l p h o l d f a s t a n d b l o c k s t h e e n t r a n c e o f i t s h i d i n g p l a c e w i t h i t s w e l l -d e v e l o p e d p i n c e r s ( p e r s . o b s . ) . A l l o f t h e s e d e f e n s e s a r e p a s s i v e , a n d o n c e a n o y s t e r c a t c h e r h a s b r o k e n t h r o u g h t h e d e f e n s e , t h e p r e y i s h e l p l e s s a n d q u i c k l y c o n s u m e d b y a n o y s t e r c a t c h e r . C h i t o n s a n d worms h a v e m o r e a c t i v e d e f e n s e s t h a n o t h e r p r e y t y p e s a n d a r e c o n s i d e r a b l y m o r e c h a l l e n g i n g f o r b i r d s t o h a n d l e . C h i t o n s c l i n g t i g h t l y t o t h e s u b s t r a t e a n d a r e o f t e n f i r m l y l o d g e d i n c r e v i c e s w h e r e i t i s d i f f i c u l t f o r b i r d s t o g a i n 34 F i g u r e 7: P r o f i t a b i l i t y (grams/second of h a n d l i n g time) of a) c r a b s , b) l i m p e t s , c) m u s s e l s , d) c h i t o n s , e) worms w i t h r e s p e c t t o prey weight (grams), ( r - v a l u e s were c a l c u l a t e d u s i n g a l i n e a r model. Quoted p r o b a b i l i t y l e v e l s a r e o n l y approximate because of n o n - l i n e a r i t i e s i n the d a t a . V e r t i c a l b a r s a r e upper 9 5 % c o n f i d e n c e l i m i t s ; numbers by each d a t a p o i n t are sample s i z e s ) . 35 A - C RABS B- LIMPETS O z o o L U 00 \ 00 < cr o 1.00' I-v' 9 .''54 0.10 4V 38 0.01" 0.0 36.0 C - M U S S E L S 1.0 0" 0.10 r = 0.55 df=102 p< 0.01 + 72.0 1-1 7 - - I I i / 0.01 I t r = 0.A1 df = 44 p<O01 97 I - - -547 0.0 D-CHI TONS l -,''3 r=0.85 df=6A8 p< 0.001 ,3.0 6.0 E-W0RMS 2 4-0.0 3.0 H— 6.0 0.0 — t -6.0 r = -0.11 df=6 ns —I V -4 r=0.03 df = 22 ns 14.0 0.0 0.6 - f -1.2 P R E Y W E I G H T ( G R A M S ) 36 s u f f i c i e n t purchase t o l e v e r them, o f f the s u b s t r a t e . Once c h i t o n s are c a p t u r e d , they c o n t r a c t on t h e i r v e n t r a l s i d e and form a t i g h t , r i g i d c u r l . B i r d s o f t e n s t r u g g l e a t l e n g t h t o f l a t t e n a c a p t u r e d c h i t o n enough t o be a b l e t o remove the f l e s h . Worms ar e removed from the s u b s t r a t e , u s u a l l y a mussel bed, by a tug-of-war between a worm and a b i r d . O f t e n a worm p u l l s f r e e of a b i r d s e v e r a l times b e f o r e a b i r d s u c c e s s f u l l y p u l l s i t f r e e of the s u b s t r a t e . A f t e r c a p t u r e , worms u s u a l l y t h r a s h and w r i t h e v i g o r o u s l y , so b i r d s must subdue worms b e f o r e they can eat them. Prey p r o f i t a b i l i t i e s a re a f f e c t e d by prey responses t o a t t a c k s by o y s t e r c a t c h e r s . P r o f i t a b i l i t i e s of r e l a t i v e l y p a s s i v e prey i n c r e a s e d w i t h prey s i z e . B e h a v i o u r of prey such as c h i t o n s and worms w i t h v i g o r o u s responses t o a t t a c k may r e s u l t i n l o n g h a n d l i n g times and r e l a t i v e l y c o n s t a n t p r o f i t a b i l i t i e s over a range of prey s i z e s . However, more data are needed t o c o n f i r m t h i s , e s p e c i a l l y f o r c h i t o n s . C u m u l a t i v e Prey Consumption The c u m u l a t i v e amount of prey o b t a i n e d by a b i r d d u r i n g an u n i n t e r r u p t e d f o r a g i n g bout v a r i e s between zones of the i n t e r t i d a l . F i g u r e 8 shows the r e l a t i o n between c u m u l a t i v e prey i n t a k e (measured as grams wet w e i g h t ) and time d u r i n g a f o r a g i n g bout w i t h i n a zone f o r f i v e i n d i v i d u a l b i r d s . (Data are shown f o r f i v e l o n g f o r a g i n g b o u t s ) . B i r d s f o r a g i n g f o r l o n g p e r i o d s of time i n one zone g e n e r a l l y o b t a i n e d g r e a t e r c u m u l a t i v e amounts of prey i n the l a m i n a r i a - p o s t e l s i a zone than the m y t i l u s zone. D i f f e r e n t c u m u l a t i v e amounts of prey consumed i n these two zones r e f l e c t z o n a l d i f f e r e n c e s i n prey abundance and s i z e . 37 Figure 8: Cumulative prey intake during uninterrupted foraging bouts for individual birds during two bouts in mytilus and three bouts in laminaria-postelsia zones. Grams of prey (wet weight) are plotted against foraging bout duration (minutes). MYTILUS ZONE CO < cr o x o LU > -L U cr Q_ LU > HO 100 60 20 UO 0 t 30 60 90 LAM.-POST. ZONE 100 ZD O 60 20 MYTILUS ZONE 30 9'0 LAM.-POST. ZONE + 30 60 M I N U T E S 90 0 LAM.-P0ST. ZONE to co 30 60 90 39 A l l o c a t i o n of F o r a g i n g Time between Zones Zonal v a r i a t i o n i n prey abundance and the a v a i l a b i l i t y of p r e f e r r e d prey t y p e s c o u l d i n f l u e n c e the way b i r d s a l l o c a t e t h e i r f o r a g i n g time between zones of the i n t e r t i d a l . D u r i n g t h i s s t u d y , o y s t e r c a t c h e r s spent 0.4% of t h e i r f o r a g i n g time i n the spray zone, 4.0% i n the fucus zone, 54.0% i n the m y t i l u s zone, and 41.0% i n the l a m i n a r i a - p o s t e l s i a zone. Prey abundances and a v a i l a b i l i t y of l a r g e and p r e f e r r e d prey were h i g h e s t i n the l a m i n a r i a - p o s t e l s i a zone, but f o r a g i n g bouts i n t h i s zone were r a r e l y l o n g e r than f o r a g i n g bouts i n o t h e r zones, and b i r d s d i d not always a c h i e v e h i g h p r o f i t a b i l i t i e s w h i l e f o r a g i n g i n the l a m i n a r i a - p o s t e l s i a zone. Data on f o r a g i n g bout d u r a t i o n s and a s s o c i a t e d average p r o f i t a b i l t i e s i n t h r e e i n t e r t i d a l zones are summarized i n T a b l e IV. Average p r o f i t a b i l i t i e s i n m y t i l u s and l a m i n a r i a - p o s t e l s i a zones were s i m i l a r . F i g u r e 9 p r e s e n t s a more d e t a i l e d l o o k a t d a t a i n T a b l e IV. Bout d u r a t i o n i n a zone i s p l o t t e d a g a i n s t average p r o f i t a b i l i t y a c h i e v e d d u r i n g the f o r a g i n g bout. Low or h i g h p r o f i t a b i l i t i e s were not c o n s i s t e n t l y a s s o c i a t e d w i t h a p a r t i c u l a r zone, and bout d u r a t i o n was not c o r r e l a t e d w i t h the p r o f i t a b i l i t y a b i r d a c h i e v e d d u r i n g a bout (r = -0.11, df = 41, NS, u n t r a n s f ormed d a t a ) . V a r i a b i l i t y ,of p r o f i t a b i l i t i e s a c h i e v e d i n f u c u s , m y t i l u s and l a m i n a r i a -p o s t e l s i a zones s u g g e s t s w i t h i n zone p a t c h i n e s s of p r e y . The s t e p - l i k e shapes of c u m u l a t i v e prey i n t a k e c u r v e s i n F i g u r e 8 a l s o suggest t h i s . TABLE IV Average' bout d u r a t i o n (seconds) and p r o f i t a b i l i t y (grams/second) d u r i n g f o r a g i n g bouts i n 3 i n t e r t i d a l zones Zone fucus N Average bout d u r a t i o n (seconds) 221.9 ( s . d. = 270.9) Average p r o f i t a b i l i t y (grams/second) 0.009 ( s . d. = 0.013) myt i l u s 14 734.7 ( s . d. = 1223.5) 0.098 ( s . d. = 0.21 ) l a m i n a r i a - 26 p o s t e l s i a 632.8 ( s . d. = 1058.3) 0.022 ( s . d. = 0.03) 41 F i g u r e 9: D u r a t i o n of f o r a g i n g bouts i n 3 i n t e r t i d a l zones p l o t t e d a g a i n s t p r o f i t a b i l i t y a c h i e v e d d u r i n g b o u t s . (Data were t r a n s f o r m e d by l o g - t r a n s f o r m a t i o n f o r purposes of g r a p h i c a l p r e s e n t a t i o n ) . 42 4.0 I 2.0T + 1.0 X ZONE X=FUCUS • =MYTI LU S + = LA M I NARIA-POS TELSIA + + 3.OT . ; ; .••. * x e • + X ° - ?4.0 TiS ^2D MX) ' OX) G R A M S / S E C O N D ( L O G 1 Q ) 43 DISCUSSION Animals f o r a g i n g i n patchy environments must s o l v e t h r e e problems as they f o r a g e : what prey to take (prey c h o i c e ) , where t o f o r a g e ( p a t c h c h o i c e ) , and how t o a l l o c a t e f o r a g i n g time between p a t c h e s . E c o l o g i c a l t h e o r y has a d d r e s s e d a l l t h r e e of these f o r a g i n g problems, and i n s i g h t s i n t o s t r e n g t h s and weaknessess of t h i s t h e o r y may be g a i n e d by a s k i n g how w e l l f o r a g i n g b e h a v i o u r of b l a c k o y s t e r c a t c h e r s i n t h i s s tudy and of o t h e r a n i m a l s r e p o r t e d i n the l i t e r a t u r e i s p r e d i c t e d by t h e o r y . Prey C h o i c e P r e d a t o r s s h o u l d become more s e l e c t i v e i n t h e i r c h o i c e of prey as prey abundances i n c r e a s e (Schoener 1971). In g e n e r a l , prey c h o i c e by b l a c k o y s t e r c a t c h e r s f o l l o w e d p r e d i c t e d t r e n d s . In zones of the i n t e r t i d a l where prey biomass and abundance are r e l a t i v e l y h i g h o y s t e r c a t c h e r s took b i g g e r , more p r o f i t a b l e prey items than they d i d i n zones where abundances and biomass were r e l a t i v e l y low ( F i g u r e 6 ) . T h i s f i n d i n g i s f a i r l y g e n e r a l , and s i m i l a r r e s u l t s have been r e p o r t e d from f i e l d s t u d i e s of redshank ( G o s s - C u s t a r d 1977a) and gray s q u i r r e l s S c i u r u s  c a r o l i n e n s i s (Lewis 1980), and l a b o r a t o r y s t u d i e s of b l u e g i l l s u n f i s h Lepomis m a c r o c h i r u s (Werner and H a l l 1974), and g r e a t t i t s (Krebs e_t a l . 1977). Cases where p r e d a t o r s do not s e l e c t prey a c c o r d i n g t o t h e i r r e l a t i v e p r o f i t a b i l i t i e s can p r o b a b l y be e x p l a i n e d i n terms of c u r r e n c i e s b e s i d e s biomass and k i l o c a l o r i e s (e. g. G o s s - C u s t a r d 1977b, D a v i e s 1977) or u n p a l a t a b l e p r o p e r t i e s of -some prey t y p e s (e. g. G l a n d e r 1-981).. 44 A f u r t h e r p r e d i c t i o n c o n c e r n i n g prey c h o i c e and d i e t b r e a d t h i s t h a t p r e d a t o r s s h o u l d not show p a r t i a l p r e f e r e n c e s ; i f a p a r t i c u l a r prey type i s e n c o u n t e r e d i t s h o u l d always or never be t a k e n , the "always or n e v e r " r u l e (MacArthur 1972; Schoener 1971). In t h i s s t u d y , the m u l t i - s p e c i e s d i s c e q u a t i o n (Murdoch and Oaten 1975) was used t o c a l c u l a t e r a t e s of e f f e c t i v e s e a r c h f o r f i v e prey t y p e s i n t h r e e zones of the i n t e r t i d a l . These c a l c u l a t i o n s ( T a b l e I I I ) i n d i c a t e t h a t p r o b a b i l i t i e s of c a p t u r e f o r d i f f e r e n t prey t y p e s may v a r y c o n s i d e r a b l y between zones, but the r e s u l t s do not s u p p o r t the "always or n e v e r " r u l e . R e s u l t s from Werner and H a l l ' s (1974) study of d i e t b r e a d t h i n b l u e g i l l s u n f i s h suggest t h a t r e l a t i v e l y u n p r o f i t a b l e prey a r e g r a d u a l l y dropped from the d i e t as abundance of b i g prey i n c r e a s e s . In t h e i r e x p e r i m e n t s , d i f f e r e n t prey t y p e s were r e p r e s e n t e d by d i f f e r e n t s i z e c l a s s e s of daphnia (Daphnia  magna). As prey d e n s i t y was i n c r e a s e d e x p e r i m e n t a l l y , s i z e c l a s s e s of daphnia were dropped s e q u e n t i a l l y from the b l u e g i l l d i e t . In a n o t h e r t e s t of the "always or never" r u l e Krebs e t a l . (1977) p r e s e n t e d g r e a t t i t s w i t h two d i f f e r e n t s i z e s of meal worms a t d i f f e r e n t encounter f r e q u e n c i e s . At h i g h encounter f r e q u e n c i e s b i r d s showed s t r o n g s e l e c t i o n f o r b i g p r e y , b u t , c o n t r a r y t o p r e d i c t i o n , s m a l l u n p r o f i t a b l e prey were o n l y g r a d u a l l y e l i m i n a t e d from the d i e t . In these l a b o r a t o r y t e s t s of the "always or never" r u l e d i f f e r e n t s i z e c l a s s e s of a s i n g l e organism were used t o r e p r e s e n t d i f f e r e n t prey t y p e s . Data from f i e l d s t u d i e s where d i f f e r e n t prey types a c t u a l l y are d i f f e r e n t organisms.do not 45 support the "always or never" r u l e e i t h e r . In t h i s study of b l a c k o y s t e r c a t c h e r s l i m p e t s were not a p r e f e r r e d prey t y p e , but they were never c o m p l e t e l y dropped from the d i e t . S i m i l a r l y i n redshank, a s m a l l amphipod c r u s t a c e a n (Corophium v o l u t a t o r ) was a p r e f e r r e d p r e y , but l e s s p r e f e r r e d n e r e i d worms were never e l i m i n a t e d from the d i e t even when Corophium d e n s i t i e s were h i g h ( G o s s - C u s t a r d 1977b). E x c e p t i o n s t o the "always or never" r u l e seem t o be g e n e r a l , and suggest t h a t the r u l e r a r e l y or never a p p l i e s i n l a b o r a t o r y or f i e l d s i t u a t i o n s . S c h l u t e r (1981) r e c e n t l y r e v iewed d i e t b r e a d t h of 44 s p e c i e s r e p o r t e d i n the l i t e r a t u r e and c o n c l u d e d t h a t d i e t t h e o r y has g e n e r a l l y f a i l e d t o p r e d i c t prey c h o i c e s of p r e d a t o r s f o r a g i n g f o r m u l t i p l e p r e y . A number of a l t e r n a t i v e s may e x p l a i n v i o l a t i o n s of the "always or never" r u l e and s h o r t c o m i n g s of d i e t t h e o r y : 1) p r e d a t o r s may f o r a g e s i m u l t a n e o u s l y f o r s e v e r a l d i f f e r e n t c u r r e n c i e s (e. g. k i l o c a l o r i e s and t r a c e n u t r i e n t s ; f o r examples see D a v i e s (1977), G o s s - C u s t a r d ( 1 9 7 7 b ) ) ; 2) d u r i n g a f o r a g i n g bout p r e d a t o r s may s w i t c h from f o r a g i n g as s p e c i a l i s t s t o f o r a g i n g as g e n e r a l i s t s ( H e l l e r 1980); 3) . p r e d a t o r s f o r a g i n g i n v a r i a b l e environments must sample t o e v a l u a t e and m o n i t o r prey abundances and p r o f i t a b i l i t i e s (e. g. Krebs e t a l . 1977); 4) p r e d a t o r s i n p a t c h y environments may .minimize the r i s k of g o i n g hungry by t a k i n g l e s s p r o f i t a b l e or l e s s p r e f e r r e d prey as they are encountered because c o n s i d e r a b l e time may e l a p s e b e f o r e the next p r e f e r r e d p r e y i s encountered (e. g. Caraco 1980). 