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Effects of food addition on a population of grey jays Delehanty, Brendan 1994

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E F F E C T S O F F O O D A D D I T I O N O N A P O P U L A T I O N O F G R E Y J A Y S by B R E N D A N DELEHANTY B.Sc , The University of Alberta, 1991 A THESIS SUBMITTED IN P A R T I A L FULFILMENT OF T H E REQUIREMENTS FOR T H E DEGREE OF MASTER OF SCIENCE in T H E F A C U L T Y OF G R A D U A T E STUDIES DEPARTMENT OFZOOLOGY We accept this thesis as conforming to the required standard T H E UNIVERSITY OF BRITISH COLUMBIA June 1995 . © Brendan Delehanty, 1995 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Z L Q - Q The University of British Columbia Vancouver, Canada DE-6 (2/88) A b s t r a c t Grey j a y s (Perisoreus canadensis). i n t h e southwest Yukon were p r o v i d e d w i t h s u p p l e m e n t a l f o o d on t h r e e s t u d y a r e a s t o s t u d y t h e e f f e c t s of f o o d on t e r r i t o r i a l i t y and f o r a g i n g s t r a t e g i e s . T e r r i t o r y s i z e s and o v e r l a p were measured i n 1993 and 1994. T e r r i t o r i e s o f f o o d a d d i t i o n g r i d s were 3 0% s m a l l e r t h a n t h o s e on g r i d s w i t h o u t a c c e s s t o added f o o d . There was o n l y s l i g h t l y more o v e r l a p between t e r r i t o r i e s on f o o d a d d i t i o n g r i d s compared t o c o n t r o l s , and the d i f f e r e n c e was 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 . Time spe n t f o r a g i n g , the r a t e a t whi c h j a y s made caches, and t h e w e i g h t s of j a y s were measured. These d a t a were used t o t e s t f o u r hypotheses about the f a c t o r s t h a t l i m i t o v e r w i n t e r body masses of j a y s . B i r d s on f o o d a d d i t i o n g r i d s made t h r e e t i m e s as many caches as c o n t r o l b i r d s i n a s i m i l a r amount o f time spent f o r a g i n g . I n w i n t e r , g r e y j a y s w i t h added f o o d spent l e s s t i m e f o r a g i n g , y e t t h e y were a b l e t o m a i n t a i n h i g h e r body c o n d i t i o n t h a n c o n t r o l b i r d s . These r e s u l t s were c o n s i s t e n t w i t h t h e h y p o t h e s i s t h a t o v e r w i n t e r w e i g h t s a re l i m i t e d by b o t h a f o o d s h o r t a g e and by c o s t s a s s o c i a t e d w i t h i n c r e a s i n g w e i g h t . S e a s o n a l t r e n d s i n body c o n d i t i o n d i f f e r e d between b i r d s on f o o d a d d i t i o n g r i d s and those w i t h o u t s u p p l e m e n t a l f o o d . B i r d s w i t h added f o o d were i n b e t t e r c o n d i t i o n y e a r round, and were a b l e t o i n c r e a s e i n weight between summer and f a l l . Control birds l o s t weight between summer and f a l l , but then increased dramatically i n winter. These d i f f e r e n t weight trends may represent d i f f e r e n t caching or cache r e t r i e v a l s t r a t e g i e s . i i i Table of Contents A b s t r a c t i i T a b l e o f C o n t e n t s i v L i s t o f T a b l e s . v L i s t o f F i g u r e s v i Acknowledgments v i C h a p t e r One G e n e r a l I n t r o d u c t i o n 1 The Kluane Food A d d i t i o n . . . . 2 C h a p t e r Two F a c t o r s L i m i t i n g O v e r w i n t e r Body Mass i n Grey J a y s 7 I n t r o d u c t i o n 7 Methods 10 Study S i t e 10 T r a p p i n g 11 C o n d i t i o n Index 13 Time Budgets 13 S t a t i s t i c a l Treatment 15 R e s u l t s 18 C o n d i t i o n . . . 18 Time Budgets 22 D i s c u s s i o n 22 R e g u l a t i o n o f W i n t e r Fat R e s e r v e s . . . 22 Trends i n Body C o n d i t i o n 31 C h a p t e r Three E f f e c t s o f Food A d d i t i o n on Grey J a y T e r r i t o r i e s 35 I n t r o d u c t i o n 35 Methods 37 Study S i t e 3 7 T e r r i t o r y Mapping 37 R e s u l t s 40 D i s c u s s i o n . 45 Methodology 45 T e r r i t o r y S i z e 46 T e r r i t o r y O v e r l a p 50 Number of T e r r i t o r i e s p e r G r i d 52 Summary 55 C h a p t e r F o u r G e n e r a l D i s c u s s i o n . .56 O v e r w i n t e r C o n d i t i o n 56 T e r r i t o r i a l i t y 58 C o n c l u s i o n 60 L i t e r a t u r e C i t e d 62 i v L i s t of Tables T a b l e 1. L i s t o f fo o d s u s e d by g r e y j a y s i n the Kluane a r e a 4 T a b l e 2. P r e d i c t e d d i r e c t i o n o f change i n response t o f o o d a d d i t i o n f o r t h e f o u r hypotheses ..9 T a b l e 3. S i g n i f i c a n c e t e s t s from ANOVA's comparing t h e v a r i a n c e i n c o n d i t i o n between s o c i a l groups t o the v a r i a n c e w i t h i n s o c i a l groups 17 T a b l e 4. S i g n i f i c a n c e t e s t s o f t - t e s t s between mean c o n d i t i o n o f f o o d and c o n t r o l b i r d s i n a l l t r a p p i n g 20 T a b l e 5. Agreement o f r e s u l t s w i t h p r e d i c t i o n s o f the f o u r h y p o t h e s e s 28 T a b l e 6. Observed and p r e d i c t e d number of t e r r i t o r i e s p e r 36-ha g r i d 43 v L i s t of Figures F i g u r e 1. Map o f t h e s t u d y s i t e showing t h e l o c a t i o n s of t r e a t m e n t and c o n t r o l g r i d s 12 F i g u r e 2. Mean c o n d i t i o n i n d i c e s f o r b i r d s on f o o d a d d i t i o n c o n t r o l b i r d s i n a l l t r a p p i n g s e s s i o n s 19 F i g u r e 3. S e a s o n a l t r e n d s i n (a) body c o n d i t i o n and (b) w e i g h t s f o r i n d i v i d u a l b i r d s on f o o d a d d i t i o n ( s o l i d l i n e s ) and c o n t r o l (dashed l i n e s ) g r i d s 21 F i g u r e 4. Summer c a c h i n g r a t e s o f food a d d i t i o n and c o n t r o l b i r d s 2 3 F i g u r e 5. P r o p o r t i o n o f t i m e spent i n v a r i o u s a c t i v i t i e s d u r i n g summer 24 F i g u r e 6. P r o p o r t i o n o f time spent i n v a r i o u s a c t i v i t i e s d u r i n g w i n t e r 25 F i g u r e 7. P r e d i c t e d w e i g h t t r e n d s a c c o r d i n g t o two models of cache use d u r i n g p e r i o d s o f f o o d s c a r c i t y 33 F i g u r e 8. Examples o f t e r r i t o r y maps f o r c o n t r o l (a) and f o o d a d d i t i o n (b) groups 39 F i g u r e 9. Diagrams showing approximate b o u n d a r i e s between t e r r i t o r i e s 42 F i g u r e 10. P e r c e n t o v e r l a p between t e r r i t o r i e s w i t h 95% c o n f i d e n c e i n t e r v a l s 44 v i Acknowledgements I would l i k e t o thank a l l t h o s e p e o p l e who h e l p e d me i n c o l l e c t i n g d a t a , e s p e c i a l l y t h o s e who were busy w i t h t h e i r own work a t t h e t i m e . F o r c a t c h i n g j a y s , i n t h e i r t r a p s , I thank t h e hare and s q u i r r e l t r a p p e r s , M. O'Donoghue, T. K a r e l s , A. Byrom, A. K l i s k y , S.. O l s e n , M. Blower, S. A n t p o e h l e r , and E . W e i l e r . C. D o y l e and F. D o y l e were e s p e c i a l l y h e l p f u l w i t h t h e i r a d v i c e and a s s i s t a n c e i n t h e f i e l d . M. J o y c e was a v a l u a b l e f i e l d a s s i s t a n t f o r b o t h summers of t h i s s t u d y . J . D e l e h a n t y was h e l p f u l i n t h e f i e l d , and good company t o have i n camp. I thank A. W i l l i a m s , base manager of t h e A r t i e I n s t i t u t e o f N o r t h A m e r i c a Kluane Lake R e s e a r c h S t a t i o n , and F. S i a s and J . S i a s f o r p r o v i d i n g p l a c e s t o l i v e and work. W. Hochachka, S. B o u t i n , F. D o y l e , C. D o y l e , and M. O'Donoghue p r o v i d e d h e l p f u l a d v i c e and d i s c u s s i o n . I thank my s u p e r v i s o r , J.N.M. Smith, f o r t a k i n g t h e time t o make such e x t e n s i v e e d i t o r i a l comments w h i c h were t r u l y h e l p f u l . I a l s o thank members of my r e s e a r c h committee f o r t h e i r comments and t i m e . F i n a n c i a l s u p p o r t was p r o v i d e d by a g r a n t f r o m t h e N a t u r a l S c i e n c e s and E n g i n e e r i n g R e s e a r c h C o u n c i l o f Canada t o J.N.M. Smith, and by N o r t h e r n S c i e n c e s T r a i n i n g G r a n t s from t h e Department o f I n d i a n A f f a i r s and N o r t h e r n Development.. S p e c i a l t h a n k s go t o C l a i r e J a r d i n e and my p a r e n t s who have p r o v i d e d h e l p and encouragement t h r o u g h o u t t h i s work. v i i Chapter 1: General Introduction The experimental addition of food, has been used to study how food supply influences a range of response va r i a b l e s from individual behaviour to community structure (e.g. Ewald and Carpenter 1978, Ewald and Rohwer 1982, Krebs et a l . 1992, Waite 1990). In Boutin's (1990) review of food supplementation experiments, he concludes that knowledge i n several areas i s s t i l l lacking despite the extensive l i t e r a t u r e already available. This i s due, i n part, to advances i n theory that require increasingly sophisticated experiments. An example i s the role of food supply i n t e r r i t o r y s i z e . Increased food density i s often correlated with smaller t e r r i t o r y sizes i n birds (e.g. Stenger 1958, Enoksson and Nilsson 1983) but as the theory of optimal t e r r i t o r y s i z e advanced from the early conceptual model of economic defe n d a b i l i t y (Brown 1964) to more e x p l i c i t mathematical models (Hixon 1980, Schoener 1983), the importance of intruder pressure was recognized and incorporated i n t o experimental food additions, and non-experimental studies (e.g. Mares et a l . 1982, Myers et al. 1979). Shortcomings i n methodology have also been recognized. For example, the role of food- supply i n demography has.long, been of i n t e r e s t (see review i n Boutin 1990). As Boutin (1990) observed, food additions have generally r e s u l t e d i n twofold increases i n density, but the r e a l increase i n l d e n s i t y may be l e s s i f a n i m a l s are commuting from o f f t h e c e n s u s i n g a r e a f o r f o o d but are not t r u l y r e s i d e n t s ( i . e . t h e e f f e c t i v e c e n s u s i n g a r e a i s l a r g e r , than c o n t r o l a r e a s ) . T h i s p r o b l e m a r i s e s when inadequate a t t e n t i o n has been p a i d t o t h e s p a t i a l s c a l e o f t h e s t u d y system. The concept o f e c o l o g i c a l n e i g h b o r h o o d s ( A d d i c o t t e t al. 1987) and r e c o g n i z i n g t h e need t o i n c o r p o r a t e m u l t i p l e s c a l e s i n s t u d i e s ( K o t l i a r and Wiens 1990) have a r i s e n from t h e m e t h o d o l o g i c a l s h o r t c o m i n g s of p a s t s t u d i e s . By i n c o r p o r a t i n g r e l e v a n t s p a t i a l s c a l e s i n t o the d e s i g n s o f f o o d a d d i t i o n s , a s h i f t from measuring q u a l i t a t i v e t o q u a n t i t a t i v e changes can be made, a l l o w i n g more p r e d i c t i v e power and t e s t i n g o f more e x p l i c i t hypotheses. The Kluane Food Addition S i n c e 1987, t h r e e 36-ha areas i n w h i t e spruce {Picea glauca) b o r e a l f o r e s t have had commercial r a b b i t chow added y e a r - r o u n d (Krebs e t a l . 1992). The f o o d i s s u p p l i e d by s p r e a d i n g p e l l e t s a l o n g f o u r rows of a 20 x 20 g r i d . Numerous a n i m a l s make use of t h i s f o o d a d d i t i o n : snowshoe h a r e s (Lepus americanus) ; r e d s q u i r r e l s (Tamiasciurus hudsonicus); a r c t i c ground s q u i r r e l s (Spermophilus parryi); moose {Alces alces); g r i z z l y b ears {Ursus arctos) ; c h i c k a d e e s {Parus atricapillus, and P., hudsonicus) ; dark-', eyed j u n c o s (Junco hyemalis) ; magpies {Pica pica) ; and g r e y j a y s ( P e r i s o r e u s canadensis) . The j u n c o s , c h i c k a d e e s , and j a y s have been o b s e r v e d s e l e c t i n g t h e g r a i n component o f t h e 2 p e l l e t s , whereas t h e o t h e r a n i m a l s appear t o use the e n t i r e p e l l e t s ( p e r s o n a l o b s e r v a t i o n ) . I u s e d t h e Kluane f o o d a d d i t i o n t o s t u d y t h e re s p o n s e o f grey, j a y s t o the f o o d a d d i t i o n . G r e y j a y s a r e common b i r d s t h r o u g h o u t b o r e a l f o r e s t i n N o r t h A m e r i c a . Because t h e y a r e r e l a t i v e l y tame, t h e y a r e e a s i l y o b s e r v e d a t most t i m e s , and r