46 P a t c h C h o i c e and Time A l l o c a t i o n between Zones P r e d a t o r s ' p a t c h c h o i c e s s h o u l d be governed by the same c r i t e r i a t h a t c o n t r o l prey c h o i c e (Pyke et a_l. 1977). P r e d a t o r s s h o u l d c o n c e n t r a t e t h e i r f o r a g i n g e f f o r t s i n the best p a t c h e s . F i e l d and l a b o r a t o r y s t u d i e s (Lewis 1980; O'Connor and Brown 1977) i n d i c a t e t h a t p r e d a t o r s g e n e r a l l y f o r a g e a c c o r d i n g t o t h i s p r e d i c t i o n , but they always spend some time f o r a g i n g i n l e s s p r o f i t a b l e p a t c h e s . In t h i s study of b l a c k o y s t e r c a t c h e r s , i n t e r t i d a l z o n a t i o n was used t o d e f i n e p a t c h e s . B i r d s spent over 95% of t h e i r f o r a g i n g time i n the m y t i l u s and l a m i n a r i a - p o s t e l s i a zones where prey abundance and biomass were g r e a t e s t . However, b i r d s a l s o spent time f o r a g i n g i n the f u c u s zone and sometimes a c h i e v e d p r o f i t a b i l i t i e s s i m i l a r t o those a t t a i n e d i n the m y t i l u s and l a m i n a r i a - p o s t e l s i a zones ( F i g u r e 9 ) . R e s u l t s from o t h e r s t u d i e s (Smith and Sweatman 1974; Zach and F a l l s 1976a,1976b) i n d i c a t e t h a t a l t h o u g h p r e d a t o r s q u i c k l y i d e n t i f y the most p r o f i t a b l e p a t c h e s and c o n c e n t r a t e t h e i r f o r a g i n g a c t i v i t i e s t h e r e , they c o n t i n u e t o v i s i t l e s s p r o f i t a b l e p a t c h e s . P r e d a t o r s use i n f o r m a t i o n b e s i d e s average prey p r o f i t a b i l i t y t o s e l e c t p a t c h e s where they w i l l f o r a g e . R e c e n t l y , i t has been shown both t h e o r e t i c a l l y (Caraco 1980) and e x p e r i m e n t a l l y (Caraco ejt a l . 1980, R e a l 1981) t h a t r e s o u r c e v a r i a b i l i t y i s an i m p o r t a n t d e t e r m i n a n t of p a t c h p r e f e r e n c e s . In v a r i a b l e environments f o r a g i n g b e h a v i o u r may r e f l e c t the v a r i a b i l i t y i n prey p r o f i t a b i l i t i e s i n a d d i t i o n t o average prey p r o f i t a b i l i t i e s , so mean prey p r o f i t a b i l i t i e s may not be s u f f i c i e n t t o c h a r a c t e r i z e p r e d a t o r s ' p a t c h p r e f e r e n c e s . 47 A p r e d a t o r s h o u l d f o r a g e i n a p a t c h as l o n g as i t s r a t e of food i n t a k e remains h i g h e r than the average r a t e f o r the environment (Charnov 1976a). T h i s r u l e f o r p a t c h s w i t c h i n g , the m a r g i n a l v a l u e theorem, i s d e t e r m i n i s t i c and assumes: 1) random movement between p a t c h e s by a p r e d a t o r , 2) prey d e p l e t i o n by a p r e d a t o r w h i l e f o r a g i n g i n a p a t c h , and 3) the a b i l i t y of a p r e d a t o r t o m o n i t o r i t s m a r g i n a l c a p t u r e r a t e as i t f o r a g e s . The m a r g i n a l v a l u e theorem has been t e s t e d under c o n t r o l l e d l a b o r a t o r y c o n d i t i o n s (Cowie 1977), but a r i g o r o u s f i e l d t e s t i s l a c k i n g . F i e l d t e s t s of the m a r g i n a l v a l u e theorem p r e s e n t some s e r i o u s d i f f i c u l t i e s . For a number of reasons b l a c k o y s t e r c a t c h e r s i n t h i s study would not be s u i t a b l e s u b j e c t s f o r a t e s t of the m a r g i n a l v a l u e theorem: 1) b l a c k o y s t e r c a t c h e r s are t e r r i t o r i a l and t h e i r f a m i l i a r i t y w i t h t e r r i t o r y topography and r e s o u r c e d i s t r i b u t i o n a f f e c t where they s e a r c h f o r p r e y ; 2) no good d a t a e x i s t s t o a d d r e s s the i s s u e of prey d e p l e t i o n i n p a t c h e s , but l a c k of c o r r e l a t i o n between d u r a t i o n of s u c c e s s f u l s e a r c h e s and amounts of time b i r d s f o r a g e d i n u n i n t e r r u p t e d bouts ( T a b l e I) suggests b i r d s do not s i g n i f i c a n t l y d e p l e t e prey d u r i n g f o r a g i n g b o u t s ; 3) the l a c k of c o r r e l a t i o n between f o r a g i n g bout d u r a t i o n and a c h i e v e d . p r o f i t a b i l i t y ( F i g u r e 9) s u g g e s t s t h a t b l a c k o y s t e r c a t c h e r s e i t h e r cannot or do not c a l c u l a t e m a r g i n a l c a p t u r e r a t e s w h i l e they f o r a g e . Other v e r t e b r a t e p r e d a t o r s , b l u e g i l l s u n f i s h (Werner et §_1. 1981), o v e n b i r d s (Zach and F a l l s 1976b, 1976c), g r e a t t i t s (Smith and Sweatman 1974), and s q u i r r e l s (Lewis 1980), a l s o f o r a g e i n ways t h a t v i o l a t e assumptions of the m a r g i n a l v a l u e theorem. The 48 f o r a g i n g b e h a v i o u r of a l l of these p r e d a t o r s and t h e i r p a t t e r n s of s e a r c h were i n f l u e n c e d by p r e v i o u s e x p e r i e n c e i n an a r e a . I n c r e a s i n g l y d e t a i l e d s t u d i e s of how v e r t e b r a t e p r e d a t o r s f o r a g e suggest t h a t the assumptions of the m a r g i n a l v a l u e theorem a r e so r e s t r i c t i v e t h a t a r i g o r o u s f i e l d t e s t w i t h a v e r t e b r a t e i s u n l i k e l y . S e v e r a l p a t c h s w i t c h i n g models e x i s t as a l t e r n a t i v e s t o Charnov's model. Cowie and Krebs (1979) suggest t h a t p r e d a t o r s s i m p l y use " r u l e s - o f - t h u m b " t o d e c i d e when t o s w i t c h p a t c h e s . The performance of an a n i m a l f o r a g i n g a c c o r d i n g t o " r u l e s - o f -thumb" may c l o s e l y approximate the s o l u t i o n p r e d i c t e d by an o p t i m a l f o r a g i n g model such as the m a r g i n a l v a l u e theorem. O l l a s o n ' s (1980) l e a r n i n g model f o r f o r a g i n g i n a patchy environment i s based on such a " r u l e - o f - t h u m b " . In O l l a s o n ' s model a p r e d a t o r s i m p l y c o n t i n u e s f o r a g i n g i n a p a r t i c u l a r p a t c h as l o n g as i t i s f e e d i n g f a s t e r than i t remembers d o i n g . The l e n g t h of memory i s a v a r i a b l e i n O l l a s o n ' s model, and f o r l o n g v a l u e s of memory the r e s u l t s from the model are c l o s e a p p r o x i m a t i o n s of p r e d i c t e d " o p t i m a l " s o l u t i o n s . The problem of when t o s w i t c h patches i s c l o s e l y r e l a t e d t o the problem of when and how much t o sample among p a t c h e s . In f a c t the b e h a v i o u r s of s w i t c h i n g and sa m p l i n g a re so s i m i l a r t h a t t h e r e i s no a p r i o r i way t o d i s t i n g u i s h between them. (An a p o s t e r i o r i way t o d i s t i n g u i s h between s a m p l i n g and s w i t c h i n g would be t o d e f i n e a b r i e f v i s i t t o a new p a t c h as samp l i n g and a l o n g v i s i t as s w i t c h i n g , but t h i s approach i s not p r e d i c t i v e and of no t h e o r e t i c a l i n t e r e s t ) . Perhaps t h i s a m b i g u i t y e x p l a i n s why t h e q u e s t i o n of how much t o sample has r e c e i v e d so l i t t l e 49 a t t e n t i o n . Krebs et a l . (1978) have d e a l t t h e o r e t i c a l l y and e x p e r i m e n t a l l y w i t h the q u e s t i o n of how much t o sample. They used a m a t h e m a t i c a l model t o p r e d i c t the o p t i m a l amount of s a m p l i n g a b i r d s h o u l d do d u r i n g a s p e c i f i e d number of t r i a l s . The r e s u l t s of t h e i r m e t i c u l u o u s l y d e s i g n e d two-patch l a b o r a t o r y experiment approximated the p r e d i c t e d sampling f r e q u e n c y , but the b i r d s ( g r e a t t i t s ) sampled more o f t e n than e x p e c t e d and t h e r e was c o n s i d e r a b l e v a r i a t i o n i n performance among i n d i v i d u a l b i r d s . A problem a r i s e s i n i n t e r p r e t i n g r e s u l t s t h a t a p p r o x i m a t e d a p r e d i c t e d o p t i m a l s o l u t i o n . Any d e v i a t i o n from a p r e d i c t i o n may be a t t r i b u t e d e i t h e r t o e x p e r i m e n t a l e r r o r or t o f a i l u r e of a p a r t i c u l a r model t o e x p l a i n a b e h a v i o u r of i n t e r e s t ( O l l a s o n 1980). So, f o r p h i l o s o p h i c a l reasons i t i s d i f f i c u l t t o a c c e p t or r e j e c t the h y p o t h e s i s of o p t i m a l s a m p l i n g . Problems encountered t e s t i n g o p t i m a l s a m p l i n g i n the l a b o r a t o r y are compounded i n the f i e l d where i t i s not p o s s i b l e to d i s t i n g u i s h f u n c t i o n a l l y between s w i t c h i n g and s a m p l i n g . C o n c l u s i o n s T h e o r e t i c i a n s f i n d i t d i f f i c u l t t o ' p r e d i c t o p t i m a l s o l u t i o n s f o r a n i m a l s f o r a g i n g i n complex n a t u r a l environments (Zach and Smith 1981), and i t i s u n l i k e l y t h a t a n i m a l s can do t h i s e i t h e r . Most n a t u r a l e nvironments a r e r e l a t i v e l y complex and v a r i a b l e , and, as a r e s u l t , the a d a p t i v e s o l u t i o n s p r e d a t o r s employ f o r f o r a g i n g i n v a r i a b l e environments are l i k e l y t o be s t o c h a s t i c i n n a t u r e . I t f o l l o w s t h a t memory and l e a r n i n g , s t o c h a s t i c p r o c e s s e s (Bateson 1979), a r e of c r i t i c a l importance 50 i n u n d e r s t a n d i n g how a n i m a l s f o r a g e i n patchy environments. R e c e n t l y , a number of workers have d e v e l o p e d models of f o r a g i n g i n v a r i a b l e environments (e. g. Caraco 1980, Green 1980, Oaten 1977), and some s p e c i f i c s o l u t i o n s a n i m a l s might employ w h i l e f o r a g i n g i n v a r i a b l e environments have a l s o been proposed. Krebs and Cowie (1979) have suggested t h a t " r u l e s - o f - t h u m b " form the bases of a n i m a l s ' f o r a g i n g d e c i s i o n s , and O l l a s o n (1980) has proposed a l e a r n i n g model t h a t e x p l a i n s p a t c h c h o i c e . In the f u t u r e new i n s i g h t s i n t o how p r e d a t o r s e x p l o i t prey i n p a t c h y environments w i l l depend on: 1) f u r t h e r development and r e f i n e m e n t of s t o c h a s t i c f o r a g i n g models; 2) b e t t e r u n d e r s t a n d i n g of the r o l e s of memory and l e a r n i n g i n p r e d a t o r b e h a v i o u r ; 3) more l a b o r a t o r y and, e s p e c i a l l y , more f i e l d i n v e s t i g a t i o n s of f o r a g i n g models and t h e i r u n d e r l y i n g a s s u m p t i o n s . 51 LITERATURE CITED Bat e s o n , G. 1979. Mind and n a t u r e . E. P. D u t t o n . New York. C a m p b e l l , R. W. and S t i r l i n g , D. 1968. Notes on the n a t u r a l h i s t o r y of C l e l a n d I s l a n d , B r i t i s h Columbia, w i t h emphasis on the b r e e d i n g b i r d f a u n a . B. C. Prov. Mus. Report f o r 1967. V i c t o r i a , B. C. pp.25-43. Caraco, T. 1980. On f o r a g i n g time a l l o c a t i o n i n a s t o c h a s t i c environment. E c o l o g y 6:119-128. Caraco, T., M a r t i n d a l e , S., and Whittam, T. S. 1980. An e m p i r i c a l d e m o n s t r a t i o n of r i s k - s e n s i t i v e f o r a g i n g , p r e f e r e n c e s . Anim. Behav. 28:820-830. C a r e f o o t , T. 1977. P a c i f i c Seashores. J . J . Douglas L t d . , Vancouver. 208 p. Charnov, E. L. 1976a. O p t i m a l f o r a g i n g , the m a r g i n a l v a l u e theorem. Theor. Pop. B i o l . 9:129-136. Charnov, E. L. 1976b. O p t i m a l f o r a g i n g : a t t a c k s t r a t e g y of a mantid. Amer. Nat. 110: 141-151. Cowie, R. J . , 1977. O p t i m a l f o r a g i n g i n Great T i t s (Parus  m a j o r ) . Nature 268:137-139. Cowie, R . J . and Krebs, J . R. 1979. O p t i m a l f o r a g i n g i n patchy e n v i r o n m e n t s . I_n . R. M. Anderson, B. D. T u r n e r , L. R. T a y l o r ( e d s . ) . P o p u l a t i o n dynamics: the 20th Symposium of the B r i t i s h E c o l o g i c a l S o c i e t y , London, 1978. B l a c k w e l l S c i e n t . P u b l . O x f o r d , pp. 183-205. D a v i e s , N. B. 1977. Prey s e l e c t i o n and the s e a r c h s t r a t e g y of the s p o t t e d f l y c a t c h e r (Muse i c a p a s t r i a t a ) , a f i e l d study on o p t i m a l f o r a g i n g . Anim. Behav. 25:1016-1033. Gass, C. L. 1977. A d i g i t a l encoder f o r f i e l d r e c o r d i n g of b e h a v i o r a l , t e m p o r a l , and s p a t i a l i n f o r m a t i o n i n d i r e c t l y c o m p u t e r - a c c e s s i b l e form. Behav. Res. Methods Ins t r u m . 9:5-11. 52 G l a n d e r , K. E. 1981. Fe e d i n g p a t t e r n s i n mantled h o w l i n g monkeys. I_n . A. C. Kam i l and T. D. Sargent ( e d s . ) . F o r a g i n g b e h a v i o r : e c o l o g i c a l , e t h o l o g i c a l , and p s y c h o l o g i c a l approaches. G a r l a n d STPM P r e s s . New York. pp. 231-257. G o s s - C u s t a r d , J . D. 1977a. O p t i m a l f o r a g i n g and the s i z e s e l e c t i o n of worms by redshank T r i n g a t o t a n u s . Anim. Behav. 25:10-29. G o s s - C u s t a r d , J . D. 1977b. The e n e r g e t i c s of prey s e l e c t i o n by redshank, Tr inga t o t a n u s ' ( L . ) , i n r e l a t i o n t o pr e y d e n s i t y . J . Anim. E c o l . 46: 1-19. Green, R. F. 1980. B a y e s i a n b i r d s : a s i m p l e example of Oaten's s t o c h a s t i c model of o p t i m a l f o r a g i n g . Theor. Pop. B i o l . 18:244-256. H a r t w i c k , E. B. 1976. F o r a g i n g s t r a t e g y of the b l a c k o y s t e r c a t c h e r ( H a e m a t o p u s bachmani Audubon). Can. J . Z o o l . 54:142-155. H e l l e r , R. 1980. On o p t i m a l d i e t i n a patchy environment. Theor. Pop. B i o l . 17:201-214. K o z l o f f , E. N. 1973. Seashore l i f e of Puget Sound, the S t r a i t of G e o r g i a and the San Juan A r c h i p e l a g o . U n i v . of Wash. P r e s s . S e a t t l e . 226 p. Kre b s , J . R. 1978. O p t i m a l f o r a g i n g : d e c i s i o n r u l e s f o r p r e d a t o r s . I_n . J . R. Krebs and N. B. D a v i e s ( e d s . ) . B e h a v i o u r a l e c o l o g y , an e v o l u t i o n a r y approach. B l a c k w e l l S c i e n t i f i c P u b l i c a t i o n s . O x f o r d , pp. 23-63. Kr e b s , J . R., E r i c h s e n , J . T., Webber, M. I . , and Charnov, E. L. 1977. O p t i m a l prey s e l e c t i o n i n the Great T i t (Parus  m a j o r ) . Anim. Behav. 25:30-38. Krebs, J . R., K a c e l n i k , A. and T a y l o r , P. J . 1978. Test of o p t i m a l s a m p l i n g by f o r a g i n g g r e a t t i t s . Nature 275:27-31. L e w i s , A. R. 1980. P a t c h used by gray s q u i r r e l s and o p t i m a l f o r a g i n g . E c o l o g y 61:1371-1379. 53 MacArthur, R. H. 1972. G e o g r a p h i c a l e c o l o g y . Harper and Row, New York. 269 p. Murdoch, W. W. and Oaten, A. 1975. P r e d a t i o n and p o p u l a t i o n s t a b i l i t y . Adv. E c o l . Res. 9: 2-131. Myers, J . P., W i l l i a m s , S. L., and P i t e l k a , F. A. 1980. An e x p e r i m e n t a l a n a l y s i s of prey a v a i l a b i l i t y f o r s a n d e r l i n g s (Aves: S c o l o p a c i d a e ) f e e d i n g on sandy beach c r u s t a c e a n s . Can. J . Z o o l . 58:1564-1574. Oaten, A. 1977. O p t i m a l f o r a g i n g i n p a t c h e s : a case f o r s t o c h a s t i c i t y . Theor. Pop. B i o l . 12:263-285. O'Connor, R. J . And Brown, R. A. 1977. Prey d e p l e t i o n and f o r a g i n g s t r a t e g y i n the o y s t e r c a t c h e r Haematopus  o s t r a l e g u s . O e c o l o g i a 27:75-92. O l l a s o n , J . G. 1980. L e a r n i n g to f o r a g e - o p t i m a l l y ? Theor. Pop. B i o l . 