e a d i l y adapt t o the p r e s e n c e o f human o b s e r v e r s . They l i v e i n s o c i a l groups c o m p r i s e d o f a m a t i n g p a i r and u s u a l l y one o r two non-b r e e d i n g b i r d s ( R u t t e r 1969, Waite and Reeve 1992) . On o c c a s i o n a group i s made up o n l y of males (Waite and Reeve 1992). The group d e f e n d s an a l l - p u r p o s e t e r r i t o r y w hich i s h e l d y e a r - r o u n d . Throughout t h e summer and i n t o t h e f a l l , j a y s s c a t t e r h o a r d s a l i v a c o a t e d b o l i of f o o d i n b a r k c r e v i c e s and c o n i f e r b r a n c h e s ( R u t t e r 1969); a l i s t o f the foods I have o b s e r v e d b i r d s u s i n g i n t h e Kluane a r e a i s p r e s e n t e d i n T a b l e 1. The d i v e r s e n a t u r e o f t h e i r d i e t l e a d s t o use o f many h a b i t a t t y p e s . I have o b s e r v e d b i r d s u s i n g mossy bogs, meadows, dense w i l l o w {Salix spp.) shr u b s , and aspen (Populus tremuloides) and w h i t e s p r u c e (Picea glauca) f o r e s t s a t v a r i o u s t i m e s t h r o u g h o u t t h e summer. J a y s w i l l a l s o cache o p p o r t u n i s t i c a l l y i n the w i n t e r , f o r .example when t h e r e i s c a r r i o n ( R u t t e r 1969, Waite.and Reeve 1992) o r , as i n t h i s s t u d y , when f o o d i s s u p p l i e d y e a r - r o u n d . The caches a r e us e d t h r o u g h o u t t h e w i n t e r , and t h e b i r d s o b s e r v e d i n t h i s s t u d y appeared t o r e l y a l m o s t 3 Table 1. List of foods used by grey jays in the Kluane area. This is only a partial list based on items that I was able to identify. Organism Notes Plant Arctostaphylos uva-ursi A. rubra Mushrooms Invertebrate Moths/Butterflies Unidentified Beetle Larvae Ants Dragonflies Spider egg masses Vertebrate Carrion, variety of species Dendroica coronata Turdus migratorius Flowers Berries Found on willow leaves Nestlings Eggs e n t i r e l y on caches (see c h a p t e r 2 ) . T h i s s t u d y took p l a c e d u r i n g t h e low o f t h e snowshoe ha r e c y c l e (Krebs et al. 1992). I n c r e a s e d a v a i l a b i l i t y o f c a r r i o n d u r i n g t h e p e r i o d of h i g h hare numbers may r e s u l t i n v e r y d i f f e r e n t o b s e r v a t i o n s . S c a t t e r h o a r d e d caches a r e s u s c e p t i b l e t o t h e f t , decay, and b e i n g f o r g o t t e n (Vander W a l l 1990). T h e f t of caches i n t h e Kluane a r e a i s p r e s u m a b l y due t o o t h e r g r e y j a y s and magpies (see Waite 1988), and r e d s q u i r r e l s have been o b s e r v e d s t e a l i n g caches (J. D e l e h a n t y , p e r s . comm.). One method t o m i t i g a t e t h e r i s k s o f l o s i n g caches o v e r the w i n t e r i s t o s t o r e energy as f a t r e s e r v e s . There i s , however, a complex system o f t r a d e o f f s between the r e l a t i v e c o s t s and b e n e f i t s o f caches and f a t r e s e r v e s (Vander W a l l 1990) w h i c h appears t o l i m i t t h e e x t e n t t o which g r e y j a y s a c c u m u l a t e f a t r e s e r v e s (Waite 1992). I n c h a p t e r two I d i s c u s s t h e s e t r a d e o f f s and t e s t s e v e r a l hypotheses t o d e t e r m i n e whether f o o d s h o r t a g e and/or c o s t s a s s o c i a t e d w i t h i n c r e a s i n g w e i g h t l i m i t w i n t e r f a t r e s e r v e s . A l s o i n c h a p t e r two I p r e s e n t the weight t r e n d s of b i r d s on f o o d a d d i t i o n and c o n t r o l g r i d s , and d i s c u s s t h e d i f f e r e n t p a t t e r n s i n terms of the f a c t o r s l i m i t i n g o v e r w i n t e r w e i g h t s . •• I go on i n c h a p t e r t h r e e t o d i s c u s s t h e response o f g r e y j a y t e r r i t o r i a l i t y t o f o o d a d d i t i o n . I measured t e r r i t o r y s i z e , p e r c e n t o v e r l a p between a d j a c e n t t e r r i t o r i e s , and t h e arrangement of t e r r i t o r i e s around f o o d 5 addition and control grids. Using these results I then discuss the relationship between territory sizes and apparent population densities on the study areas. 6 C h a p t e r 2 : F a c t o r s L i m i t i n g O v e r w i n t e r Body Mass i n G r e y J a y s Introduction A n i m a l s o v e r w i n t e r i n g i n temperate and s u b - a r c t i c r e g i o n s can e x p e r i e n c e f o o d s h o r t a g e d u r i n g t i m e s when m a i n t a i n i n g adequate energy r e s e r v e s i s p a r t i c u l a r l y i m p o r t a n t . Because w i n t e r i n n o r t h e r n b o r e a l f o r e s t b r i n g s s h o r t days, extreme c o l d , and reduced a v a i l a b i l i t y o f f o o d , a n i m a l s must be a b l e t o s t o r e enough energy i n e i t h e r f a t r e s e r v e s o r f o o d caches t o l a s t t h e p e r i o d o f f o o d s h o r t a g e . A l t h o u g h d i f f e r e n t energy s t o r a g e s t r a t e g i e s have been i d e n t i f i e d , l i t t l e r e s e a r c h has been done on t h e f a c t o r s i n f l u e n c i n g the a d a p t i v e v a l u e o f a p a r t i c u l a r s t r a t e g y . Vander W a l l (1990, p. 26) s u g g e s t s t h a t because f a t d e p o s i t i o n i s a f u n c t i o n o f body mass and f o o d s t o r a g e i s n o t , s m a l l a n i m a l s can s t o r e more energy t h r o u g h f o o d h o a r d i n g . I n s m a l l b i r d s , t h i s e x p l a n a t i o n a l o n e i s i n s u f f i c i e n t as t h e y have an e x t r a o r d i n a r i l y h i g h a b i l i t y t o s t o r e energy as f a t (Blem 1976) y e t a c t u a l w i n t e r f a t mass i s g e n e r a l l y l e s s than what i s p o s s i b l e ( K i n g 1972). I n t h i s s t u d y , I t e s t e d f o u r hypotheses c o n c e r n i n g t h e f a c t o r s l i m i t o v e r w i n t e r body mass i n ' g r e y . j a y s (Perisoreus canadensis). I f i r s t d e s c r i b e p a t t e r n s o f w e i g h t change i n c o n t r o l b i r d s and those w i t h added fo o d . I t h e n compare t i m e budgets of b i r d s w i t h added f o o d t o c o n t r o l b i r d s . 7 These h y p o t h e s e s and t h e i r p r e d i c t i o n s are d e s c r i b e d below and a r e summarized i n T a b l e 2. I a l s o d i s c u s s t h e o b s e r v e d w e i g h t t r e n d s i n t h e c o n t e x t of these f i n d i n g s . Hypothesis 1: Winter fat reserves are not l i m i t e d by food o r weight-related costs. I f t h i s i s the case, f o o d a d d i t i o n s h o u l d n o t a f f e c t body c o n d i t i o n , nor s h o u l d b i r d s w i t h e x t r a f o o d spend more time f o r a g i n g i n the w i n t e r , n o r s h o u l d t h e y cache more f o o d i n the summer o r f a l l . Hypothesis 2: Food alone l i m i t s grey jay f a t reserves. I f j a y s a r e f o o d l i m i t e d , j a y s w i t h added foo d s h o u l d m a i n t a i n b e t t e r body c o n d i t i o n o v e r w i n t e r by s p e n d i n g more t i m e f o r a g i n g , and cache more i n the summer o r f a l l . T h i s h y p o t h e s i s a l l o w s some c o s t t o added weight, b u t i t i s a l w a y s l e s s t h a n t h e c o s t a s s o c i a t e d w i t h s t o r i n g t h e same amount of e n ergy as caches. Hypothesis 3: Costs associated with increasing weight are the sole l i m i t i n g factors. The c o s t s of added w e i g h t a r e i n t h e f orm o f p r e d a t i o n r i s k w h i l e f o r a g i n g o r i n t h e m e t a b o l i c expense of t r a n s p o r t i n g and m a i n t a i n i n g t h e e x t r a w e i g h t . I n t h i s case, b i r d s w i t h added f o o d w i l l m a i n t a i n • h i g h e r w i n t e r body c o n d i t i o n t h a n c o n t r o l b i r d s (Lima-1986), b u t not because t h e y have cached more i n t h e summer, s i n c e f o o d i s n o t l i m i t e d . R ather, b i r d s a r e a b l e t o r e duce t h e i r f o r a g i n g t i m e i n w i n t e r because the f o o d a d d i t i o n p r o v i d e s 8 Table 2. Predicted direction of change in response to food addition for the four hypotheses being tested. A n increase is denoted by +, decrease by -, and no difference between food and control animals is =. Winter Summer Hypothesis Condition Time Spent Foraging Amount Cached 1: Not Limited = 2: Food Limited + + + 3: Weight Limited + - = 4: Food and Weight + - + Limited f o o d w i t h l e s s s e a r c h t i m e i n v o l v e d . S i n c e f o r a g i n g time and t h e a s s o c i a t e d c o s t s have been reduced, b i r d s w i t h added f o o d c a n m a i n t a i n h i g h e r w e i g h t s t h a n c o n t r o l b i r d s w i t h o u t i n c u r r i n g a h i g h e r t o t a l c o s t . Hypothesis 4: A l i m i t e d food supply i n winter and a cost of increasing weight combine to l i m i t winter f a t reserves. I n summer, f o o d a d d i t i o n a l l o w s j a y s t o cache more f o o d t h a n c o n t r o l b i r d s w h i c h , i n t u r n , a l l o w s them t o m a i n t a i n h i g h e r w e i g h t s i n w i n t e r . Once a g a i n , though, because t h e r e i s a c o s t t o i n c r e a s e d w e i g h t , t h e j a y s w i t h added f o o d must be a b l e t o b a l a n c e t h o s e c o s t s by d e c r e a s i n g the c o s t s of f o r a g i n g i n t h e w i n t e r (Lima 1986) . As w i t h t h e w e i g h t - o n l y h y p o t h e s i s t h i s g a i n can be had by u s i n g t h e added f o o d a v a i l a b l e on t h e ground i n t h e w i n t e r . I n c o n t r a s t t o the w e i g h t o n l y h y p o t h e s i s , f o r a g i n g time can a l s o be reduced by h a v i n g more caches from t h e summer. Methods Study S i t e T h i s s t u d y was conducted n e a r Kluane Lake, i n the so u t h w e s t Yukon (61°N, 138°W) between May 1993 and November 1994. G rey j a y s a r e found t h r o u g h o u t t h e s t u d y s i t e w hich i s d o m i n a t e d by. w h i t e s p r u c e (Picea -glauca) o v e r s t o r y and a., s h r u b w i l l o w u n d e r s t o r y . {Salix spp.) . A more complete d e s c r i p t i o n o f t h e s i t e i s found i n Krebs e t a l . (1986). Commercial r a b b i t chow (16% crude p r o t e i n , 2% f a t , 18% 10 f i b r e ) i s added y e a r - r o u n d t o t h r e e 36-ha g r i d s as p a r t of th e K l u a n e B o r e a l F o r e s t E c o s y s t e m P r o j e c t (Krebs e t a l . 1992). Grey j a y s use t h e g r a i n component of t h i s f o o d , w h i c h o f t e n s e p a r a t e s from t h e p e l l e t s ( p e r s o n a l o b s e r v a t i o n ) . J a y s on t h e f o o d a d d i t i o n g r i d s and i n o t h e r a r e a s o f s i m i l a r h a b i t a t b u t w i t h no a c c e s s t o f o o d a d d i t i o n were used i n t h e s t u d y ( F i g u r e 1 ) . Trapping J a y s were caught t h r o u g h o u t t h e s t u d y i n Tomahawk l i v e t r a p s d u r i n g h a r e (Lepus americanus) and s q u i r r e l (Tamiasciurus hudsonicus and Spermophilus parryi) t r a p p i n g s e s s i o n s of o t h e r p r o j e c t s , and a l s o when t h e s e t r a p s were s e t e x p l i c i t l y f o r j a y s . Depending on the n a t u r e o f the t r a p p i n g s e s s i o n , t r a p s were b a i t e d w i t h e i t h e r peanut b u t t e r , a p p l e , r a i s i n s , o r r a b b i t chow. The b i r d s were banded w i t h numbered aluminum bands and w i t h one t o t h r e e c o l o u r e d p l a s t i c bands i n u n i q u e c o m b i n a t i o n s t o make b i r d s i n d i v i d u a l l y r e c o g n i z a b l e . J a y s were weighed t o t h e n e a r e s t gram u s i n g a 300 g s p r i n g s c a l e and t h e i r t a r s u s l e n g t h was measured t o the n e a r e s t 0.1 mm from t h e j o i n t w i t h t h e t i b i o t a r s u s t o the l a s t u n d i v i d e d s c u t e a t t h e d i s t a l end o f t h e t a r s u s . The average t a r s u s measurement-over a l l t r a p p i n g s e s s i o n s f o r • each b i r d was t h e n u s e d w i t h i t s a v e r a g e w e i g h t o v e r a s i n g l e t r a p p i n g s e s s i o n t o c a l c u l a t e a c o n d i t i o n i n d e x (see b e l o w ) . 