18:44-56. Pyke, G. H., P u l l i a m , H. R.,. Charnov, E. L. 1977. O p t i m a l f o r a g i n g : a s e l e c t i v e review of t h e o r y and t e s t s . Q. Rev. B i o l . 52:137-154. R e a l , L. A. 1981. U n c e r t a i n t y and p o l l i n a t o r - p l a n t i n t e r a c t i o n s : the f o r a g i n g b e h a v i o r of bees and wasps on a r t i f i c i a l f l o w e r s . E c o l o g y 62:20-26. R i c k e t t s , E. W. and C a l v i n , J . 1968. Between P a c i f i c T i d e s , 4th ed., r e v i s e d by J.W. Hedgepeth. S t a n f o r d U n i v . P r e s s , S t a n f o r d , C a l i f . 614 p. Royama, T. 1970. F a c t o r s g o v e r n i n g the h u n t i n g b e h a v i o u r and s e l e c t i o n of food by the Great T i t (Parus major L . ) . J . Anim. E c o l . 39:619-668. S c h l u t e r , D. 1981. Does the t h e o r y of o p t i m a l d i e t s a p p l y i n complex environments? Amer. Nat. 118:139-147. Schoener, T. W. 1971. Theory of f e e d i n g s t r a t e g i e s . Ann. Rev. E c o l . S y s t . 2:369-404. 54 Smith, J . N. M. and Sweatman, H. P. 1974. Food s e a r c h i n g b e h a v i o u r of t i t m i c e i n patchy e n v i r o n m e n t s . E c o l o g y 55:1216-1232. Werner, E. E. and H a l l , D. J . 1974. O p t i m a l f o r a g i n g and the s i z e s e l e c t i o n of prey by the b l u e g i l l s u n f i s h (Lepomis  m a c r o c h i r u s ) . E c o l o g y 55:1042-1052. Werner, E. E., M i t t e l b a c h , G. G., and H a l l , D. J . 1981. T h e , r o l e of f o r a g i n g p r o f i t a b i l i t y and e x p e r i e n c e i n h a b i t a t use by the b l u e g i l l s u n f i s h . E c o l o g y 62:116-125. Zach, R. and F a l l s , J . B. 1976a. O v e n b i r d (Aves: P a r u l i d a e ) h u n t i n g b e h a v i o r i n a pa t c h y environment: an e x p e r i m e n t a l s t u d y . Can. J . Z o o l . 54:1863-1879. Zach, R. and F a l l s , J . B. 1976b. F o r a g i n g b e h a v i o r , l e a r n i n g , and e x p l o r a t i o n by c a p t i v e o v e n b i r d s (Aves: P a r u l i d a e ) . Can. J . Z o o l . 54:1880-1893. Zach, R. and F a l l s , J . B. 1976c. Do o v e n b i r d s (Aves: P a r u l i d a e ) hunt by e x p e c t a t i o n ? Can. J . Z o o l . 54: 1894-1903. Zach, R. and F a l l s , J . B. 1978. Prey s e l e c t i o n by c a p t i v e o v e n b i r d s (Aves: P a r u l i d a e ) . J . Anim. E c o l . 47:929-943. Zach, R. and Smith, J . N. M. 1981. O p t i m a l f o r a g i n g i n w i l d b i r d s ? I_n . A. C. Ka m i l and T. D. Sargent ( e d s . ) . F o r a g i n g b e h a v i o r : e c o l o g i c a l , e t h o l o g i c a l , and p s y c h o l o g i c a l approaches. G a r l a n d STPM P r e s s , New York. pp. 95-109. 55 DEVELOPMENT OF FORAGING SKILLS IN YOUNG BLACK OYSTERCATCHERS INTRODUCTION Young a n i m a l s o f t e n have few or p o o r l y d e v e l o p e d f o r a g i n g s k i l l s (e. g. B u c k l e y and B u c k l e y 1974, Dunn 1972, Groves 1978, O r i a n s 1969, Recher and Recher 1969, Verbeek 1977a) and may r e l y on p a r e n t a l f e e d i n g u n t i l they d e v e l o p f o r a g i n g p r o f i c i e n c y . Growth, p h y s i c a l m a t u r a t i o n , and e a r l y e x p e r i e n c e a re a l l components i n the development of f o r a g i n g i n young a n i m a l s . S u r p r i s i n g l y , s t u d i e s of development of f o r a g i n g s k i l l s a r e r a r e (e. g. Cadrnan 1980, D a v i e s 1976, D a v i e s and Green 1 976, N o r t o n - G r i f f i t h s 1968, R a b i n o w i t c h 1968, 1969, V i n c e 1960). I s t u d i e d development of f o r a g i n g s k i l l s i n b l a c k o y s t e r c a t c h e r s and a t t e m p t e d t o i d e n t i f y i m p o r t a n t p r o c e s s e s i n the development of f o r a g i n g s k i l l s . I s t u d i e d m o r p h o l o g i c a l and b e h a v i o u r a l components of development of f o r a g i n g i n young b i r d s and a n a l y z e d prey c h o i c e , p a t c h c h o i c e , s e a r c h i n g b e h a v i o u r , and prey h a n d l i n g by young b i r d s . STUDY AREAS The summer p o r t i o n of t h i s s tudy was conducted on C l e l a n d I s l a n d , an E c o l o g i c a l Reserve o f f the west c o a s t of Vancouver I s l a n d , B. C. The w i n t e r . p o r t i o n of t h i s s tudy was conducted on 56 t i d a l m u d f l a t s i n Lemmens I n l e t ( L a t . 49°10' N., Long. 125°50' W. ) near T o f i n o , B. C. D u r i n g the w i n t e r months mixed semi-d i u r n a l t i d e s 'in the a r e a range from 0.2 metre t o 3.9 metres. Low t i d e exposes e x t e n s i v e m u d f l a t s where f l o c k s of b l a c k o y s t e r c a t c h e r s come t o f e e d . STUDY ANIMAL B r e e d i n g b l a c k o y s t e r c a t c h e r s and t h e i r c h i c k s were the s u b j e c t s of t h i s s t u d y . From l a t e A p r i l u n t i l the end of August b r e e d i n g o y s t e r c a t c h e r s on C l e l a n d I s l a n d defended t e r r i t o r i e s t h a t i n c l u d e d n e s t s i t e s and i n t e r t i d a l f e e d i n g a r e a s . B l a c k o y s t e r c a t c h e r c h i c k s a re p r e c o c i a l , but they remain on t h e i r p a r e n t s ' t e r r i t o r i e s u n t i l they can f l y ( a p p r o x i m a t e l y 35 days of age) and u s u a l l y l o n g e r (50 or more days of. a g e ) . D u r i n g t h i s p e r i o d most prey items eaten by c h i c k s a re p r o v i d e d f o r them by t h e i r p a r e n t s . S h o r t l y a f t e r h a t c h i n g , a l l c h i c k s on C l e l a n d I s l a n d were i n d i v i d u a l l y marked w i t h unique c o m b i n a t i o n s of c o l o r e d p l a s t i c l e g bands. Through at l e a s t t h e i r f i r s t y e a r , young b i r d s can be r e a d i l y d i s t i n g u i s h e d from a d u l t s by t h e i r dark brown b i l l t i p s and d u l l i r i s c o l o r . A d u l t s have b r i l l i a n t r e d b i l l s and chrome y e l l o w i r i s e s . By l a t e August, most a d u l t s and f l y i n g c h i c k s spent v e r y l i t t l e time on C l e l a n d . D u r i n g the f a l l and w i n t e r months o y s t e r c a t c h e r s f o r a g e d on s h e l t e r e d r e e f s and t i d a l f l a t s i n the ar e a of T o f i n o , B. C. Some o y s t e r c a t c h e r s f o r a g e d on t i d a l m u d f l a t s i n Lemmens I n l e t , and these b i r d s were the s u b j e c t s of the w i n t e r f o r a g i n g o b s e r v a t i o n s r e p o r t e d h e r e . 57 METHODS I n t e r t i d a l Prey Organisms The d i s t r i b u t i o n and abundance of organisms taken as prey by b l a c k o y s t e r c a t c h e r s on C l e l a n d I s l a n d were a s s e s s e d by 176 50 x 50 cm quadr a t samples made between h i g h and low water l i n e d u r i n g extreme low t i d e s . The abundance of s m a l l i n v e r t e b r a t e s l i v i n g i n spaces between mussels i n mussel beds was a s s e s s e d by e l e v e n 25 x 25 cm q u a d r a t s . ( D e t a i l s of the sa m p l i n g scheme are o u t l i n e d i n the p r e v i o u s c h a p t e r ) . The abundance of mussels ( M y t i l u s e d u l i s ) i n the w i n t e r f o r a g i n g a r e a , Lemmens I n l e t , was a l s o a s s e s s e d by quadrat s a m p l i n g . M u s s e l s o c c u r r e d i n l a r g e p a t c h e s (up t o 50 m long) on the m u d f l a t s . F i f t e e n q u a d r a t s (50 x 50 cm) i n mussel patches were randomly s e l e c t e d , and the numbers and l e n g t h s of a l l mussels w i t h i n the q u a d r a t s were r e c o r d e d . A l e n g t h - w e i g h t r e g r e s s i o n (Menge 1972) was used t o e s t i m a t e mussel biomass. C h i c k Growth C h i c k s were weighed and measured about every t h i r d day from h a t c h i n g u n t i l they c o u l d no l o n g e r be caught (about 35 days of age when they began t o f l y ) . B i l l l e n g t h (exposed culmen) was measured i n m i l l i m e t r e s . Weight was r e c o r d e d t o the n e a r e s t gram. 58 F o r a g i n g B e h a v i o u r I n d i v i d u a l a d u l t s and c h i c k s (0-55 days of age) i n e i g h t o y s t e r c a t c h e r f a m i l i e s were observed f o r a g i n g d u r i n g low t i d e p e r i o d s from 3 hours b e f o r e u n t i l 3 hours a f t e r low t i d e . C o n tinuous r e c o r d s of f o r a g i n g b e h a v i o u r were encoded u s i n g a d i g i t a l event r e c o r d e r . Records of f o r a g i n g b e h a v i o u r i n c l u d e d the f o l l o w i n g : b i r d i d e n t i f i c a t i o n and zone of the i n t e r t i d a l ; d u r a t i o n of p r e y - h a n d l i n g and c h i c k - f e e d i n g e v e n t s , prey t y p e , and prey s i z e w i t h r e s p e c t t o the b i r d ' s b i l l l e n g t h ; d u r a t i o n of s u c c e s s f u l s e a r c h e s ( p e r i o d s of c o n t i n u o u s s e a r c h ending w i t h a prey c a p t u r e ) and u n s u c c e s s f u l s e a r c h e s ( p e r i o d s of c o n t i n u o u s s e a r c h e n d i n g i n a n o n - s e a r c h i n g b e h a v i o u r w i t h o u t a prey c a p t u r e ) ; d u r a t i o n of n o n - f o r a g i n g b e h a v i o u r s such as s i t t i n g , s t a n d i n g , and f l y i n g t h a t occur d u r i n g p e r i o d s of f o r a g i n g . Data were a n a l y z e d by F o r t r a n programs t h a t I wrote and by the MIDAS s t a t i s t i c a l package. ( D e t a i l s of d a t a c o l l e c t i o n a r e summarized i n the p r e v i o u s c h a p t e r ) . F o r a g i n g b i r d s , e s p e c i a l l y c h i c k s , sometimes s e l e c t e d prey items so s m a l l t h a t the prey type c o u l d not be i d e n t i f i e d by an o b s e r v e r . These prey items were c l a s s i f i e d as "unknown" and a s s i g n e d a wet weight of 0.02 g, the weight of the s m a l l e s t prey items measured i n the quad r a t samples. Data on f o r a g i n g b e h a v i o u r p r e s e n t e d here were c o l l e c t e d i n two d i f f e r e n t p l a c e s : 1) on C l e l a n d I s l a n d b r e e d i n g t e r r i t o r i e s from l a t e June through August, 1977, when c h i c k s were 0-55 days o l d ; 2) on Lemmens I n l e t m u d f l a t s d u r i n g January and F e b r u a r y , 1-978, -when c h i c k s were a p p r o x i m a t e l y 6 months o l d . 59 RESULTS Prey D i s t r i b u t i o n and Abundance Prey abundance, d i v e r s i t y , and range of s i z e s were q u i t e h i g h on the C l e l a n d I s l a n d study s i t e and v e r y low a t the Lemmens I n l e t s i t e . On C l e l a n d I s l a n d f o u r i n t e r t i d a l zones were i d e n t i f i e d between the h i g h and low water l i n e s . In orde r from h i g h t o low water they a r e : 1) spray zone (SPZO), 2) fucus zone (FUCU), 3) m y t i l u s zone (MYTL), 4) l a m i n a r i a - p o s t e l s i a zone (LAPO). The abundance.and biomass of prey p r e s e n t i n each zone i n c r e a s e d markedly between the h i g h and low water l i n e s (see F i g u r e s 3 and 4 ) . The g r e a t e s t abundance and -g r e a t e s t biomass of prey o c c u r r e d i n the lowe s t zones of the i n t e r t i d a l . C e r t a i n p r o f i t a b l e p rey t y p e s (e. g. c h i t o n s , K a t h a r i n a t u n i c a t a , and c r a b s , Oedignathus i n e r m i s ) were r e c o r d e d o n l y i n the l o w e s t zones. In Lemmens I n l e t , mussels ( M y t i l u s e d u l i s ) were the most abundant i n v e r t e b r a t e s o c c u r r i n g on the m u d f l a t s and the o n l y i n v e r t e b r a t e s t h a t b i r d s were obse r v e d to e a t . The number of l i v e mussels i n each of the f i f t e e n 50 x 50 cm quadrat was h i g h l y v a r i a b l e . An average of 100.4 mussels ( s . d. = 102.6) was found i n each q u a d r a t . L i v e mussels were r e l a t i v e l y u n i f o r m i n s i z e w i t h an average l e n g t h of 40.0 mm ( s . d. = 3.3). C h i c k Growth •Black o y s t e r c a t c h e r c h i c k s grew from 30-35 grams a t 60 h a t c h i n g t o 350 grams or more by the time of t h e i r f i r s t f l i g h t . A d r a m a t i c change i n b i l l l e n g t h accompanied t h i s change i n body w e i g h t . C h i c k s ' b i l l s grew from about 15 mm a t h a t c h i n g t o 45 mm or more by 35 days of age. Average c h i c k b i l l l e n g t h s and w e i g h t s f o r 10 day increments of c h i c k age are shown i n F i g u r e 10. Average a d u l t w e i g h t s and b i l l l e n g t h s a r e i n c l u d e d f o r compar i son. A l l o c a t i o n of F o r a g i n g Time between Zones C h i c k s and t h e i r p a r e n t s f o r a g e d t o g e t h e r and, c o n s e q u e n t l y , spent s i m i l a r p r o p o r t i o n s of t h e i r f o r a g i n g time i n each zone of the i n t e r t i d a l . Data on p r o p o r t i o n s of f o r a g i n g time 'spent i n each zone of the i n t e r t i d a l by a d u l t s and c h i c k s a r e summarized i n T a b l e V. C h i c k s f e d s l i g h t l y but not s i g n i f i c a n t l y more o f t e n i n upper i n t e r t i d a l zones than d i d t h e i r p a r e n t s , and p a r e n t s never f o r a g e d i n the uppermost zone, the spray zone. For c o m p a r i s o n , d a t a on p r o p o r t i o n of t h e i r f o r a g i n g time a d u l t s w i t h o u t c h i c k s spent i n each zone are a l s o i n c l u d e d i n T a b l e V. A d u l t s w i t h and w i t h o u t c h i c k s d i d not d i f f e r s i g n i f i c a n t l y i n t h e i r a l l o c a t i o n of f o r a g i n g time between zones. Prey S e l e c t i o n by C h i c k s and t h e i r P a r e n t s Food d e l i v e r e d by p a r e n t s and s m a l l "unknown" prey items t h a t c h i c k s p i c k e d out of mussel beds were f r e q u e n t l y i n c l u d e d i n the c h i c k d i e t . Young o y s t e r c a t c h e r s have s h o r t b i l l s and t h e r e f o r e l a c k e f f e c t i v e t o o l s f o r p r o c u r i n g and h a n d l i n g marine 61 F i g u r e 10: Average c h i c k b i l l l e n g t h s and w e i g h t s over ten day i n t e r v a l s from . h a t c h i n g t o f l e d g i n g . ( V e r t i c a l b a r s are 95% c o n f i d e n c e l i m i t s ; numbers b e s i d e d a t a p o i n t s are sample s i z e s ) . 62 TABLE V Average p r o p o r t i o n of f o r a g i n g time spent i n each zone of the i n t e r t i d a l * Zone SPZO FUCU MYTL LAPO C h i c k s (N = 22) 0.05 0.19 0.58 0.18 A d u l t s 1 w i t h C h i c k s (N = 25) 0.00 2 0.05 3 0.70" 0.24 5 A d u l t s 6 w i t h o u t C h i c k s (N = 12) 0.03 7 0.03 s 0.52 9 0.42 1 0 * A r c s i n t r a n s f o r m a t i o n was a p p l i e d t o a l l d a t a b e f o r e a n a l y s i s of v a r i a n c e . 