11 Figure 1. Map of the study site showing the location of food addition (Fl - F3) and control grids (CI - C 4 ). The three food addition grids and three of the control grids were used to estimate the number of territories per grid. Control grid 2 was not used due to incomplete information 12 Condition Index To e s t i m a t e r e l a t i v e amounts of f a t r e s e r v e s a c o n d i t i o n i n d e x based on weight and body s i z e (as r e p r e s e n t e d by t a r s u s l e n g t h ) was used (see Krebs and S i n g l e t o n 1993 f o r a d i s c u s s i o n of t h i s t e c h n i q u e ) . The f i r s t s t e p i n g e n e r a t i n g a c o n d i t i o n i n d e x was t o use w e i g h t s and mean t a r s u s l e n g t h s of a l l c o n t r o l j a y s caught i n t r a p p i n g s e s s i o n s i n May and e a r l y June, 1993 and 1994. Because t h e r e l a t i o n s h i p was l i n e a r and t h e r e were measurement e r r o r s i n b o t h weight and t a r s u s , t h e G e o m e t r i c Mean R e g r e s s i o n was c a l c u l a t e d (Krebs 1989 p. 462) f r o m t h i s d a t a . The r e s u l t i n g r e g r e s s i o n e q u a t i o n (WEIGHT = 3.86TARSUS - 50.51) was used t o c a l c u l a t e a p r e d i c t e d w e i g h t f o r a b i r d w i t h a g i v e n t a r s u s l e n g t h . I t h e n u s e d t h e r a t i o o f ob s e r v e d w e i ght t o p r e d i c t e d w e i g h t as a c o n d i t i o n i n d e x . T h i s c o n d i t i o n i n d e x assumes t h a t changes i n mass a r e due t o changes i n f a t r e s e r v e s , and t h a t t h i s i n t u r n r e p r e s e n t s an i n c r e a s e i n body c o n d i t i o n . Time Budgets To e s t i m a t e the p r o p o r t i o n of time spent f o r a g i n g i n w i n t e r , t i m e budgets were conducted between J a n u a r y 17, 1994 and March 11, 1994. A t o t a l o f 36 s e s s i o n s were r e c o r d e d i n v o l v i n g .5 c o n t r o l groups and 7 f o o d a d d i t i o n g r o u p s . Once a f o c a l b i r d was s e l e c t e d , a watch and a hand h e l d t a p e r e c o r d e r were used t o r e c o r d time o f day ( t o t h e n e a r e s t s e c o n d ) , and whether t h e b i r d was pe r c h e d , f o r a g i n g , 13 c a c h i n g , o r f l y i n g . F l i g h t s l e s s than two seconds were not i n c l u d e d i n t h e f l i g h t t i me as such s h o r t e v e n t s were i m p r a c t i c a l t o r e c o r d . I f b i r d s were l o s t f o r more th a n 30 s t h e s e s s i o n was ended." I n p r a c t i c e , however, when b i r d s were l o s t , t h e y were always l o s t f o r much more t h a n 30 s. O r i g i n a l l y , s e s s i o n s between 5 min and 30 min were t o be used, b u t t o i n c r e a s e sample s i z e , the l o w e r l i m i t was s e t a t 4 min. An e f f o r t was made t o sample each group a t e v e n l y d i s t r i b u t e d t i m e s o f day. To e s t i m a t e t h e amount of f o o d cached i n the summer I c o l l e c t e d 55 t i m e budget s e s s i o n s between June 19, 1994 and August 6, 1994 ( u s i n g 3 f o o d a d d i t i o n groups and 3 c o n t r o l g r o u p s ) . Summer ti m e budgets were s i m i l a r t o the w i n t e r s e s s i o n s , e x c e p t t h a t d i s t i n g u i s h i n g between food g a t h e r i n g and f o o d c a c h i n g was more d i f f i c u l t t h a n i n w i n t e r . These two a c t i v i t i e s were t h e r e f o r e grouped t o g e t h e r as f o r a g i n g . I a l s o r e c o r d e d t h e number of caches made. To a v o i d c o u n t i n g " f a l s e " c a c hes, o n l y cases where th e j a y was seen m a n i p u l a t i n g t h e b o l u s i n i t s beak b e f o r e making c a c h i n g m o t i o n s were co u n t e d . As a r e s u l t , the a c t u a l c a c h i n g r a t e i s p r o b a b l y s l i g h t l y h i g h e r than r e p o r t e d h e r e , but t h e r e i s no r e a s o n t o s u s p e c t t h a t an unequal p r o p o r t i o n o f f o o d and c o n t r o l b i r d s ' caches were r e c o r d e d . S e s s i o n s l e s s t h a n 5 min were d i s c a r d e d , as were -sessions where t h e b i r d - w a s l o s t f o r more t h a n 30 s. Based on w i n t e r r e s u l t s , the maximum s e s s i o n l e n g t h was r e d u c e d t o 20 min. 14 S t a t i s t i c a l Treatment 1. G e n e r a l P r o c e d u r e s and Co n v e n t i o n s : A l l t e s t s a r e o n e - t a i l e d f o r hypotheses two t h r o u g h f o u r where d i r e c t i o n a l p r e d i c t i o n s a re made. The t e s t s f o r h y p o t h e s i s one, a l t h o u g h u s i n g the same d a t a , a r e t w o - t a i l e d because d i f f e r e n c e s between foo d and c o n t r o l b i r d s i n e i t h e r d i r e c t i o n w i l l l e a d t o r e j e c t i o n of the h y p o t h e s i s . The s i g n i f i c a n c e l e v e l f o r a l l t e s t s was s e t a t 0.05. I n t e s t s where d a t a were t r a n s f o r m e d , t h e r e s u l t s a r e p r e s e n t e d as back t r a n s f o r m e d means w i t h 95% c o n f i d e n c e i n t e r v a l s s i n c e t h e s t a n d a r d e r r o r has no meaning i f back t r a n s f o r m e d ( S o k a l and R o h l f 1981, p. 421; Krebs 1989, p. 447) . N o n - t r a n s f o r m e d d a t a a r e p r e s e n t e d as means w i t h s t a n d a r d e r r o r s . 2. C o n d i t i o n Index: Because j a y s l i v e i n s o c i a l groups, t h e r e i s t h e q u e s t i o n whether i n d i v i d u a l c o n d i t i o n measurements a r e ind e p e n d e n t , and t h a t u s i n g them as such would be p s e u d o r e p l i c a t i o n . To t e s t the independence o f i n d i v i d u a l b i r d s , an F - r a t i o was c a l c u l a t e d u s i n g the mean square between s o c i a l groups o v e r the mean square w i t h i n s o c i a l g r oups. I f t h e ANOVA i s s i g n i f i c a n t , t h e n i n d i v i d u a l s a r e not i n d e p e n d e n t and group means s h o u l d be used i n f u t u r e a n a l y s e s . There was no s i g n i f i c a n t e f f e c t f o r e i t h e r f o r f o o d o r c o n t r o l b i r d s i n any of the s e s s i o n s (Table 3 ) . Based on t h i s , t h e e f f e c t o f group membership appears t o be 15 s m a l l , so t h e c o n d i t i o n a n a l y s i s was done u s i n g i n d i v i d u a l s as i n d e p e n d e n t measurements. 3. Time Budget A n a l y s i s : I n w i n t e r , l e g bands were r a r e l y v i s i b l e because t h e b i r d s u s u a l l y k e p t t h e i r l e g s t u c k e d up i n t h e i r f e a t h e r s . Based on t r a p p i n g , o c c a s i o n a l c o n f i r m a t i o n of a b i r d ' s i d e n t i t y , and t h e l o c a t i o n and b e h a v i o u r o f the b i r d s , I am c e r t a i n t h a t I was a b l e t o a c c u r a t e l y i n f e r the group a b i r d b e l o n g e d t o a l t h o u g h i n d i v i d u a l s w i t h i n groups were not d i s t i n g u i s h e d . F o r t h i s reason/ one-way ANOVA's were c a l c u l a t e d u s i n g group means of the a r c s i n e t r a n s f o r m e d p r o p o r t i o n t i m e spent i n each b e h a v i o u r d u r i n g a s e s s i o n . I n t h e summer, t h e p r o p o r t i o n of time spent i n each a c t i v i t y was c a l c u l a t e d , and then a r c s i n e t r a n s f o r m e d . Because i n d i v i d u a l s c o u l d be r e c o g n i z e d i n the summer s e s s i o n s t h i s d a t a was a n a l y z e d u s i n g a n e s t e d ANOVA, where m u l t i p l e o b s e r v a t i o n s o f i n d i v i d u a l s were n e s t e d w i t h i n s o c i a l groups w i t h i n t r e a t m e n t s . The c a c h i n g r a t e s were c a l c u l a t e d f o r each s e s s i o n , square r o o t t r a n s f o r m e d (because t h e r a t e s were based on counts of observed c a c h e s ; S o k a l and R o h l f 1981, p. 421), and th e n a n a l y z e d w i t h a n e s t e d ANOVA. 16 Table 3: Significance tests from ANOVA's comparing the variance in condition between social groups to the variance within social groups for both treatments in each trapping session. Non-significant results indicate that individual birds can be used as the experimental units in the condition analyses. Trapping Session Treatment Test Statistics Spring 93 Food (May 11-June 8) Control F 7 > 8 = 0.680 p=0.688 F M =0.898 p=0.569 Summer 93 Food F 6 > 1 3=1.86 p=0.164 (Aug. 1 4 - A u g 26) Control F 4, 8=0.142 p=0.962 Fall 93 Food (Oct. 16 - Nov. 13) Control Winter 94 Food (Jan. 1 - Mar. 30) Control Spring 94 Food (May 6 - June 9) Control Summer 94 Food (Aug. 8 - Aug 23) Control F 9 > 9=2.62 p=0.084 F 1 > 2=1.02 p=0.419 Fi3,is=1-46 p=0.241 F 2 > 7=0.27 p=0.771 F 7 i 6=0.713 p=0.668 F 9 , 1 0=1.30 p=0.342 F 1 0 , , 2=1.33 p=0.314 F8,io=0.481 p=0.841 17 Results Condition Mean t a r s u s l e n g t h s were s i m i l a r between f o o d a d d i t i o n and c o n t r o l g r i d s ( C o n t r o l mean (SE)=32.0 mm (0 . 2 2 ) ; Food A d d i t i o n mean (SE)= 31.8 mm ( 0 . 1 4 ) ; t - t e s t t=0.60, df=86, p=0.55). J a y s w i t h added f o o d were i n s i g n i f i c a n t l y b e t t e r c o n d i t i o n t h a n c o n t r o l b i r d s i n a l l t r a p p i n g s e s s i o n s ( F i g u r e 2, T a b l e 4 ) . The d i f f e r e n c e s i n c o n d i t i o n i n d i c e s between f o o d a d d i t i o n and c o n t r o l b i r d s r anged between 3% i n the w i n t e r s e s s i o n , and 15% i n t h e f a l l s e s s i o n , a l t h o u g h t h e r e were o n l y f o u r b i r d s i n t h e c o n t r o l group f o r t h a t s e s s i o n . I n b o t h 1993 and 1994 S p r i n g and Summer s e s s i o n s , the d i f f e r e n c e i n c o n d i t i o n was c o n s i s t e n t l y between 5% and 7% h i g h e r on f o o d a d d i t i o n g r i d s . The mean c o n d i t i o n o f c o n t r o l b i r d s f e l l s h a r p l y between the Summer 1993 and F a l l 1993 s e s s i o n s , whereas t h e mean c o n d i t i o n o f b i r d s on f o o d a d d i t i o n s r o s e d u r i n g t h a t p e r i o d . T h i s p a t t e r n was m i r r o r e d by t r e n d s o f i n d i v i d u a l b i r d s . I caught t e n b i r d s i n a l l t h r e e s e s s i o n s from Summer 1993 t h r o u g h W i n t e r 1994. Of t h e s e , t h r e e were c o n t r o l b i r d s . A l l c o n t r o l b i r d s were i n p o o r e r c o n d i t i o n i n the f a l l s e s s i o n t h a n i n e i t h e r t h e p r e v i o u s summer s e s s i o n o r the f o l l o w i n g w i n t e r s e s s i o n ( F i g u r e 3 a ) . A l l t h e b i r d s on f o o d a d d i t i o n g r i d s i n c r e a s e d , f r o m t h e summer s e s s i o n to. t h e f a l l s e s s i o n ( F i g u r e 3 a ) . F o r c o m p a r i s o n w i t h o t h e r work (see D i s c u s s i o n ) , t h e we i g h t t r e n d s o f t h e s e i n d i v i d u a l b i r d s a r e a l s o p r e s e n t e d ( F i g u r e 3 b ) . 18 0.9 0.85 + 0.8 - s - - Control — • - - Food Addition Spring 93 Summer 93 Fall 93 Trapping Session Winter 94 Spring 94 Summer 94 Figure 2. Mean (±SE) condition indices for birds on food addition and control grids in all trapping sessions. Results of significance tests are in Table 4. 19 Table 4: Significance tests of t-tests between mean condition of food and control birds in all trapping sessions. Asterisks indicate significant differences (p<0.05) Session Treatment Mean (SE) t-statistic Spring 93 Food 1.04(0.015) 16 Control 0.975 (0.023) 1 1 2.66* Summer 93 Food 1.03(0.012) 21 Control 0.978(0.021) 13 2.1 3* Fall 93 Food 1.06(0.015) 20 Control 0.918(0.021) 4 4.03* Winter 94 Food Control 1.09(0.010) 1.05(0.013) 30 11 2.66* Spring 94 Food Control 1.06(0.015) 1.00(0.011) 15 20 3.51 Summer 94 Food 1.02(0.012) 23 Control 0.973(0.012) 17 2.45* 20 1 .2 "T 1.15 1.1 "D c .1 1 3 0.95 0.9 0.85 4-0.8 a) 1 1 . 1 Summer 93 Fall 93 Winter 94 Spring 94 Trapping Session 90 85 80 CO 2 70 65 4-60 b) 4-Summer 1993 Fall Winter 1993 1994 Trapping Session Spring 1994 Figure 3. Seasonal trends in in (a) body condition and (b) weights for individual birds on food addition (solid lines) and control (dashed lines) grids. 21 Time Budgets J a y s w i t h a d d e d f o o d m a d e t h r e e t i m e s a s m a n y c a c h e s p e r h o u r i n t h e s u m m e r a s c o n t r o l b i r d s ( N e s t e d A N O V A , F 1 4 = 2 5 . 5 4 , p = 0 . 0 1 ; F i g u r e 4 ) . A n a l y s i s o f s u m m e r t i m e b u d g e t s s h o w e d t h a t t h e r e w a s n o e f f e c t o f f o o d a d d i t i o n o n t i m e s p e n t f o r a g i n g , p e r c h i n g , o r f l y i n g ( F i g u r e 5 ) . I n w i n t e r , b i r d s o n f o o d g r i d s s p e n t l e s s o f t h e i r t i m e f o r a g i n g t h a n c o n t r o l b i r d s ( F i g u r e 6 ) . T h e d i f f e r e n c e w a s s i g n i f i c a n t f o r t h e o n e - t a i l e d t e s t (ANOVA F 1 ( 1 0 = 4 . 