1 F - v a l u e s f o r comparison of c h i c k s w i t h t h e i r p a r e n t s 2 F = 2.12, df = 1,45, NS; 3 F = 1.91, df = 1,45, NS ; • F = 2.24, df = 1,45, NS; 5 F = 0.79, df = 1,45, NS. 6 F - v a l u e s f o r comparison of a d u l t s w i t h c h i c k s and a d u l t s w i t h o u t c h i c k s : 7 F = 3.65, df = 1,35, NS; 8 F = 0.34, df = 1,35, NS; 9 F = 2.56, df = 1,35, NS; 1 0 F = 1.16, df = 1,35, NS 64 i n v e r t e b r a t e s ( l i m p e t s , c h i t o n s , m u s s e l s , c r a b s , worms) eaten by a d u l t b l a c k o y s t e r c a t c h e r s i n the st u d y a r e a . C o n s e q u e n t l y , c h i c k s depend h e a v i l y on t h e i r p a r e n t s f o r food e a r l y i n l i f e . The p r o p o r t i o n s of f e e d i n g s c h i c k s o b t a i n e d on t h e i r own and the p r o p o r t i o n of t o t a l food biomass th e s e f e e d i n g s r e p r e s e n t a re p l o t t e d i n F i g u r e 11. As c h i c k s got o l d e r and b i g g e r they o b t a i n e d an i n c r e a s i n g p r o p o r t i o n of prey f o r them s e l v e s but c o n t i n u e d t o r e c e i v e some food from t h e i r p a r e n t s f o r an extended t i m e . F l y i n g c h i c k s 50 days of age and o l d e r r e c e i v e d 15% of t h e i r f e e d i n g s and about 57% of t h e i r t o t a l food biomass from t h e i r p a r e n t s . In each zone, the average biomass of i n d i v i d u a l p rey s e l e c t e d by: 1) c h i c k s , 2) p a r e n t s , and 3) p a r e n t s f e e d i n g c h i c k s a re shown i n F i g u r e 12 • f o r t e n day incr e m e n t s of c h i c k age. The l a r g e s t prey items i n the c h i c k d i e t were those items f e d t o c h i c k s by t h e i r p a r e n t s . Prey items taken by c h i c k s were g e n e r a l l y much s m a l l e r than prey items taken by a d u l t s e i t h e r f o r themselves or f o r t h e i r c h i c k s . The l a r g e s t prey items taken by c h i c k s were o b t a i n e d i n the m y t i l u s zone where the abundance of s m a l l prey items i s g r e a t e s t (see F i g u r e 4 ) . (Average prey w e i g h t s , w i t h s t a n d a r d d e v i a t i o n s , taken by c h i c k s i n each zone a r e : SPZO - 0.02g ( 0 . 0 1 ) , N = 26; FUCU - 0.05g ( 0 . 1 4 ) , N = 22; MYTL - 0.43g (1 . 5 5 ) , N = 78; LAPO -0.02g ( 0 . 0 7 ) , N = 86;, F = 3.09, df = 3,211, p.< 0.05). Sm a l l and weakly a t t a c h e d or u n a t t a c h e d prey items such as amphipod c r u s t a c e a n s , v e r y s m a l l sea cucumbers and v e r y s m a l l c h i t o n s were o f t e n taken by c h i c k s p e c k i n g among the m u s s e l s . O f t e n , these s m a l l , s o f t - b o d i e d prey items l a c k i n g d i s t i n c t i v e hard p a r t s were d i f f i c u l t t o i d e n t i f y . a c c u r a t e l y i n the f i e l d . These 65 F i g u r e 11: P r o p o r t i o n s of f e e d i n g s and food biomass p r o c u r e d c h i c k s . C H I C K A G E ( D A Y S ) 67 F i g u r e 12: Average biomass of i n d i v i d u a l prey 1) taken by c h i c k s , 2) t a k e n by a d u l t s , and 3) f e d t o c h i c k s by a d u l t s i n each zone of the i n t e r t i d a l . (Numbers below h i s t o g r a m bars a r e s a r p l e s i z e s . V e r t i c a l bars a r e 95% c o n f i d e n c e l i m i t s ) . 68 0.1 OT . 0.01-1.00 ^ 0.1 0' to < 0.01' cr o h- 10.0 0 X o U J 1.0 0 >" 0.10 LU cr 0.01 10.00 1.0 0 0.10 0.01 SPRAY ZONE 10 1 1 12 FUCUS ZONE 82 31 1 7 3 19 1 99 2 4 44 MYTILUS ZONE t i f a i f - i • ^ -> ^  ' ir- <i i tr • C / A r 1 • in i r\ 11 o • 52 2 ' 2 3 8 2 2 16 74 26 ' 5 40 53 47 49 118 8 6 10 T LAM INAR IA-POSTELSIA ZONE 13 2 1 1 93 20 * 1 1 7 5 " 2 9 8 1 2 20 86 ' 76 6 1 - 9 1 0 - 1 9 2 0 - 2 9 3 0 - 3 9 A O - 4 9 5 0 - 5 5 C H I C K A G E (DAYS) = PREY TAKEN BY CHICKS r Z PREY TAKEN BY ADULTS JX, PREY FED TO CHICKS BY ADULTS 69 "unknown" prey items were common i n the c h i c k d i e t . F i g u r e 13 shows the t o t a l f r e q u e n c i e s w i t h which each prey type was taken by 1) c h i c k s , 2) a d u l t s , 3) a d u l t s f e e d i n g c h i c k s . C h i c k s seldom took l i m p e t s or mu s s e l s , the most p r o f i t a b l e items i n t h e i r d i e t t h a t a d u l t s p r o v i d e d f o r them. The f r e q u e n c i e s of prey t y p e s taken, by c h i c k s and those taken by a d u l t s f o r themselves d i f f e r e d s i g n i f i c a n t l y ( X 2 = 238.7, df = 6, p < 0.001). A f u r t h e r breakdown of prey t y p e s taken by c h i c k s i s p r e s e n t e d i n Ta b l e V I . From.10 through 55 days of age, s m a l l "unknown" prey items a re the most common prey items taken by c h i c k s . S e a r c h i n g B e h a v i o u r C h i c k s ' s e a r c h i n g b e h a v i o u r changed as they grew. I n t e r -peck i n t e r v a l s (seconds between'pecks) measure how f r e q u e n t l y f o r a g i n g b i r d s i n v e s t i g a t e d p o t e n t i a l prey i t e m s . I n t e r - p e c k i n t e r v a l s of young c h i c k s f o r a g i n g i n each zone of the i n t e r t i d a l were s i g n i f i c a n t l y l o n g e r than those of a d u l t s f o r a g i n g i n the same zone. Data on i n t e r - p e c k i n t e r v a l s f o r c h i c k s (averaged over 10 day in c r e m e n t s of age) f o r a g i n g i n each zone of the i n t e r t i d a l a r e shown i n F i g u r e 14 a l o n g w i t h average i n t e r - p e c k i n t e r v a l s of a d u l t s f o r a g i n g w r t h t h e i r c h i c k s . As c h i c k s became o l d e r t h e i r average i n t e r - p e c k i n t e r v a l s s h o r t e n e d and approached a d u l t performance. W i n t e r F o r a g i n g By m i d - w i n t e r young and a d u l t o y s t e r c a t c h e r s d i d not d i f f e r much i n t h e i r f o r a g i n g b e h a v i o u r . D u r i n g t h e w i n t e r , young and 70 F i g u r e 13: F r e q u e n c i e s w i t h which 7 prey t y p e s were 1) taken c h i c k s , 2) taken by a d u l t s , and 3) f e d t o c h i c k s by a d u l t s . 1.0 >-LU cr Q_ o O 0.5 t— cr o Q_ o cr a. 0.0 I BARNACLE CHITON CRAB LIMPET MUSSEL N=164 N=15 N=68 N=688 N=104 P R E Y T Y P E WORM N=52 UNKNOWN N=333 = P R E Y TAKEN BY CHICKS, N= 212 " P R E Y TAKEN BY ADULTS, N=495 ^ > ^ P R E Y FED TO CHICKS / / BY ADULTS, N = 717 TABLE VI Numbers of v a r i o u s prey t y p e s taken by c h i c k s of d i f f e r e n t ages Chi c k age (days) Prey Type 10-19 20-29 30-39 40-49 50-55 b a r n a c l e s 0 0 0 1 7 4 c r a b s 0 3 1 0 1 amphipod c r u s t a c . 0 0 0 2 0 1impets 1 5 2 8 1 mussels 0 0 0 2 0 sea-cucumbers 0 1 1 1 0 worms 0 1 0 0 0 unknown 1 2 10 22 43 82 Hours* 0.29 1 .09 1 .35 4.09 1 .22 T o t a l o b s e r v a t i o n time of c h i c k s f o r a g i n g on t h e i r own. 73 F i g u r e 1 4 : A v e r a g e d u r a t i o n o f i n t e r - p e c k i n t e r v a l s o f c h i c k s o v e r t e n d a y i n t e r v a l s o f a g e i n e a c h i n t e r t i d a l z o n e . A v e r a g e i n t e r - p e c k i n t e r v a l s o f a d u l t s f o r a g i n g i n e a c h z o n e a r e i n c l u d e d f o r c o m p a r i s o n w i t h c h i c k s . ( V e r t i c a l b a r s a r e 9 5 % c o n f i d e n c e l i m i t s ) . I N T E R - P E C K I N T E R V A L S (SECONDS) o < > o TT (/I < CO Q a. $ X J / \ C3 O O o L O , o , 10-19 $ 20-29 w 30-39 40-49 £ 50-55 & ADULT 10-19 20-29 $ 30-39 S 40-49 ^ 50-55 ADULT S o X o > o m a 10"19 > -< CD cn oo 20-29 S 30-39 £ 40-49 o 50-55 ^ ADULT » 10-19 £ cn 20-29 30-39 g z.0-49 * 50-55 ADULT g f c> ^ 1 f i i i i i t * oo o en "0 X) M O z m -n c o cz CO M O z m cr to NI O z m 3 8 m 75 a d u l t o y s t e r c a t c h e r s f o r a g e d t o g e t h e r i n f l o c k s on t i d a l m u d f l a t s i n Lemmens I n l e t . U n i f o r m l y s i z e d mussels d i s t r i b u t e d a c r o s s the m u d f l a t s were the o n l y prey items b i r d s took i n t h i s a r e a . By m i d w i n t e r , January and F e b r u a r y , average i n t e r - p e c k i n t e r v a l s f o r young b i r d s d i d not d i f f e r s t a t i s t i c a l l y from average i n t e r - p e c k i n t e r v a l s of a d u l t s f o r a g i n g i n the same a r e a . ( a d u l t s : average i n t e r - p e c k i n t e r v a l = 4.39 s e c , s. d. = 4.1, N = 512; young: average i n t e r - p e c k i n t e r v a l = 4.55 s e c , s. d. = 4.8, N = 596; F = 0.33, df = 1,1107, NS) . Young b i r d s took s l i g h t l y l o n g e r t o handle mussels than d i d a d u l t s , but the d i f f e r e n c e i s not s t a t i s t i c a l l y s i g n i f i c a n t ( a d u l t s : average h a n d l i n g time = 29.5 s e c , s. d. = 17.9, N = 89; young: average h a n d l i n g time = 33.5 s e c , s. d. = 22.3, N = 104; F = 1.90, df'= 1,192, NS). Prey S t e a l i n g Prey s t e a l i n g was e x h i b i t e d by some young o y s t e r c a t c h e r s . Young b i r d s f o r a g i n g on m u d f l a t s sometimes atte m p t e d t o s t e a l opened mussels from f e e d i n g b i r d s . T y p i c a l l y , a young b i r d would approach a f e e d i n g i n d i v i d u a l and suddenly lunge towards i t i n an apparent attempt t o s t e a l a p a r t i a l l y eaten m u s s e l . Sometimes a young b i r d succeeded i n - s t e a l i n g a m u s s e l , but o f t e n the f e e d i n g b i r d would run or f,ly away c a r r y i n g the m u s s e l . -A t o t a l of 22 chases were obse r v e d d u r i n g seven low t i d e o b s e r v a t i o n p e r i o d s ; 19 of these chases were i n i t i a t e d by young b i r d s (13 chases towards a d u l t s , 6 chases towards o t h e r young b i r d s ) . S i x of the 13 times young b i r d s chased a d u l t s , young b i r d s 'succeeded i n s t e a l i n g p a r t i a l l y eaten m u s s e l s . (On one o c c a s i o n when a 76 young b i r d chased a young b i r d , the c h a s e r succeeded i n s t e a l i n g a m u s s e l ) . Three of the observ e d chases were i n i t i a t e d by a d u l t s (2 chases towards young b i r d s , 1 chase towards an a d u l t ) , a p p a r e n t l y as d e f e n s i v e responses t o i n t r u s i o n s i n t o f e e d i n g a d u l t s ' i n d i v i d u a l d i s t a n c e s . On none of these o c c a s i o n s d i d a d u l t s s t e a l p r e y . Observed and e x p e c t e d numbers of chases based on the age c o m p o s i t i o n of f o r a g i n g f l o c k s are summarized i n Tab l e V I I . Young b i r d s i n i t i t a t e d c h a s e s , apparent a t t e m p t s t o s t e a l food, more o f t e n than a d u l t s . I made o b s e r v a t i o n s of u n i n t e r r u p t e d f o r a g i n g bouts by 21 a d u l t s and 22 young b i r d s on the m u d f l a t s . Among the young b i r d s 10 were observed t o i n i t i a t e c h a ses, a t t e m p t s t o s t e a l p r e y , and the o t h e r 12 were not. Young b i r d s i n these two groups, c h a s e r s and n o n - c h a s e r s , d i d not d i f f e r i n p r e y - h a n d l i n g times (F =0.76, df = 1,103, NS) or d u r a t i o n of s u c c e s s f u l s e a r c h (F =0.06, df = 1,103, NS), but the two groups d i d d i f f e r i n d u r a t i o n of u n s u c c e s s f u l s e a r c h ( c h a s e r s : 40.1 s e c , s. d. = 36.3; non-chasers: 19.3 s e c , s. d. = 16.4; F = 8.41, df = 1,67, p < 0.01). The l o n g d u r a t i o n s of u n s u c c e s s f u l s e a r c h f o r c h a s e r s r e f l e c t the time these i n d i v i d u a l s spent i n p u r s u i t . DISCUSSION Growth and p h y s i c a l m a t u r a t i o n a re im p o r t a n t i n the development of f o r a g i n g s k i l l s i n young b l a c k o y s t e r c a t c h e r s . Other s t u d i e s suggest t h a t e a r l y e x p e r i e n c e and c r i t i c a l p e r i o d s a r e a l s o i m p o r t a n t i n b e h a v i o u r a l development of young a n i m a l s (Bateson 1976). TABLE V I I Chases on w i n t e r f o r a g i n g a r e a * (Numbers i n p a r e n t h e s e s are p r o p o r t i o n s ) Group Chases i n i t i a t e d by: C o m p o s i t i o n A d u l t Young Date A d u l t s Young Obs. (Exp.) Obs. (Exp.) 29/1/78 19(0.56) 15(0.44) 2 (6.16) 9 (4.84) 30/1/78 17(0.65) 9(0.35) 0 (3.25) 5 ( 1 . 7 5 ) 13/2/78 10(0.50) 10(0.50) 1 (3.00) 5. (3.00) * R e s u l t s of C h i - s q u a r e o n e - t a i l e d t e s t : 1 X 2 = 6.38, df = 1, p < 0.01 2 X 2 = 9.29, df = 1, p < 0.005 3 X 2 = 2.67, df = 1, NS 78 C h i c k Growth B l a c k o y s t e r c a t c h e r c h i c k s undergo g r e a t m o r p h o l o g i c a l change from h a t c h i n g a t 35 grams t o t h e i r f i r s t f l i g h t about 35 days l a t e r a t 350 grams or more. E l o n g a t i o n of the b i l l i n t o a more v e r s a t i l e f o r a g i n g t o o l and i n c r e a s e d s t r e n g t h are two i m p o r t a n t a s p e c t s of m a t u r a t i o n and development of f o r a g i n g p r o f i c i e n c y i n o y s t e r c a t c h e r c h i c k s . However, even a f t e r an o y s t e r c a t c h e r ' s b i l l has reached a d u l t l e n g t h , a b i r d may l a c k the p h y s i c a l s t r e n g t h of an a d u l t u n t i l a t l e a s t two y e a r s of age due t o i n c o m p l e t e s k u l l o s s i f i c a t i o n and, p r o b a b l y , l e s s muscle mass (Cadman 1980). F u r t h e r m o r e , j u v e n i l e o y s t e r c a t c h e r s ' p o i n t e d b i l l t i p s h i n d e r t h e i r a b i l i t y t o handle prey such as m u s s e l s . N o r t o n - G r i f f i t h s (1968) amputated the p o i n t e d b i l l t i p s of two 54 day o l d o y s t e r c a t c h e r c h i c k s and recorded- 11% and 15% d e c r e a s e s i n the time b i r d s took t o handle mussels ( M y t i l u s  e d u l i s ) . Both a w e l l - d e v e l o p e d , b l u n t e d b i l l and p h y s i c a l s t r e n g t h are n e c e s s a r y b e f o r e a b i r d can p r o f i c i e n t l y handle the f u l l a r r a y of prey t y p e s taken by a d u l t o y s t e r c a t c h e r s . A l l o c a t i o n of F o r a g i n g Time between Zones E a r l y exposure t o p a r t i c u l a r h a b i t a t t y p e s appears i m p o r t a n t i n d e v e l o p i n g h a b i t a t p r e f e r e n c e s t h a t p e r s i s t i n l a t e r l i f e . The e a r l y e x p e r i e n c e of c h i c k s f o r a g i n g w i t h t h e i r p a r e n t s and b e i n g f e d i n d i f f e r e n t zones of the i n t e r t i d a l may be i m p o r t a n t i n e s t a b l i s h i n g p a t t e r n s of zone use by c h i c k s . O y s t e r c a t c h e r c h i c k s f o r a g e d w i t h t h e i r p a r e n t s and a l l o c a t e d t h e i r f o r a g i n g time between zones i n s i m i l a r p r o p o r t i o n s t o 79 t h e i r p a r e n t s . (These p a t t e r n s of zone use were s i m i l a r t o those of a d u l t o y s t e r c a t c h e r s w i t h o u t c h i c k s ) . K l o p f e r (1963) has shown e x p e r i m e n t a l l y t h a t e a r l y e x p e r i e n c e does a f f e c t h a b i t a t p r e f e r e n c e i n l a t e r l i f e . In h i s e x p e r i m e n t s , c a p t i v e c h i p p i n g sparrows ( S p i z e l l a p a s s e r i n a ) p r e f e r r e d l a b o r a t o r y h a b i t a t s t h a t they were exposed t o from e a r l y l i f e t o n o v e l h a b i t a t s . Prey Choice E a r l y e x p e r i e n c e of young o y s t e r c a t c h e r c h i c k s i s p r o b a b l y v e r y i m p o r t a n t i n the development of prey r e c o g n i t i o n , prey p r e f e r e n c e s , and prey h a n d l i n g t e c h n i q u e s . A l t h o u g h young c h i c k s d i d not take many of the p r o f i t a b l e prey types common i n the a d u l t d i e t ( F i g u r e 13), s e a r c h i n g f o r and h a n d l i n g s m a l l prey i n mussel beds gave c h i c k s e x p e r i e n c e i n r e c o g n i z i n g and m a n i p u l a t i n g p r e y . As c h i c k s grew and began t o o b t a i n some prey on t h e i r own, the p a r e n t a l c o n t r i b u t i o n of food d e c l i n e d i n a p a t t e r n s i m i l a r t o t h a t noted by D a v i e s (1976) i n s p o t t e d f l y c a t c h e r s (Muse i c a p a s t r i a t a ) . However, even as the freq u e n c y w i t h which c h i c k s o b t a i n e d prey f o r themselves i n c r e a s e d , p a r e n t s c o n t i n u e d t o p r o v i d e the l a r g e s t prey items consumed by c h i c k s ( F i g u r e 12). P a r e n t a l f e e d i n g s p r o b a b l y gave c h i c k s e x p e r i e n c e i n h a n d l i n g p r e y , e s p e c i a l l y when a d u l t s brought them prey such as l i m p e t s and mussels t h a t had not had the e d i b l e and i n e d i b l e p a r t s s e p a r a t e d . S e a r c h i n g B e h a v i o u r and Prey H a n d l i n g S e a r c h i n g b ehaviour changed a s . c h i c k s matured. Young c h i c k s 80 i n v e s t i g a t e d prey items (as measured by i n t e r - p e c k i n t e r v a l s ) a t much sl o w e r r a t e s than d i d t h e i r p a r e n t s f o r a g i n g w i t h them i n the same zones ( F i g u r e 14). As c h i c k s got o l d e r , the r a t e at which they s e a r c h e d i n c r e a s e d and approached a d u l t r a t e s s h o r t l y a f t e r c h i c k s began f l y i n g . S i m i l a r p a t t e r n s of development of s e a r c h i n g b e h a v i o u r i n young b i r d s have been r e p o r t e d by D a v i e s and Green (1976) i n young reed w a r b l e r s ( A c r o c e p h a l u s  sc i rpaceus) and by D a v i e s (1976) i n s p o t t e d f l y c a t c h e r s . In t h e s e two s p e c i e s , number of movements per minute and c a p t u r e a t t e m p t s per minute, r e s p e c t i v e l y , i n c r e a s e d w i t h age and approached a d u l t performance. By m i d - w i n t e r , s e a r c h i n g and p r e y - h a n d l i n g b e h a v i o u r of young b l a c k o y s t e r c a t c h e r s on the m u d f l a t s was s i m i l a r t o t h a t of a d u l t o y s t e r c a t c h e r s . Young b i r d s i n m i d - w i n t e r took s l i g h t l y , but not s i g n i f i c a n t l y , l o n g e r t o handle prey items (mussels) than a d u l t s . T h i s r e s u l t i s s u r p r i s i n g i n view of N o r t o n - G r i f f i t h s ' (1968) c l a i m t h a t o y s t e r c a t c h e r s take up t o 3 y e a r s t o a t t a i n a d u l t p r o f i c i e n c y h a n d l i n g prey i t e m s . Data from a p o p u l a t i o n of o y s t e r c a t c h e r s (H. p a l l i a t u s ) i n V i r g i n i a i n d i c a t e t h a t prey s i z e i s the key f a c t o r i n comparing prey h a n d l i n g t i m e s of a d u l t and immature b i r d s . Cadman (1980) found t h a t a d u l t s and immatures had s i m i l a r h a n d l i n g t i m e s f o r s m a l l o y s t e r s ( C r a s s o s t r e a v i r g i n i c a ) l e s s than 1 ml i n volume, but t h a t a d u l t s took s i g n i f i c a n t l y l e s s time t o handle l a r g e r o y s t e r s . Quinney and Smith (1980) o b t a i n e d s i m i l a r r e s u l t s i n a study of a d u l t and j u v e n i l e g r e a t b l u e herons (Ardea h e r o d i a s ) f e e d i n g on f i s h . S i m i l a r i t i e s i n h a n d l i n g t i m e s of a d u l t and f i r s t w i n t e r b l a c k o y s t e r c a t c h e r s f o r a g i n g on mussels i n Lemmens 81 I n l e t p r o b a b l y o c c u r r e d because the a v a i l a b l e mussels were r e l a t i v e l y s m a l l and u n i f o r m i n s i z e . There were no l a r g e mussels a t the Lemmens I n l e t s i t e t o a l l o w e v a l u a t i o n of p r o f i c i e n c y of f i r s t w i n t e r b i r d s over a broad range of prey s i z e s . Prey S t e a l i n g Young b i r d s sometimes i n i t i a t e d chases i n apparent a t t e m p t s t o s t e a l prey from o t h e r f o r a g i n g b i r d s , m o s t l y a d u l t s . Prey s t e a l i n g by f o r a g i n g o y s t e r c a t c h e r s a l s o o c c u r r e d i n w i n t e r i n V i r g i n i a , and t h e r e , a l s o , a d u l t s were more o f t e n the v i c t i m s ( a d u l t s l o s t 1 of 5 prey i t e m s , immatures l o s t 1 of 14 prey i t e m s ; Cadman 1980). Prey s t e a l i n g has a l s o been r e p o r t e d i n g u l l s (Verbeek 1977b) and h e rons, e g r e t s , and i b i s e s i n w i n t e r f o r a g i n g a g g r e g a t i o n s (Kushlan 1978). B i r d s may chase and s t e a l prey f o r s e v e r a l r e a s o n s . 1) Some young b i r d s may not o b t a i n s u f f i c i e n t r a t i o n s on t h e i r own (e. g. Dare 1977) and may chase f e e d i n g b i r d s t o make up food d e f i c i t s . 2) Young b i r d s may s t i l l r e c o g n i z e f e e d i n g a d u l t s as p r o v i d e r s of f o o d , and chases may r e s u l t because an a d u l t does not o f f e r food as a p a r e n t would. In t h i s and o t h e r s t u d i e s of o y s t e r c a t c h e r s (Cadman 1980; N o r t o n - G r i f f i t h s 1969) young from 4 t o 10 months of age s u c c e s s f u l y begged from t h e i r p a r e n t s . Important P r o c e s s e s i n Development of F o r a g i n g S k i l l s In o y s t e r c a t c h e r s , development of f o r a g i n g s k i l l s t h a t •enable b i r d s t o e x p l o i t d i f f i c u l t - t o - h a n d l e -prey items r e s u l t s 82 from m o r p h o l o g i c a l and b e h a v i o u r a l changes. Young c h i c k s are i l l - e q u i p p e d m o r p h o l o g i c a l l y t o handle prey types common i n d i e t s of a d u l t s , but p h y s i c a l m a t u r a t i o n a l o n e does not bestow a f u l l complement of f o r a g i n g s k i l l s on young b i r d s . L e a r n i n g and e a r l y e x p e r i e n c e p r o b a b l y p l a y i m p o r t a n t r o l e s i n development of f o r a g i n g b e h a v i o u r as c h i c k s grow. The importance of l e a r n i n g i n the development of f e e d i n g b e h a v i o u r i n o y s t e r c a t c h e r c h i c k s was demonstrated by H o r l y k and L i n d (1978). They c o n c l u d e d from e x p e r i m e n t s w i t h newly ha t c h e d o y s t e r c a t c h e r s (H. o s t r a l e g u s ) t h a t l e a r n i n g a b i l i t y and e a r l y e x p e r i e n c e e x p l a i n e d the development of c h i c k responses d u r i n g p a r e n t a l f e e d i n g . In another experiment they demonstrated t h a t a 30 h o u r - o l d c h i c k c o u l d d i s t i n g u i s h between e d i b l e and i n e d i b l e o b j e c t s . The c h i c k d e v e l o p e d a c l e a r p r e f e r e n c e f o r e d i b l e o b j e c t s by the end of the f i r s t f i v e minute t e s t p e r i o d ( H o r l y k and L i n d 1978). The importance of l e a r n i n g and e x p e r i e n c e i n a l l phases of development of f o r a g i n g b e h a v i o u r i n young o y s t e r c a t c h e r s i s f u r t h e r demonstrated i n N o r t o n - G r i f f i t h s ' (1967) study of m u s s e l - f e e d i n g t e c h n i q u e s i n o y s t e r c a t c h e r s . S t u d i e s of o t h e r s p e c i e s suggest t h a t e a r l y e x p e r i e n c e (e. g. W e i g l and Hanson 1980) and c r i t i c a l p e r i o d s (e. g. R a b i n o w i t c h 1968, 1969; Smith 1972; V i n c e 1960) a r e a l s o i m p o r t a n t i n the development of f o r a g i n g b e h a v i o u r i n young a n i m a l s . However, these a r e a s of development were not a d d r e s s e d i n t h i s s t u d y . An i m p o r t a n t d e v e l o p m e n t a l p e r i o d a p p a r e n t l y o c c u r s between the time c h i c k s a r e about 50 days o l d and l e a v e the n a t a l a r e a and m i d - w i n t e r when c h i c k s a r e observed f o r a g i n g on t h e i r own on t i d a l m u d f l a t s . 83 LITERATURE CITED Bateson, P. P. G. 1976. R u l e s and r e c i p r o c i t y i n b e h a v i o u r a l development. I n . P. P. G. Bateson and R. A. Hinde ( e d s . ) . Growing p o i n t s i n e t h o l o g y . Cambridge U n i v e r s i t y P r e s s . Pp.401-421. B u c k l e y , F. G. and B u c k l e y , P. A. 1974. Comparative f e e d i n g e c o l o g y of w i n t e r i n g a d u l t and j u v e n i l e R o y a l Terns ( A v e s : L a r i d a e , S t e r n i n a e ) . E c o l o g y 55:1053-1063. Cadman, M. D. 1980. A g e - r e l a t e d f o r a g i n g e f f i c i e n c y of the American O y s t e r c a t c h e r (Haematopus p a l l i a t u s ) . M.Sc. t h e s i s , U n i v e r s i t y of T o r o n t o . Dare, P. J . 1977. Se a s o n a l changes i n body-weight of o y s t e r c a t c h e r s Haematopus o s t r a l e q u s . I b i s 119:494-506. D a v i e s , N.. B. 1976. P a r e n t a l c a r e and the t r a n s i t i o n t o independent f e e d i n g i n the young "spotted f l y c a t c h e r (Museicapa s t r i a t a ) . B e h a v i o u r 59:280-295. D a v i e s , N. B. and Green, R. E. 1976. The development and e c o l o g i c a l s i g n i f i c a n c e of f e e d i n g t e c h n i q u e s i n the Reed Warbler ( A c r o c e p h a l u s s c i r p a c e u s ) . Anim. Behav. 24:213-229. Dunn, E. K. 1972. E f f e c t of age on f i s h i n g a b i l i t y of Sandwich Terns ( S t e r n a s a n d v i c e n s i s ) . I b i s 114: 360-366. Groves, S. 1978. A g e - r e l a t e d d i f f e r e n c e s i n ruddy t u r n s t o n e f o r a g i n g and a g g r e s s i v e b e h a v i o r . Auk 95:95-103. H o r l y k , N.-O. and L i n d , H. 1978. P e c k i n g response of a r t i f i c i a l l y hatched O y s t e r c a t c h e r Haematopus o s t r a l e q u s young. O r n i s Scand. 9:138-145. K l o p f e r , P. 1963. B e h a v i o r a l a s p e c t s of h a b i t a t s e l e c t i o n : the r o l e of e a r l y e x p e r i e n c e . W i l s o n B u l l . 75:15-22. K u s h l a n , J . A. 1978. N o n r i g o r o u s f o r a g i n g by r o b b i n g e g r e t s . E c o l o g y 59:649-653. 84 Menge, B. A. 1972. F o r a g i n g s t r a t e g y of a s t a r f i s h i n r e l a t i o n t o a c t u a l prey a v a i l a b i l i t y and e n v i r o n m e n t a l p r e d i c t a b i l i t y . E c o l . Monogr. 42:25-50. N o r t o n - G r i f f i t h s , M. 1967. Some e c o l o g i c a l a s p e c t s of f e e d i n g b e h a v i o u r of the O y s t e r c a t c h e r (Haematopus o s t r a l e g u s ) on the e d i b l e mussel M y t i l u s e d u l i s . I b i s 109:412-424. N o r t o n - G r i f f i t h s , M. 1968. The f e e d i n g b e h a v i o u r of the o y s t e r c a t c h e r (Haematopus o s t r a l e g u s ) . Ph.D. t h e s i s . O x f o r d . N o r t o n - G r i f f i t h s , M. 1969. The o r g a n i s a t i o n , c o n t r o l , and development of p a r e n t a l f e e d i n g i n the o y s t e r c a t c h e r (Haematopus o s t r a l e g u s ) . Behaviour 34:55-114. O r i a n s , G. H. 1969. Age and h u n t i n g s u c c e s s i n the Brown P e l i c a n ( P e l e c a n u s occ i d e n t a l i s ) . Anim. Behav. 17:316-319. Quinney, T. E. and Smith, P. C. 1980. Comparative f o r a g i n g b e h a v i o u r and e f f i c i e n c y of a d u l t and j u v e n i l e g r e a t b l u e h e r o n s . Can. J . Z o o l . 58:1168-1173. R a b i n o w i t c h , V. E. 1968. The r o l e of e x p e r i e n c e i n the development of food p r e f e r e n c e s i n g u l l c h i c k s . Anim. Behav. 16:425-428. R a b i n o w i t c h , V. E. 1969. The r o l e of e x p e r i e n c e i n the development and r e t e n t i o n of seed p r e f e r e n c e s i n Zebra F i n c h e s . Behaviour 33:222-236. Recher, H. F. and Recher, J . A. 1969. Comparative f o r a g i n g b e h a v i o u r of a d u l t and immature l i t t l e b l u e herons ( F l o r i d a  c a e r u l e a ) . Anim. Beh. 17:320-322. Sm i t h , S. M. 1972. The ontogeny of i m p a l i n g b e h a v i o u r i n the "Loggerhead S h r i k e L a n i u s l u d o v i c i - a n u s L. B e h a v i o u r -42:232-242. Verbeek, N. A. M. 1977a. Age d i f f e r e n c e s i n the d i g g i n g f r e q u e n c y of H e r r i n g G u l l s on a dump. Condor 79:123-125. Verbeek, N. A, M. 1977b. Comparative f e e d i n g b e h a v i o r of immature -and a d u l t g u l l s . W i l s o n B u l l . 89:415-421. 85 V i n c e , M. A. 1960. Developmental changes i n r e s p o n s i v e n e s s of the Great T i t (Parus m a j o r ) . B e h a v i o u r 15:219-243. W e i g l , P. D. and Hanson, E. V. 1980. O b s e r v a t i o n a l l e a r n i n g and the f e e d i n g . b e h a v i o r of the r e d s q u i r r e l T a m i a s c i u r u s  h u d s o n i c u s : the ontogeny of o p t i m i z a t i o n . E c o l o g y 61:213-218. 