8 8 , p = . 0 2 6 , o n e - t a i l e d ) . N o s i g n i f i c a n t d i f f e r e n c e s w e r e f o u n d i n t h e p r o p o r t i o n o f t i m e s p e n t p e r c h i n g (ANOVA F ^ 1 r j = 1 . 9 2 , p = . 2 0 ) o r f l y i n g ( A N O V A F 1 / 1 0 = . l l , p = . 7 5 ) . B i r d s o n f o o d g r i d s w o u l d r e g u l a r l y , t h o u g h i n f r e q u e n t l y , c a c h e t h e r a b b i t c h o w , w h e r e a s n o c o n t r o l b i r d s w e r e e v e r s e e n m a k i n g new c a c h e s ( F i g u r e 6 ) . Discussion Regulation of Winter Fat Reserves E n e r g y s t o r a g e s t r a t e g i e s c a n b e r e p r e s e n t e d o n a c o n t i n u u m r a n g i n g f r o m p u r e f o o d c a c h i n g t o p u r e f a t s t o r a g e . W h i l e f o o d c a c h e s a n d f a t r e s e r v e s a r e a n a l o g o u s , e v o l u t i o n a r y a n d e c o l o g i c a l f a c t o r s m a y m a k e a p a r t i c u l a r s t o r a g e s t r a t e g y o p t i m a l ( M c N a m a r a e t al. 1 9 9 0 ; V a n d e r W a l l . 199 .0 , p . 2 7 ) . . W r a z e n a n d . W r a z e n ( 1 9 8 2 ; s e e a l s o P a n u s k a 1 9 5 9 ) f o u n d t h a t i n d i v i d u a l e a s t e r n c h i p m u n k s (Tamias s t r i a t u s ) c o u l d e i t h e r g a i n n o m a s s b e f o r e w i n t e r a n d s u b s i s t o n s t o r e d f o o d , o r g a i n m a s s a t t h e o n s e t o f w i n t e r , 22 18 T 16 14 12 + 10 8 6 4 2 4-0 Control Food Figure 4. Summer caching rates of food addition and control birds. Means, shown with 95% confidence intervals, are significantly different (Nested ANOVA F 1 4 =25.54, p=0.01). 23 1 0.9 + 0.8 . °-7 E H- 0.6 § 0.5 g. 0.4 o 0.3 0.2 0.1 0 II 1| Foraging Q Control ^ Food Addition Perching Activity Flying Figure 5. Proportion of time spent in various activities during summer. Means are shown with 9 5 % confidence intervals. Two-tailed p-values of treatment effects from nested ANOVA 1 s are as follows: Foraging p=0.086; Perching p=0.1 2; Flying p=0.36. 24 0.9 0.8 0.7 «j 0.6 *&• 0.5 c g t 0.4 o CL O rfc 0.3 0.2 0.1 0 1 i Foraging Perching • Control ^ Food Addition 4-Flying Caching Activity Figure 6. Proportion of time spent in various activities during winter. Means are shown with 9 5 % confidence intervals. 25 b e c o m e s e m i t o r p i d , a n d r e l y l e s s o n s t o r e d f o o d . S i m i l a r l y , a d u l t b e a v e r s {Castor canadensis) r e l y o n f a t s t o r a g e f o r o v e r w i n t e r s u r v i v a l ( N o v a k o w s k i 1 9 6 7 , A l e k s i u k 1 9 7 0 ) w h i l e t h e k i t s a n d y e a r l i n g s r e l y m o r e o n c a c h e d f o o d i n o r d e r t o g a i n w e i g h t o v e r t h e w i n t e r ( N o v a k o w s k i 1 9 6 7 ) . I n b i r d s , R o g e r s (19 87) f o u n d t h e a m o u n t o f f a t r e s e r v e s w a s i n v e r s e l y r e l a t e d t o t h e r e l i a b i l i t y o f f o o d s u p p l y . N i l s s o n e t a l . ( 1 9 9 3 ) f o u n d t h a t n u t h a t c h e s r e l i e d l e s s o n c a c h e s i n g o o d w e a t h e r w h e n o t h e r s o u r c e s o f f o o d w e r e a v a i l a b l e . T h e y a l s o f o u n d t h e t h a t n u m b e r o f c a c h e s c o n s u m e d w a s i n v e r s e l y p r o p o r t i o n a l t o a m b i e n t t e m p e r a t u r e . E v a n s ( 1 9 6 9 ) f o u n d t h a t f a t r e s e r v e s o f Y e l l o w B u n t i n g s (Emberiza c i t r i n e l l a ) w e r e n e g a t i v e l y c o r r e l a t e d w i t h t h e l o n g t e r m m i n i m u m t e m p e r a t u r e f o r t h e d a y o f c a p t u r e . A l t h o u g h t h e r e i s e v i d e n c e o f s o m e s y s t e m o f b e n e f i t s a n d c o s t s t o s t o r i n g e n e r g y a s e i t h e r f a t o r c a c h e s ( M c N a m a r a e t al. 1 9 9 0 , V a n d e r W a l l 1 9 9 0 p . 2 6 , L u c a s 1 9 9 4 ) , f e w s t u d i e s h a v e t e s t e d t h e n a t u r e o f t h e s e t r a d e o f f s . W a i t e ( 1 9 9 2 ) s t u d i e d g r e y j a y s i n c e n t r a l A l a s k a a n d f o u n d e v i d e n c e t h a t j a y s i n c r e a s e t h e i r f a t r e s e r v e s a s m i d w i n t e r a p p r o a c h e s . H e s u g g e s t e d t h a t t h i s i s e v i d e n c e t h a t t h e b i r d s a r e r e g u l a t i n g b o d y m a s s i n a w a y t h a t b a l a n c e s h i g h e r p r e d a t i o n r i s k a n d l o w e r r i s k o f s t a r v a t i o n . S i m i l a r t r e n d s i n o t h e r s p e c i e s ( R o g e r s 1 9 8 7 , R o g e r s a n d S m i t h 1 9 9 3 , B l e m 1 9 7 6 , B l e m a n d P a g e l s 1 9 8 4 ) h a v e l i k e w i s e b e e n a s c r i b e d t o a t r a d e o f f b e t w e e n f a t r e s e r v e s a n d p r e d a t i o n r i s k ( B l e m 1 9 7 5 , 2 6 Lima 1986, McNamara and Houston 1990) o r i n c r e a s e d m e t a b o l i c expense (Blem 1975, 1976). W h i l e Rogers (1987) and Waite (1992) p r o v i d e s u p p o r t i v e e v i d e n c e f o r t h e p r e d a t i o n r i s k h y p o t h e s i s , e x p e r i m e n t a l work i s l a c k i n g . Moreover, a l t e r n a t i v e h y p otheses o f what l i m i t s o v e r w i n t e r w e i g h t s (or even i f t h e y a r e l i m i t e d a t a l l ) have not been t e s t e d . T h i s s t u d y was a b l e t o t e s t t h e p r e d i c t i o n s of s e v e r a l hypotheses on t h e f a c t o r s t h a t may l i m i t w i n t e r body mass i n grey j a y s . My r e s u l t s s u p p o r t h y p o t h e s i s f o u r , t h a t t h e o v e r w i n t e r w e i g h t s o f the j a y s I s t u d i e d a re l i m i t e d b o t h by c o s t s a s s o c i a t e d w i t h i n c r e a s i n g weight, and by a s h o r t a g e o f f o o d (Table 5 ) . Food l i m i t a t i o n f o r the g r e y j a y s appears t o have i t s r o o t i n t h e summer, s i n c e b i r d s on f o o d g r i d s d i d no t spend a s i g n i f i c a n t l y l a r g e r p r o p o r t i o n o f time f o r a g i n g b u t made t h r e e t i m e s as many caches. T h i s s u g g e s t s t h a t c o n t r o l j a y s cache as much as they can t h r o u g h t h e summer, but b i r d s w i t h added f o o d cache more because o f t h e g r e a t e r abundance o f f o o d i n t h e i r t e r r i t o r i e s . T h i s r e s u l t i s c o n s i s t e n t w i t h Waite and Ydenberg's (1994) f i n d i n g t h a t j a y s maximize the r a t e a t w h i c h t h e y s t o r e r e c o v e r a b l e energy. Rate m a x i m i z a t i o n i s e x p e c t e d when a n i m a l s a r e t i m e c o n s t r a i n e d , r a t h e r t h a n c o n s t r a i n e d by an e n e r g y - e x p e n d i t u r e c e i l i n g (Ydenberg e t a l . 