86 GROWTH, SIBLING RIVALRY, AND CHICK PRODUCTION IN BLACK OYSTERCATCHERS INTRODUCTION Growth and s u r v i v a l of young a n i m a l s a r e a f f e c t e d by how p a r e n t a l investment i s p a r t i t i o n e d among o f f s p r i n g and how much energy i n i n v e s t e d i n each o f f s p r i n g (Smith and F r e t w e l l 1974). Animals w i t h o u t p a r e n t a l c a r e p a r t i t i o n energy among eggs, and t h i s energy a l l o c a t i o n a f f e c t s subsequent growth and s u r v i v a l of o f f s p r i n g . Energy i s a l s o p a r t i t i o n e d among o f f s p r i n g t h r o u g h p a r e n t a l c a r e , and i t s a l l o c a t i o n a f f e c t s o f f s p r i n g growth and s u r v i v a l (Brockelman 1975). When o f f s p r i n g a re h e a v i l y dependent on p a r e n t a l p a r e n t a l f e e d i n g , p a r e n t a l f o r a g i n g performance and f i t n e s s (number or q u a l i t y of o f f s p r i n g ) s h o u l d be r e l a t e d . In p r i n c i p l e , i t s h o u l d be p o s s i b l e t o r e l a t e p a r e n t a l f o r a g i n g performance t o r e p r o d u c t i v e o u t p u t . In the case a n a l y z e d h e r e , the r e l a t i o n s h i p between p a r e n t a l f o r a g i n g performance and f i t n e s s was examined i n d i r e c t l y by s t u d y i n g c h i c k growth and c h i c k p r o d u c t i o n i n b l a c k o y s t e r c a t c h e r s (Haematopus bachmani). STUDY AREA C l e l a n d I s l a n d near T o f i n o , B r i t i s h Columbia was the s i t e of t h i s s t u d y . The c l i m a t e i s m i l d and wet w i t h an average •annual temperature of 9°C.and average annual p r e c i p i t a t i o n - . . o f 87 309 cm r e c o r d e d f o r T o f i n o ( C l i m a t e of B r i t i s h C olumbia, 1976). Major storms sometimes occur d u r i n g the summer. D u r i n g storms, heavy seas c o n t i n u a l l y wash the i n t e r t i d a l zone, even a t low t i d e , and poor d r a i n a g e on the i s l a n d r e s u l t s i n f l o o d i n g and a c c u m u l a t i o n of water t h a t e v a p o r a t e s s l o w l y . STUDY ANIMAL About 30 p a i r s of b l a c k o y s t e r c a t c h e r s h o l d b r e e d i n g t e r r i t o r i e s on C l e l a n d . B i r d s b u i l d s i m p l e nest cups l i n e d w i t h b i t s of broken s h e l l i n b a s a l t c r e v i c e s or l a y t h e i r eggs i n sc r a p e s made on beaches of broken s h e l l . C l u t c h s i z e i s 1-3 eggs. I n c u b a t i o n b e g i n s when the c l u t c h i s complete and l a s t s about 26 days. Eggs h a t c h w i t h i n a few hours of each o t h e r . (The l o n g e s t h a t c h i n g i n t e r v a l o bserved d u r i n g t h i s s t u d y was of 12-18 h o u r s ) . F i r s t hatched c h i c k s a r e brooded w h i l e the r e s t of the c l u t c h h a t c h e s . Egg l a y i n g b e g i n s i n mid-May, and replacement c l u t c h e s may be l a i d i f eggs are l o s t . In many seasons whole c l u t c h e s a re l o s t due t o f l o o d i n g d u r i n g storms, and every year n o r t h w e s t e r n crows (Corvus b r a c h y r h y n c h o s ) d e s t r o y c l u t c h e s of o y s t e r c a t c h e r eggs. Glaucous-winged g u l l s ( L a r u s g l a u c e s c e n s ) are c h i c k p r e d a t o r s , but they were never observed t o ta k e o y s t e r c a t c h e r eggs. Many b i r d s r e p l a c e the f i r s t c l u t c h i f i t i s l o s t , and some b i r d s r e l a y a f t e r l o s i n g t h e i r second c l u t c h . In no year were b i r d s observed t o l a y replacement c l u t c h e s a f t e r l o s i n g c h i c k s nor has e g g - i a y i n g c o n t i n u e d a f t e r 15 J u l y . B l a c k o y s t e r c a t c h e r c h i c k s a re p r e c o c i a l but dependent ,on p a r e n t a l f e e d i n g f o r an extended p e r i o d ( F i g u r e 11). Pare n t 88 o y s t e r c a t c h e r s f e e d t h e i r c h i c k s d u r i n g low t i d e p e r i o d s . Very young c h i c k s remain near the n e s t , and p a r e n t s take t u r n s g u a r d i n g the c h i c k s and c a r r y i n g food item by item from the i n t e r t i d a l t o the c h i c k s . As they become b i g g e r and more a g i l e , c h i c k s f o l l o w t h e i r p a r e n t s i n t o the i n t e r t i d a l t o be f e d . Around the time they begin t o f l y (35+ d a y s ) , c h i c k s b e g i n t o c a p t u r e s m a l l prey items f o r t h e m s e l v e s , but the most p r o f i t a b l e prey items a r e s t i l l p r o v i d e d by t h e i r p a r e n t s (see F i g u r e 13). METHODS Bl a c k O y s t e r c a t c h e r T e r r i t o r i e s B l a c k o y s t e r c a t c h e r t e r r i t o r i e s and nest s i t e s were mapped from 1975 thr o u g h 1978. T e r r i t o r i e s were d e l i n e a t e d by r e c o r d i n g l o c a t i o n s of boundary d i s p u t e s . In 1976 and 1977 I censused the i s l a n d d a i l y (weather p e r m i t t i n g ) from May th r o u g h August t o gath e r d a t a f o r t e r r i t o r y mapping. In 1975, t e r r i t o r y and nest data were g a t h e r e d i n censuses made d u r i n g two fo u r - d a y v i s i t s t o the i s l a n d i n m i d - J u l y and mid-August, and 1978 da t a were g a t h e r e d over 5 t r i p s between the end of June and the end of August. Three b r e e d i n g b i r d s banded on t h e i r t e r r i t o r i e s i n 1971 by E. B. H a r t w i c k ( p e r s . comm.) were p r e s e n t on the same t e r r i t o r i e s from 1975 through 1978. and S u r v i v a l o f c h i c k .growth and s u r v i v a l were r e c o r d e d i n 1-976 Chick Growth .Detail-s 89 and 1977. Nests were l o c a t e d as c l u t c h e s were commenced and checked e v e r y t h r e e days t o o b t a i n n e s t h i s t o r i e s . A f t e r eggs p i p p e d , n e s t s were checked d a i l y t o dete r m i n e the date of h a t c h i n g . Unique c o m b i n a t i o n s of c o l o r e d p l a s t i c l e g bands and numbered m e t a l w i l d l i f e s e r v i c e bands were put on each c h i c k t o ensure p o s i t i v e i d e n t i f i c a t i o n . C h i c k w e i g h t s and exposed culmen l e n g t h s were measured every t h i r d day (as weather p e r m i t t e d ) u n t i l c h i c k s f l e w or d i s a p p e a r e d . A l l w e i g h i n g s were done w i t h i n 1 1/2 hours of h i g h t i d e , when most i n t e r t i d a l f e e d i n g a r e a s had been under water f o r a few h o u r s . T h i s m i n i m i z e d the c o n t r i b u t i o n of r e c e n t l y f i l l e d c r o p s t o c h i c k w e i g h t s . C h i c k F e e d i n g Records of p a r e n t s f e e d i n g c h i c k s were made i n 1977. O b s e r v a t i o n s were made w i t h a 20 X s p o t t i n g scope over low t i d e p e r i o d s . When a c h i c k was f e d the prey t y p e , p r e y s i z e , c h i c k i d e n t i f i c a t i o n , p a r e n t i d e n t i f i c a t i o n , and time of f e e d i n g were r e c o r d e d . I n t e r a c t i o n s between s i b l i n g s were a l s o r e c o r d e d . The dat a p r e s e n t e d here f o c u s on f o u r o y s t e r c a t c h e r f a m i l i e s w i t h t w o - c h i c k broods. They were observed f o r 43.2 hours d u r i n g 14 o b s e r v a t i o n p e r i o d s . The c h i c k s observed ranged i n age from 14 to 53 days (day of h a t c h i n g i s day 0 ) . RESULTS 90 T e r r i t o r y f i d e l i t y , c l u t c h s i z e , and b r e e d i n g s u c c e s s B l a c k o y s t e r c a t c h e r s on C l e l a n d I s l a n d showed a h i g h degree of t e r r i t o r y f i d e l i t y . From 1975 through 1978 no s h i f t s i n t e r r i t o r y b o u n d a r i e s were o b s e r v e d , and i n many c a s e s , the same nest s i t e s were used i n two or more y e a r s . Three a d u l t s c o l o r -banded i n 1971 o c c u p i e d the same t e r r i t o r i e s i n 1975-78 t h a t they had o c c u p i e d a t the time of c o l o r - b a n d i n g . Four a d u l t s banded on t e r r i t o r i e s i n 1976 defended the same t e r r i t o r i e s t h r o u g h 1978. Another c o l o r - b a n d e d b i r d and i t s mate o c c u p i e d the same t e r r i t o r y i n 1976 and 1977, but i n 1978 they had d i s a p p e a r e d and the t e r r i t o r y was v a c a n t . A s i x t h c o l o r - b a n d e d b i r d l e f t i t s t e r r i t o r y , a f t e r b e i n g banded i n 1976, and the t e r r i t o r y was unoccupied u n t i l 1980 when the banded b i r d r eappeared and bred on the same t e r r i t o r y . The t e r r i t o r i a l f i d e l i t y of these marked b i r d s , c o n s t a n c y of t e r r i t o r i a l b o u n d a r i e s of unmarked b i r d s , l o n g l i f e s p a n , and f r e q u e n t reuse of n e s t s i t e s i n s u c c e s s i v e y e a r s suggest l o n g term f i d e l i t y by b i r d s t o t e r r i t o r i e s . B l a c k o y s t e r c a t c h e r c l u t c h s i z e ranges from 1 t o 3 eggs. Data on 310 c l u t c h e s g a t h e r e d i n t h r e e seasons of t h i s study and t h r e e seasons of H a r t w i c k ' s (1974) study a r e p r e s e n t e d i n Table V I I I . The mean s i z e of a l l 310 c l u t c h e s was 2.04 eggs t s . d. = 0.66). B r e e d i n g s u c c e s s of i n d i v i d u a l p a i r s of b l a c k o y s t e r c a t c h e r s was h i g h l y v a r i a b l e between y e a r s . From nest r e c o r d s and t e r r i t o r y maps i t i s p o s s i b l e t o i d e n t i f y 32 t e r r i t o r i e s on which b i r d s a t t e m p t e d b r e e d i n g i n more than one year from 197 5-T'978. For e l e v e n of these t e r r i t o r i e s p resence of TABLE V I I I B l a c k o y s t e r c a t c h e r c l u t c h s i z e i n 6 y e a r s , C l e l a n d I s l a n d Year T eqq Number of 2 eqqs C l u t c h e s 3 eggs T o t a l • c l u t c h e s Mean c l u t c h s i z e s.d. 1 970* 1 4 38 9 61 1 .92 0.61 1 97 1 * 8 26 1 4 48 2.13 0.67 1 972* 11' 34 1 3 58 2.03 0.65 1 976 1 1 25 18 54 2.13 0.73 1 977 1 2 38 1 0 60 1 .97 0.61 1978** 5 1 5 9 29 2.14 0.69 T o t a l 61 176 73 310 2.04 0.66 * Data from E. B. H a r t w i c k (1974) ** Data based on 5 v i s i t s t o the i s l a n d . 92 c o l o r - b a n d e d a d u l t s and annual reuse of the same nest s i t e i n d i c a t e t h a t the same p a i r s bred on t h e s e t e r r i t o r i e s f o r the f o u r y e a r s of o b s e r v a t i o n s . Data on c h i c k p r o d u c t i o n of these p a i r s i s shown i n Table IX. These d a t a show t h a t : 1) f o u r p a i r s d i d not s u c c e s s f u l l y r a i s e any young i n 4 y e a r s ; 2) one p a i r s u c c e s s f u l l y r a i s e d young i n a l l 4 y e a r s ; 3) s i x p a i r s were s u c c e s s f u l i n some y e a r s and f a i l e d i n o t h e r y e a r s ; 4) t h e r e was no tendency f o r s u c c e s s of a p a i r t o i n c r e a s e from one year t o the next over the f o u r y e a r s of o b s e r v a t i o n . Brood s i z e and c h i c k growth B l a c k o y s t e r c a t c h e r c h i c k growth from h a t c h i n g t o time of f i r s t f l i g h t depends on brood s i z e and a c h i c k ' s rank i n the brood ( F i g u r e 15). A n a l y s i s of v a r i a n c e was used t o a n a l y z e c h i c k w e i g h t s . Data were a n a l y z e d a c c o r d i n g t o brood s i z e f o r t h r e e day i n t e r v a l s of age as shown i n F i g u r e 15. Weights of c h i c k s i n o n e - c h i c k and t w o - c h i c k broods d i d not d i f f e r s i g n i f i c a n t l y u n t i l c h i c k s were .14-16 days o l d . At t h a t t i m e , average w e i g h t s of c h i c k s i n o n e - c h i c k broods were g r e a t e r than w e i g h t s of c h i c k s i n t w o - c h i c k broods (F = 8.89, df = 1,22, p < 0.005), and these d i f f e r e n c e s p e r s i s t e d u n t i l c h i c k s c o u l d f l y . These d i f f e r e n c e s p e r s i s t e d at l e a s t u n t i l c h i c k s began f l y i n g . Average w e i g h t s of s i b l i n g s i n t w o - c h i c k broods d i d not d i f f e r s t a t i s t i c a l l y , but i n some broods, the l a r g e r c h i c k weighed up t o 48% more when c h i c k s began t o f l y . The average weight d i f f e r e n c e at the time s i b l i n g s began f l y i n g was 84.2 grams (N = 4, s. d. = 53.2 g; t h i s i n c l u d e s one brood i n which the s m a l l e r • s i b never f l e w ) . The two extreme c a s e s of s i b l i n g TABLE IX Number of c h i c k s produced on 11 b l a c k o y s t e r c a t c h e r t e r r i t o r i e s , 1975-1978 T e r r i t o r y 1975 1 976 1 977 1 978 T o t a l 1 (B) 1 1 1 2 5 2(B) ' 1 0 1 0 2 3 3 0 . 1 0 4 4 3 0 o' 1 4 5 0 0 0 ' 0 0 6(B) 1 0 -0 - X 1 7 1 1 0 2 4 8(2B) 0 0 0 0 0 9(B) 0 0 0 0 0 10 1 0 0 1 2 1 1 0 0 0 0 0 B .= 1 banded p a r e n t 2B = 2 banded p a r e n t s X = a d u l t s ' absent from t e r r i t o r y 94 F i g u r e 15: Average c h i c k w e i g h t s ( l o g s c a l e ) a r e - p l o t t e d a g a i n s t c h i c k age i n days. Data are p l o t t e d a c c o r d i n g t o b r o o d - s i z e and c h i c k rank i n brood ( f o r t w o - c h i c k b r o o d s ) . ( V e r t i c a l b a r s are 95% c o n f i d e n c e l i m i t s ) . 95 5 0 0 T t o < tr '100 o 7t t i 9 T 8 "t 1 I 2 0 ' 0-3 ' 5-9 ' 10-12 'u-16 ' 17-19 ' 20-22 ' 2 3- 2 5 26-29 34-37 A G E (DAYS) X SINGLE CHICKS B IGSIBS * LITTLE SIBS 96 weight d i f f e r e n c e s are i l l u s t r a t e d i n F i g u r e 16. The c h i c k t h a t was 152 grams l i g h t e r than i t s s i b l i n g never f l e w and d i e d of apparent s t a r v a t i o n at 49 days of age, n i n e days a f t e r i t s s i b began f l y i n g . The s m a l l s i b was unable t o f o l l o w i t s p a r e n t s and f l y i n g s i b t o nearby o f f s h o r e r e e f s t o f o r a g e and be f e d . Weight D i f f e r e n c e s i n Two-chick Broods Weight d i f f e r e n c e s between s i b l i n g s i n t w o - c h i c k broods were p o s s i b l y a f f e c t e d by dominance r e l a t i o n s between s i b s and r e s u l t i n g unequal d i s t r i b u t i o n of f o o d . On one o c c a s i o n I o b s e r v e d the e s t a b l i s h m e n t or r e i n f o r c e m e n t of dominance between s i b s . The 15 day o l d c h i c k s weighed 158 grams and 128 grams. D u r i n g p r e v i o u s o b s e r v a t i o n s no. c o n f l i c t between, th e s e s i b s had been o b s e r v e d . The s i b l i n g s were s i t t i n g near the nest s i t e w i t h one p a r e n t . The second p a r e n t had been f o r a g i n g and r e t u r n e d c a r r y i n g a p i e c e of food. Both c h i c k s approached the r e t u r n i n g p a r e n t and reached i t t o g e t h e r . The b i g s i b t u r n e d and began p e c k i n g the l i t t l e s i b on the head and neck. The l i t t l e s i b ran away pursued by the b i g s i b . A f t e r c h a s i n g f o r s e v e r a l meters, the b i g s i b r e t u r n e d t o the p a r e n t and took the f o o d . The l i t t l e s i b s t a y e d away. S u b s e q u e n t l y , whenever both c h i c k s approached a p a r e n t t o be f e d , a lunge by the b i g s i b towards the l i t t l e s i b sent the l a t t e r r u n n i n g away from the p a r e n t w i t h f o o d . Sometimes l u n g i n g and c h a s i n g were o v e r t and v i g o r o u s , but o f t e n merely the b i g s i b t u r n i n g towards the l t t l e s i b r e s u l t e d i n the l i t t l e - s i b moving away or s t a y i n g away from a p a r e n t w i t h f o o d . In t h i s and o t h e r t w o - c h i c k broods, dominance r e l a t i o n s 97 F i g u r e 16: Growth of s i b l i n g s i n the two t w o - c h i c k broods w i t h a) minimum and b) maximum weight d i f f e r e n c e s a t the time c h i c k s began f l y i n g . • AGE (DAYS) 99 p e r s i s t e d a f t e r the c h i c k s were f l y i n g and s t i l l b e i n g f e d by t h e i r p a r e n t s . O c c a s i o n a l l y l i t t l e s i b s i n broods chased b i g s i b s , but 36 of 38 o v e r t chases were i n i t i a t e d by b i g s i b s ( X 2 = 30.42, df = 1, p < 0.005). D u r i n g o b s e r v a t i o n s e s s i o n s when c h a s i n g was obser v e d , l i t t l e s i b s were f e d much l e s s o f t e n than b i g s i b s ( F i g u r e 17, X 2 = 85.78, df = 1, p < 0.001). However, d u r i n g s e s s i o n s when c h a s i n g d i d not o c c u r , l i t t l e s i b s were fed more o f t e n than b i g s i b s ( X 2 = 40.03, df = 1, p < 0.005). As a r e s u l t of c h a s i n g and dominance b i g s i b s c o u l d c o n t r o l a c c e s s t o p a r e n t a l f e e d i n g s . C h i c k s u r v i v a l H e a v i e r o y s t e r c a t c h e r c h i c k s had a b e t t e r chance than l i g h t e r c h i c k s of s u r v i v i n g t o ta k e t h e i r f i r s t f l i g h t . By, twenty days of age, weight d i f f e r e n c e s r e l a t e d t o brood s i z e were w e l l d e v e l o p e d ( F i g u r e 15). C h i c k s t h a t s u r v i v e d t o f l y were h e a v i e r a t 20 days of age than c h i c k s t h a t d i d not ( F i g u r e 18, t=2.36, df=21, p<0.05). A l l c h i c k s h e a v i e r than 200 grams a t 20 days of age e v e n t u a l l y f l e w e xcept one t h a t was p r o b a b l y eaten by a r i v e r o t t e r , L u t r a c a n a d e n s i s ( p e r s . o b s . ) . C h i c k s u r v i v a l from h a t c h i n g v a r i e d w i t h brood s i z e and rank i n brood ( F i g u r e 19). M o r t a l i t y was h i g h e s t d u r i n g the f i r s t week a f t e r h a t c h i n g . In o n e - c h i c k broods a l l of the m o r t a l i t y o c c u r r e d d u r i n g t h i s p e r i o d . In t w o - c h i c k broods m o r t a l i t y o c c u r r e d u n t i l c h i c k s began f l y i n g . Sometimes c h i c k s v a n i s h e d , and the cause of c h i c k m o r t a l i t y c o u l d not be det e r m i n e d . C h i c k s t h a t d i s a p p e a r e d d u r i n g b i g 100 F i g u r e 17: Frequency of p a r e n t a l f e e d i n g s and s i b l i n g chases i n t w o - c h i c k broods. 320 LITTLE SIB 1 02 F i g u r e 18: C h i c k s u r v i v a l t o f l y and weight at 20 days of age. 103 .1 151-175 175-200 200-225 225-250 250-275 275-300 300-325 G R A M S SURVIVED TO FLY T DID NOT FLY 1 04 F i g u r e 19: C h i c k s u r v i v o r s h i p from h a t c h i n g v e r s u s c h i c k age i n days. Data a r e p l o t t e d a c c o r d i n g t o b r o o d - s i z e and c h i c k rank ( i n t w o - c h i c k b r o o d s ) . 105 X to cr o > > cr ZD 00 1.0 0.8 0.6 0.4 0.2 X i — + 0.0' o U 21 2 8 A G E (DAYS) 35 kl 49 X S INGLE CHICKS N = 10 • BIG SIBS N = 16 + LITTLE SIBS N= 16 106 storms were p r o b a b l y swept away by storm surge and r a i n r u n - o f f . Crows preye d on o y s t e r c a t c h e r eggs and p r o b a b l y a l s o took some ver y young c h i c k s t h a t were s t i l l b e i n g brooded by t h e i r p a r e n t s . G u l l s were observed t o a t t a c k c h i c k s , and p a r e n t o y s t e r c a t c h e r s responded by p e r s i s t e n t l y f l y i n g towards and s t r i k i n g a t t a c k i n g g u l l s . However, c h i c k s were s t i l l l o s t t o g u l l s . In August, 1978, a c o l o r - b a n d e d l e g of a c h i c k about 4 weeks o l d and over 300 grams i n weight was found i n a g u l l p e l l e t . S u r v i v a l a f t e r c h i c k s c o u l d f l y was s i m i l a r f o r c h i c k s from d i f f e r e n t brood s i z e s . S i m i l a r p r o p o r t i o n s of c o l o r - b a n d e d b l a c k o y s t e r c a t c h e r s from o n e - c h i c k and t w o - c h i c k broods were r e s i g h t e d as one and two year o l d s ( T able X ) . There was no d i f f e r e n c e i n weight a t the time of f i r s t f l i g h t between b i r d s r e s i g h t e d one or two y e a r s l a t e r and those not r e s i g h t e d ( r e s i g h t e d : 351 g, s. d. = 45.1; not r e s i g h t e d : 350 g, s. d. =68.7; t = 0.32, df = 15). C l u t c h s i z e and c h i c k p r o d u c t i o n One-egg and two-egg c l u t c h e s produced s i m i l a r numbers of c h i c k s per c l u t c h . S u r v i v a l of c h i c k s t o f l y from one and two-egg c l u t c h e s i n which a l l eggs hat c h e d i s shown i n Table X I . The number of c h i c k s s u r v i v i n g t o f l y d i d not d i f f e r s i g n i f i c a n t l y w i t h c l u t c h - s i z e ( X 2 = 0.57, df = 1, NS). An average of 0.50 (± 0.53) c h i c k s per c l u t c h s u r v i v e d t o f l y from one-egg c l u t c h e s w h i l e 0.75 (± 0.77) c h i c k s per c l u t c h s u r v i v e d t o f l y from two--egg c l u t c h e s . TABLE X R e s i g h t i n g s of b l a c k o y s t e r c a t c h e r s banded as c h i c k s i n 1976 and 1977* Number Number of c h i c k s r e s i g h t e d Brood s i z e f l e d g e d by Sept.1978 1 c h i c k 7 3(0.43) 2 c h i c k s 10 6(0.60) * P = 0.30, F i s h e r E x a c t Test ( S i e g e l 1956). TABLE XI B l a c k o y s t e r c a t c h e r c h i c k p r o d u c t i o n ( p r o p o r t i o n i n p a r e n t h e s e s ) ONE-EGG CLUTCHES 1 976 1 977 T o t a l H a t c h i n g 1 c h i c k F l e d g i n g 1 c h i c k F l e d g i n g 0 c h i c k s 2(1.00) 8(1.00) 10(1.00) 1(0.50) 4(0.50) 5(0.50) 1(0.50) 4(0.50) 5(0.50) TWO-EGG CLUTCHES 1 976 1 977 T o t a l H a t c h i n g 2 c h i c k s F l e d g i n g 2 c h i c k s F l e d g i n g 1 c h i c k F l e d g i n g 0 c h i c k s 7(1.00) 9(1.00) 16(1.00) 2(0.29) 1(0.11) 3(0.19) 1(0.14) 5(0.56) 6(0.38) 4(0.57) 3(0.33) 7(0.44) 109 DISCUSSION C h i c k growth and s i b l i n g r i v a l r y Dependence of b l a c k o y s t e r c a t c h e r c h i c k growth on brood s i z e ( F i g u r e 15) s u g gests t h a t a v a i l a b i l i t y of food p r o v i d e d by p a r e n t s l i m i t s c h i c k growth. Two-chick broods a r e more demanding of p a r e n t s ' time and energy than o n e - c h i c k broods, and d i f f e r e n c e s i n c h i c k w e i g h t s r e l a t e d t o brood s i z e appeared by the time c h i c k s were two weeks o l d . C h i c k growth p a t t e r n s i n d i c a t e t h a t p a r e n t a l investment i n c h i c k s i n c r e a s e s w i t h brood s i z e but t h a t the i n c r e a s e i s not p r o p o r t i o n a l t o brood s i z e . L ess than p r o p o r t i o n a l i n c r e a s e i n p a r e n t a l investment w i t h i n c r e a s i n g brood s i z e has been r e p o r t e d i n numerous o t h e r b i r d s p e c i e s and these r e p o r t s are summarized by Klomp (1970). An h y p o t h e s i s t h a t i s c o n s i s t e n t w i t h the l e s s than p r o p o r t i o n a l i n c r e a s e i n p a r e n t a l f e e d i n g w i t h i n c r e a s e d brood s i z e was proposed by N o r t o n - G r i f f i t h s (1969). He h y p o t h e s i z e d t h a t p a r e n t a l f e e d i n g of o y s t e r c a t c h e r c h i c k s i s m o t i v a t e d by p a r e n t a l hunger and t h a t c h i c k f e e d i n g w i l l o n l y o ccur d u r i n g p e r i o d s when p a r e n t s are m o t i v a t e d t o feed t h e m s e l v e s . Data on c h i c k growth p r o v i d e i n d i r e c t e v i d e n c e t h a t p a r e n t a l f e e d i n g s - a r e a l i m i t e d r e s o u r c e f o r c h i c k s i n t w o - c h i c k broods. S i b l i n g r i v a l r y i s a response t o t h i s food l i m i t a t i o n , and i t may a f f e c t c h i c k growth and s u r v i v a l . Prey a v a i l a b i l i t y t o c h i c k s i s c o n t r o l l e d by p a r e n t a l d e l i v e r y of p r e y , and c h i c k s , e s p e c i a l l y l a r g e c h i c k s , a r e p r o b a b l y r a r e l y s a t i a t e d . Hunger and the s i g h t of a p a r e n t c a r r y i n g food a r e l i k e l y 1 10 s t i m u l i f o r the o c c u r r e n c e of s i b l i n g r i v a l r y . Hunger a l o n e does not s t i m u l a t e dominance b e h a v i o u r i n b l a c k o y s t e r c a t c h e r c h i c k s s i n c e s i b l i n g s showing w e l l - e s t a b l i s h e d dominance r e l a t i o n s i n the presence of a p a r e n t w i t h food w i l l spend hours crouched s i d e - b y - s i d e i n h i d i n g p l a c e s between f e e d i n g s . However, i n cases of severe food l i m i t a t i o n hunger a l o n e may s t i m u l a t e dominance. An extreme example of t h i s i s c a n n i b a l i s m i n n e s t l i n g s h o r t - e a r e d owls ( A s i o flammeus). Ingram (1959) r e p o r t e d t h a t c a n n i b a l i s m between s i b l i n g s was most f r e q u e n t i n y e a r s of food s h o r t a g e when p a r e n t s were unable t o p r o v i s i o n t h e i r nest by c a c h i n g food w i t h i n reach of b i g g e r c h i c k s . A s i m i l a r s i t u a t i o n was r e p o r t e d f o r South P o l a r skuas ( C a t h a r a c t a m a c c o r m i c k i ) . P r o c t e r (1975) demonstrated t h a t n u t r i t i o n a l c o n d i t i o n of skua c h i c k s (measured as p e r c e n t of s t a n d a r d weight a t a p a r t i c u l a r age) was p o s i t i v e l y c o r r e l a t e d w i t h the o c c u r r e n c e and i n t e n s i t y of a g g r e s s i o n between s i b s i n n a t u r a l l y and a r t i f i c i a l l y c r e a t e d t w o - c h i c k broods. Once e s t a b l i s h e d , dominance r e l a t i o n s between s i b l i n g s p e r s i s t e d . In t h i s s tudy s m a l l s i b s ( i n f o u r broods) chased dominant b i g s i b s i n o n l y 2 of 38 o v e r t chases and i n no case was the dominance o r d e r o b s e r v e d t o r e v e r s e . S t u d i e s of dominance i n Cactus wrens, Campy1orhynchus b r u n n e i c a p i l l u s ( R i c k l e f s and H a i n s w o r t h 1967) and e a g l e t s (Meyburg 1978) suggest t h a t dominance i s q u i c k l y and i r r e v e r s i b l y d e c i d e d i n one f i g h t . Dominance i s u s u a l l y based on weight or age d i f f e r e n c e s a t t r i b u t e d t o asynchronous h a t c h i n g ( e . g. Bryant 1978, Ingram 1962, Meyburg 1973, M i l l e r 1973, P r o c t e r 1975). However, dominance has been reported•between e a g l e • c h i c k s of the 111 same weight (Meyburg 1978), and R i c k l e f s and H a i n s w o r t h (1967) r e p o r t t h a t the younger of two c a c t u s wrens emerged from a f i g h t as the dominant. In b l a c k o y s t e r c a t c h e r s h e a v i e r c h i c k s were dominant. C h i c k s u r v i v a l Brood s i z e and c h i c k weight a f f e c t s u r v i v a l u n t i l c h i c k s are a b l e t o f l y . In o n e - c h i c k broods a l l c h i c k l o s s e s o c c u r r e d d u r i n g the f i r s t week a f t e r h a t c h i n g w h i l e i n t w o - c h i c k broods c h i c k l o s s e s c o n t i n u e d t h rough the time c h i c k s began f l y i n g ( F i g u r e 19). These p a t t e r n s of c h i c k l o s s may be due to be h a v i o u r of p a r e n t o y s t e r c a t c h e r s . C h i c k s a r e f r e q u e n t l y brooded by t h e i r p a r e n t s d u r i n g the f i r s t week or l o n g e r a f t e r h a t c h i n g , and p r e d a t o r s , crows and g u l l s , a r e a b l e t o l o c a t e c h i c k s by o b s e r v i n g b e h a v i o u r of p a r e n t s . As c h i c k s grow o l d e r and a re brooded l e s s , p r e d a t o r s a r e no l o n g e r a b l e t o l o c a t e c h i c k s d i r e c t l y by o b s e r v i n g t h e i r p a r e n t s (Y. Yom-Tov, p e r s . comm.). I n s t e a d , p r e d a t o r s must s e a r c h f o r c r y p t i c a l l y c o l o r e d c h i c k s . A p r e d a t o r p r o b a b l y has a b e t t e r chance of f i n d i n g a c h i c k on a t e r r i t o r y w i t h a t w o - c h i c k brood than on a t e r r i t o r y w i t h a o n e - c h i c k brood. H e a v i e r c h i c k s had a b e t t e r chance of s u r v i v i n g t o f l y . Twelve of t h i r t e e n c h i c k s w e i g h i n g over 200 grams a t ,20 days of age s u r v i v e d t o f l y w h i l e o n l y 5 of 10 c h i c k s w e i g h i n g 200 grams or l e s s e v e n t u a l l y f l e w ( F i g u r e 18). There were no d i f f e r e n c e s i n weight a t time of f i r s t f l i g h t between b i r d s t h a t were and were not r e s i g h t e d as one and two-year o l d s , nor were t h e r e any d i f f e r e n c e s r e l a t e d t o brood s i z e i n the p r o p o r t i o n of b i r d s 1 1 2 r e s i g h t e d one and two y e a r s l a t e r ( T a ble X ) . T h i s s u g g e s t s the c r i t i c a l l i f e h i s t o r y p e r i o d f o r b l a c k o y s t e r c a t c h e r s i s s u r v i v a l from h a t c h i n g t o f i r s t f l i g h t . Once a b i r d can f l y i t s p r o s p e c t s f o r s u r v i v a l a r e a p p a r e n t l y q u i t e good a l t h o u g h no data i s a v a i l a b l e on e v e n t u a l r e c r u i t m e n t of f l y i n g young t o the b r e e d i n g p o p u l a t i o n . C h i c k p r o d u c t i o n and p a r e n t a l investment C h i c k p r o d u c t i o n by b l a c k o y s t e r c a t c h e r s was low and v a r i e d between y e a r s . One hundred and f o u r t e e n c l u t c h e s of eggs s u r v i v e d a t l e a s t t o the b e g i n n i n g of i n c u b a t i o n i n 1976 and 1977 (Table V I I I ) . Only 23% (26 c l u t c h e s ) s u r v i v e d t o ha t c h c h i c k s , and j u s t 12% (14 c l u t c h e s ) produced f l y i n g c h i c k s (Table X I ) . C h i c k p r o d u c t i o n by any p a i r of o y s t e r c a t c h e r s v a r i e d c o n s i d e r a b l y between y e a r s , and over f o u r seasons some p a i r s d i d not produce any f l y i n g c h i c k s a t a l l ( T a b l e I X ) . P r e d a t i o n and u n p r e d i c t a b l e bouts of se v e r e weather were c r i t i c a l f a c t o r s a f f e c t i n g c h i c k p r o d u c t i o n . These f a c t o r s , u n r e l a t e d t o p a r e n t a l f o r a g i n g performance, p l a c e d c o n s t r a i n t s on c h i c k p r o d u c t i o n . B r e e d i n g s u c c e s s and c h i c k p r o d u c t i o n f o r C l e l a n d I s l a n d b l a c k o y s t e r c a t c h e r s were much lower than H a r r i s (1967) r e p o r t e d f o r o y s t e r c a t c h e r s (Haematopus o s t r a l e q u s ) on Skokholm I s l a n d , -Wales. There, 37% and 59% of eggs produced f l y i n g young i n two c o n s e c u t i v e y e a r s . On C l e l a n d I s l a n d H a r t w i c k (1974) r e p o r t e d t h a t 13%, 9%, and 13% of eggs produced f l y i n g young i n t h r e e y e a r s (1970-1972), and d u r i n g t h i s s tudy o n l y 5% and 9% of eggs produced f l y i n g young.in 1976 and 1977. One and t w o - c h i c k broods d u r i n g the p e r i o d of t h i s study 1 1 3 produced s i m i l a r numbers of f l y i n g young (Table X I ) . In two-c h i c k broods lower c h i c k growth and o c c u r r e n c e of s i b l i n g r i v a l r y suggest t h a t c h i c k s i n t w o - c h i c k broods were not as w e l l p r o v i s i o n e d by t h e i r p a r e n t s as c h i c k s i n o n e - c h i c k broods. In o n l y one brood i n t h i s study (see "Brood s i z e and c h i c k growth") d i d t h e s e f a c t o r s c o n t r i b u t e t o s t a r v a t i o n of a c h i c k . However, i n seasons of u n u s u a l l y stormy weather t h a t s e v e r e l y l i m i t s a c c e s s to f e e d i n g a r e a s , weight d i f f e r e n c e s m a i n t a i n e d by s i b l i n g r i v a l r y c o u l d e f f e c t c h i c k p r o d u c t i o n . S i b l i n g s of d i f f e r e n t s i z e s r e p r e s e n t d i f f e r e n t amounts of p a r e n t a l i n vestment ( T r i v e r s 1972), and s i b l - i n g r i v a l r y can m a i n t a i n or i n c r e a s e these d i f f e r e n c e s (O'Connor 1978). D i f f e r e n c e s i n s i z e s of s i b l i n g s f a c i l i t a t e r a p i d responses t o sudden changes i n food a v a i l a b i l i t y t h r o u g h brood r e d u c t i o n ( R i c k l e f s 1965). H e a v i e r c h i c k s have more energy r e s e r v e s t o support them, e s p e c i a l l y t h r o u g h p e r i o d s of food s h o r t a g e (O'Connor 1976). For i n s t a n c e , i n skua broods t h a t f l e d g e d 2 c h i c k s the b i g g e r c h i c k s were h e a v i e r than the b i g g e r c h i c k s i n t w o - c h i c k broods t h a t e v e n t u a l l y f l e d g e d o n l y one c h i c k ( P r o c t e r 1975). S i b l i n g r i v a l r y i s a mechanism t h a t may p r e s e r v e some c h i c k p r o d u c t i o n under c o n d i t i o n s of severe food s h o r t a g e . 1 1 4 LITERATURE CITED Brockelman, W. Y. 1975. C o m p e t i t i o n , the f i t n e s s of o f f s p r i n g , and o p t i m a l c l u t c h s i z e . Amer. Nat. 109: 677-699. B r y a n t , D. M. 1978. E s t a b l i s h m e n t of weight h i e r a r c h i e s i n the broods of house m a r t i n s D e l i c h o n u r b i c a . I b i s 120: 16-26. C l i m a t e of B r i t i s h Columbia, 1976: T a b l e s of t e m p e r a t u r e , p r e c i p i t a t i o n , and s u n s h i n e . B r i t i s h Columbia M i n i s t r y of A g r i c u l t u r e . V i c t o r i a , B. C , 82 pp. H a r r i s , M. P. 1967. The b i o l o g y of o y s t e r c a t c h e r s Haematopus  o s t r a l e q u s on Skokholm I s l a n d , S. Wales. I b i s 109: 180-193. H a r t w i c k , E. B. 1974. B r e e d i n g e c o l o g y of the b l a c k o y s t e r c a t c h e r (Haematopus bachmani Audubon). S y e s i s .7:83-92. Ingram, C. 1959. The importance of j u v e n i l e c a n n i b a l i s m i n the b r e e d i n g b i o l o g y of c e r t a i n b i r d s of p r e y . Auk 76: 218-226. Ingram, C. 1962. C a n n i b a l i s m by n e s t l i n g S h o r t - e a r e d Owls. Auk 79: 715. Klomp, H. 1970. The d e t e r m i n a t i o n of c l u t c h - s i z e i n b i r d s : a r e v i e w . Ardea 58: 1-151. Meyburg, B. 1973. S i b l i n g a g g r e s s i o n and m o r t a l i t y among n e s t l i n g e a g l e s . I b i s 116: 224-228. Meyburg, B. 1978. S i b l i n g a g g r e s s i o n and c r o s s - f o s t e r i n g of e a g l e s . I n . Endangered B i r d s : Management Techniques f o r P r e s e r v i n g Threatened S p e c i e s . S. A. Temple ( e d . ) . U n i v . of W i s c o n s i n P r e s s , Madison. M i l l e r , R. S. 1973. The brood s i z e of c r a n e s . W i l s o n B u l l . 85: 436-441. N o r t o n - G r i f f i t h s , M. 1969. The- o r g a n i s a t i o n , c o n t r o l , and development of p a r e n t a l f e e d i n g i n the o y s t e r c a t c h e r (Haematopus ostr-alegus) . 'Behaviour 34 : 55-1 1 2 . 1 1 5 O'Connor, R. J . 1976. Weight and body c o m p o s i t i o n i n n e s t l i n g B lue T i t s Parus c a e r u l u s . I b i s 118: 108-112. O'Connor, R. J . 1978. Brood r e d u c t i o n i n b i r d s : s e l e c t i o n f o r f r a t r i c i d e , i n f a n t i c i d e , and s u i c i d e . Anim. Behav. 26:79-96. P r o c t e r , D. L. C. 1975. The problem of c h i c k l o s s i n the South P o l a r Skua C a t h a r a c t a maccormicki . I b i s 117: 452-459. R i c k l e f s , R. E. 1965. Brood r e d u c t i o n i n the C u r v e - b i l l e d T h r a s h e r . Condor 67: 505-510. R i c k l e f s , R. E. and F. R. H a i n s w o r t h . 1967. The temporary e s t a b l i s h m e n t of dominance between two h a n d - r a i s e d j u v e n i l e C a c tus Wrens (Campylorhynchus b r u n n e i c a p i 1 l u s ) . Condor 69: 528. S i e g e l , S. 1956. Nonparametric S t a t i s t i c s f o r the B e h a v i o r a l S c i e n c e s . M c G r a w - H i l l Book Co., New York. Smith, C. C. and F r e t w e l l , S. D. 1974. The o p t i m a l b a l a n c e between s i z e and number of o f f s p r i n g . Amer. Nat. 108:499-506. T r i v e r s , R. L. 1972. P a r e n t a l i nvestment and s e x u a l s e l e c t i o n . I n . B. Campbell ( e d . ) . S e x u a l s e l e c t i o n and the descent of man. A l d i n e , C h i c a g o , pp. 139-179. 1 16 GENERAL CONCLUSION C u r i o s i t y about how a n i m a l s f o r a g e i n pat c h y environments has g e n e r a t e d e x t e n s i v e t h e o r e t i c a l , l a b o r a t o r y , and f i e l d work i n f o r a g i n g e c o l o g y . Work on f o r a g i n g e c o l o g y a d d r e s s e s t h r e e main i s s u e s - f u n c t i o n , mechanism, and consequences. F u n c t i o n a l d e s c r i p t i o n of f o r a g i n g has been of c o n s i d e r a b l e i n t e r e s t t o t h e o r e t i c i a n s and e m p i r i c i s t s . L a b o r a t o r y and f i e l d s t u d i e s have been the t e s t i n g grounds f o r f o r a g i n g t h e o r y . These s t u d i e s have shown t h a t a n i m a l s a r e very good at s o l v i n g f o r a g i n g problems they c o n f r o n t i n the l a b o r a t o r y and f i e l d , but 1) d i s c r e p a n c i e s e x i s t between observed and t h e o r e t i c a l l y p r e d i c t e d performance, and 2) b e h a v i o u r of f o r a g i n g a n i m a l s v i o l a t e s a ssumptions of f o r a g i n g t h e o r y i n a l l but the most r e s t r i c t i v e l a b o r a t o r y s e t - u p s . T h i s s t u d y r e p o r t s d i s c r e p a n c i e s between o b s e r v e d f o r a g i n g performance of b l a c k o y s t e r c a t c h e r s and t h e o r e t i c a l p r e d i c t i o n s c o n c e r n i n g prey c h o i c e , p a t c h c h o i c e , and a l l o c a t i o n of f o r a g i n g time between p a t c h e s . These d i s c r e p a n c i e s o c c u r r e d because the rocky i n t e r t i d a l i s a v a r i a b l e environment, b i r d s f o r a g e d f o r m u l t i p l e d i f f e r e n t prey s p e c i e s , and b i r d s p r o b a b l y f o r a g e d s i m u l t a n e o u s l y f o r s e v e r a l d i f f e r e n t c u r r e n c i e s ( e. g. c a l o r i e s , t r a c e n u t r i e n t s , e t c . ) . F u r t h e r m o r e , because o y s t e r c a t c h e r s a r e t e r r i t o r i a l , assumptions about random movement through the f o r a g i n g a r e a a re p r o b a b l y v i o l a t e d . Mechanisms of f o r a g i n g have r e c e i v e d r e l a t i v e l y l i t t l e a t t e n t i o n from e c o l o g i s t s . - L a b o r a t o r y e x p e r i m e n t s t h a t 1 1 7 m a n i p u l a t e s i n g l e v a r i a b l e s have been c r i t i c a l i n d e t e r m i n i n g the range of d i s c r i m i n a t o r y a b i l i t i e s of a n i m a l s (e. g. l a r g e v s . s m a l l p r e y , G o s s - C u s t a r d 1970; r i c h v s . poor p a t c h e s , Smith and Sweatman 1974), but these e x p e r i m e n t s have p r o v i d e d l i t t l e i n s i g h t i n t o p r o c e s s e s i m p o r t a n t i n f o r a g i n g a n i m a l s , e s p e c i a l l y p r o c e s s e s concerned w i t h making d e c i s i o n s about f o r a g i n g . These ex p e r i m e n t s do not a i d i n the u n d e r s t a n d i n g of mechanisms of f o r a g i n g l a r g e l y because the problems they pose are u n r e a l i s t i c a l l y s i m p l e compared t o the problems f o r a g i n g a n i m a l s s o l v e d a i l y i n n a t u r e . For i n s t a n c e , b l a c k o y s t e r c a t c h e r s f o r a g e as though s i m u l t a n e o u s l y s o l v i n g problems of prey c h o i c e and p a t c h c h o i c e , but n e i t h e r t h e o r y nor l a b o r a t o r y s t u d i e s suggest what p r o t o c o l ( i f any) an o y s t e r c a t c h e r f o r a g i n g i n the rocky i n t e r t i d a l uses to s o l v e these problems. B e t t e r u n d e r s t a n d i n g of. f o r a g i n g mechanisms w i l l p r o v i d e u s e f u l i n s i g h t s f o r i m p r o v i n g f u n c t i o n a l d e s c r i p t i o n s of f o r a g i n g . S t u d i e s of b e h a v i o u r a l development i n young a n i m a l s are one way t o g a i n an u n d e r s t a n d i n g of i m p o r t a n t p r o c e s s e s i n f o r a g i n g a n i m a l s . Young, u n s k i l l e d a n i m a l s a r e good s u b j e c t s f o r s t u d y i n g the a c q u i s i t i o n and r e f i n e m e n t of f o r a g i n g s k i l l s and t e c h n i q u e s , i d e n t i f y i n g c r i t i c a l d e v e l o p m e n t a l p e r i o d s f o r f o r a g i n g s k i l l s , and a s s e s s i n g the development of d i s c r i m i n a t o r y c a p a c i t i e s . R e s u l t s from t h i s s t u d y of young o y s t e r c a t c h e r s suggest t h a t growth and p h y s i c a l m a t u r a t i o n are i m p o r t a n t i n e a r l y b e h a v i o u r a l development, and i t i s suggested t h a t l e a r n i n g and e a r l y e x p e r i e n c e are a l s o i m p o r t a n t . In a d u l t s , l e a r n i n g and r e c e n t e x p e r i e n c e a r e p r o b a b l y i m p o r t a n t p r o c e s s e s i n the f i n e -t u n i n g of f o r a g i n g b e h a v i o u r . 1 18 R e l a t i n g f i t n e s s t o f o r a g i n g performance i s a s i g n i f i c a n t c h a l l e n g e f o r t h e o r e t i c i a n s , l a b o r a t o r y and f i e l d w o r k e r s . F o r a g i n g performance i s an im p o r t a n t d e t e r m i n a n t of an a n i m a l ' s f i t n e s s because i t a f f e c t s how much energy an a n i m a l has a v a i l a b l e t o i n v e s t i n o f f s p r i n g and p a r e n t a l c a r e (Schoener 1971). However, the t r a n s l a t i o n of f o r a g i n g performance i n t o f i t n e s s i s f r a u g h t w i t h d i f f i c u l t i e s . To e v a l u a t e f o r a g i n g performance i n terms of f i t n e s s i t i s f i r s t n e c e s s a r y t o r e l a t e some measure of f o r a g i n g performance t o r e p r o d u c t i v e output of an i n d i v i d u a l and i t s progeny, but t h i s i s p a r t i c u l a r l y d i f f i c u l t i n l o n g - l i v e d b i r d s such as o y s t e r c a t c h e r s ( b r e e d i n g b i r d s of 27-34+ y e a r s of age have been r e p o r t e d from Europe, N i c e 1962). F u r t h e r m o r e , many c o n s t r a i n t s a f f e c t ' r e p r o d u c t i v e s u c c e s s and f i t n e s s ( M c F a r l a n d 1976). U n p r e d i c t a b l e e n v i r o n m e n t a l c o n d i t i o n s and ha z a r d s t o eggs and young (e. g. crows and g u l l s ) r e s u l t i n low and v a r i a b l e annual p r o d u c t i o n i n C l e l a n d I s l a n d o y s t e r c a t c h e r s and have n o t h i n g t o do w i t h a ..bird's f o r a g i n g performance. T e r r i t o r y a v a i l a b i l i t y and q u a l i t y may a f f e c t r e p r o d u c t i v e o u t p u t , but they a r e a l s o u n r e l a t e d t o f o r a g i n g performance. F i n a l l y , f o r a g i n g performance may not be a good measure of f i t n e s s - b e c a u s e i n t e n s e s e l e c t i o n a c t i n g on f o r a g i n g performance may be i n f r e q u e n t (e. g. Boag and Grant 1981). Because f i t n e s s i s s u b j e c t t o so many c o n s t r a i n t s , i t may not be p o s s i b l e i n most c a s e s t o use f o r a g i n g performance as a measure of f i t n e s s . In f u t u r e s t u d i e s of f o r a g i n g e c o l o g y , the i n t e r f a c e between f u n c t i o n and mechanism s h o u l d be a p a r t i c u l a r l y p r o d u c t i v e area of r e s e a r c h . U n d e r s t a n d i n g mechanisms -and 119 p r o c e s s e s i n f o r a g i n g a n i m a l s w i l l p r o v i d e new f o u n d a t i o n s f o r f u n c t i o n a l d e s c r i p t i o n s of f o r a g i n g . The t r a n s l a t i o n of f o r a g i n g performance i n t o f i t n e s s i s l i k e l y t o remain an e l u s i v e g o a l because f i t n e s s i s the i n t e g r a t i o n of a l l f a c e t s of an a n i m a l ' s e x i s t e n c e , not merely f o r a g i n g performance, but i n p r i n c i p l e i t i s p o s s i b l e t o r e l a t e f o r a g i n g performance t o f i t n e s s . 1 20 LITERATURE CITED Boag, P. T. and G r a n t , P. R. 1981. I n t e n s e n a t u r a l s e l e c t i o n i n a p o p u l a t i o n of Darwin's F i n c h e s ( G e o s p i z i n a e ) i n the Galapagos. S c i e n c e 214:82-85. G o s s - C u s t a r d , J . D. 1970. F a c t o r s a f f e c t i n g the d i e t and f e e d i n g of the redshank ( T r i n q a t o t a n u s ) . In . A. Watson ( e d . ) . Animal p o p u l a t i o n s i n r e l a t i o n t o t h e i r food r e s o u r c e s . B l a c k w e l l S c i e n t i f i c P u b l i c a t i o n s , O x f o r d , pp. 101-110. M c F a r l a n d , D. J . 1976. Form and f u n c t i o n i n t e m p o r a l o r g a n i z a t i o n of b e h a v i o u r . I_n . P. P. G. Bateson and R. A. Hinde ( e d s . ) . Growing p o i n t s i n e t h o l o g y . Cambridge U n i v e r s i t y P r e s s , Cambridge, pp. 55-93. N i c e , M. M. 1962. O y s t e r c a t c h e r 34 y e a r s o l d - the o l d e s t r i n g e d b i r d t o date of the V o g e l w a r t e H e l g o l a n d . B i r d - B a n d i n g 33:205. Smith, J . N. M. and Sweatman, H. P. be h a v i o u r of t i t m i c e i n patchy 55:1216-1232. 1974. Food s e a r c h i n g e n v i r o n m e n t s . E c o l o g y 121 APPENDIX A L e n g t h ( x ) - w e i g h t ( y ) r e g r e s s i o n s used t o c a l c u l a t e wet w e i g h t s of i n v e r t e b r a t e s measured i n i n t e r t i d a l q u a d r a t s and consumed by b l a c k o y s t e r c a t c h e r s SPECIES EQUATION N N e r e i s v e x i l l o s a  K a t h a r i n a t u n i c a t a  T o n i c e l l a 1 i n e a t a  C o l l i s e l l a d i g i t a l i s and C^ P e l t a ( l e s s than 20mm) Notoacmea scutum ( g r e a t e r than 20mm) Hemigrapsus nudus 1 y = 0. 199x - 0.398 l o g ( y ) = 2.451og(x) - 3.71 l o g ( y ) = 0.03x - 1.11 l o g ( y ) = 2.771og(x) - 4.33 l o g ( y ) = 5.861og(x) - 8.31 y = 1.5(1 . 11x - 0.4 2 ) 2 O r c h e s t o i d e s c a l i f o r n i a n a 2 y = (9.37x - 2.6 6 ) 2 1 6 22 11 86 44 1 4 1 5 M y t i l u s c a l i f o r i a n u s : y = (0.233x - 0.27) 1 A l t h o u g h o y s t e r c a t c h e r s were never ob s e r v e d e a t i n g Hemigrapsus  nudus, t h i s s p e c i e s was used t o o b t a i n an a p p r o x i m a t i o n of the wet weight of Oediqnathus i n e r m i s because i t was not p o s s i b l e t o c o l l e c t enough of the l a t e r s p e c i e s f o r use i n d e t e r m i n i n g a l e n g t h - w e i g h t r e g r e s s i o n . 2 U n p u b l i s h e d d a t a from L. J . R i c h a r d s . 3 Harger (1970) 1 22 LITERATURE CITED H a r g e r , J . R. E. 1970. The e f f e c t of wave impact on some a s p e c t s of the b i o l o g y of sea musse l s . V e l i g e r 12:401-414. 1 23 APPENDIX B Va l u e s used f o r v a r i a b l e s i n m u l t i s p e c i e s d i s c e q u a t i o n Prey Type ( i ) Limpet M u s s e l C h i t o n Worm c r a b Average 8.3 34.9 53.5 6.8 19.1 h a n d l i n g s.d.=4.4 s.d.=32.5 s.d.=35.9 s.d.=3.0 s.d.= time (s) N = 282 N=48 N = 4 N=9 N=49 Prey D e n s i t y ( D j ) . a n d Number Eaten (N; ) i n Three I n t e r t i d a l Zones (Prey d e n s i t i e s a r e per 50 x 50 cm q u a d r a t ) Limpet Mussel C h i t o n Worm Crab Zone 9-i Mi P-i N-—i D-—i M| D-— l N-—i D-— i N fucus 0 .20 20 0, . 1 3 1 a. .00 0 0. .00 0 0. .00 0 m y t i l u s 1 .32 202 0. .72 37 0, .03 1 0. .02 5 0, .00 12 lam.post. 1 .42 60 0, .10 1 0 3, .34 3 0, .00 4 0, .05 37 T o t a l f o r a g i n g t i m e , seconds (T ) fucu s 8 5 3 m y t i l u s 1 0 7 7 9 lam.post. 8 5 9 4 

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