1994) . I t app e a r s t h a t c o n t r o l b i r d s a r e l i m i t e d by t h e i r c a p a c i t y t o cache r a t h e r t h a n by t h e a v a i l a b i l i t y o f f o o d . B i r d s w i t h 27 Table 5. Agreement of results with predictions of the four hypotheses. An asterisk by the prediction indicates the results met the prediction. No asterisk indicates that the data did not fit the prediction statistically. Where directional predictions were made, tests were one-tailed. Winter Summer Hypothesis Condition Time Spent Foraging Amount Cached 1: Not Limited =* 2: Food Limited +* + +* 3: Weight Limited +* -* 4: Food and Weight +* -* +* Limited 28 added f o o d , however, a r e a b l e t o cache a t a h i g h e r r a t e because o f t h e g r e a t e r f o o d d e n s i t y . I n t e r r i t o r i e s w i t h added f o o d , t h e s h o r t a g e of f o o d was f u r t h e r r e d u c e d by r e g u l a r w i n t e r c a c h i n g . C o n t r o l b i r d s may g e t t h e o p p o r t u n i t y t o cache i n w i n t e r when c a r r i o n i s f o u n d , bu t i n t h i s s t u d y no such events were o b s e r v e d ( F i g u r e 6 ) . A l s o i n w i n t e r , b i r d s on f o o d a d d i t i o n g r i d s were a b l e t o r e duce th e amount of time spent f o r a g i n g f o r f o o d . T h i s c o u l d r e f l e c t b o t h the i n c r e a s e d d e n s i t y of caches f r o m t h e summer, and t h e a v a i l a b i l i t y o f r a b b i t chow on t h e g r o u n d . T h i s s t u d y was n o t a b l e t o d e s c r i b e t h e n a t u r e of t h e f o o d s h o r t a g e . The v a l u e o f added f o o d c o u l d be due t o e i t h e r an a b s o l u t e s c a r c i t y o f f o o d o r a s t r o n g i n c e n t i v e t o k e e p i n g as many caches as p o s s i b l e u n t i l t h e end of w i n t e r . Such i n c e n t i v e s may i n c l u d e r e p r o d u c t i v e b e n e f i t s (Jansson et al. 1981) o r t h e u n c e r t a i n l e n g t h of w i n t e r . The a l l o c a t i o n o f caches t o p r e s e n t and f u t u r e needs, and t h e f i t n e s s consequences o f such d e c i s i o n s a r e i n t e r e s t i n g p roblems f o r m o d e l l i n g and f i e l d e x p e r i m e n t s , and w h i c h have not been e x p l o r e d i n d e t a i l t o my knowledge (but see N i l s s o n et al. 1993) . The t r e n d i n c o n d i t i o n i n d i c e s shows t h a t b i r d s on f o o d a d d i t i o n g r i d s were i n b e t t e r c o n d i t i o n i n a l l t r a p p i n g s e s s i o n s . The w i n t e r t i m e budgets show t h a t c o n t r o l b i r d s spent more t i m e f o r a g i n g t h a n b i r d s w i t h added f o o d . H i g h e r 29 c o n d i t i o n and l o w e r f o r a g i n g t i m e s i n w i n t e r were p r e d i c t e d i n h y p o t h e s e s t h r e e and f o u r and a r e e v i d e n c e of weight-r e l a t e d l i m i t s t o w i n t e r f a t r e s e r v e s . To e x p l a i n why many temperate b i r d s p e c i e s do not m a i n t a i n maximum f a t r e s e r v e s , Lima (1986) and McNamara and Houston (1990) have p r o p o s e d models where t h e s u r v i v a l v a l u e of f a t r e s e r v e s ( i . e . t h e a b i l i t y t o s u r v i v e p e r i o d s of f o o d i n a c c e s s i b i l i t y ) i s b a l a n c e d by i n c r e a s i n g r i s k o f p r e d a t i o n because o f i n c r e a s e d f o r a g i n g t i m e o r by. d e c r e a s e d a g i l i t y i n e s c a p i n g p r e d a t o r s . Three o f f o u r p r e d i c t i o n s made by Lima's model have been c o n f i r m e d : Rogers (1987) found t h a t f a t r e s e r v e s i n c r e a s e as t h e f r e q u e n c y o r ha r s h n e s s of f o o d s h o r t a g e i n c r e a s e s ; W a i t e (1992) found t h a t f a t r e s e r v e s i n c r e a s e w i t h d e c r e a s i n g t e m p e r a t u r e s ; and t h i s s t u d y shows t h a t w i n t e r w e i g h t s i n c r e a s e w i t h i n c r e a s i n g f o o d r e s o u r c e s (Table 4) . Rogers' (1987) and W a i t e ' s (1992) r e s u l t s a r e a l s o c o n s i s t e n t w i t h McNamara and Houston's model, but the i n c r e a s e i n body c o n d i t i o n i n r e s p o n s e t o i n c r e a s e d f o o d abundance found i n t h i s s t u d y a r e o p p o s i t e t o the p r e d i c t i o n s i n McNamara and Houston (1990). As the a u t h o r s o f t h i s model p o i n t o u t , t h e d i s a g r e e m e n t between t h e i r model and Lima's (1986) i s due t o d i f f e r e n t assumptions about t h e n a t u r e o f e n v i r o n m e n t a l v a r i a b i l i t y . I n Lima's model a day was e i t h e r good o r bad, whereas i n McNamara and Houston's model f o r a g i n g s u c c e s s was a c o n t i n u o u s , s t o c h a s t i c f u n c t i o n . Under t h e c o n d i t i o n s t h a t each model 30 was r u n , t h i s d i f f e r e n c e l e d t o o p p o s i t e p r e d i c t i o n s . Because g r e y j a y s a r e r e t r i e v i n g caches from t r e e b r a n c h e s , f o r a g i n g s u c c e s s i s l i k e l y t o dependable (Rogers 1987) exc e p t on days w i t h extreme weather, and more a c c u r a t e l y r e p r e s e n t e d by Lima's model. F o r t h i s r e a s o n , I based my p r e d i c t i o n s on Lima's model, r a t h e r t h a n McNamara and Houston's model. A l t h o u g h Lima (1986) argues a g a i n s t t h e r e l e v a n c e o f c o s t s t o added w e i g h t o t h e r t h a n p r e d a t i o n r i s k , and McNamara and Houston (1990) do n o t ad d r e s s a l t e r n a t e c o s t s , i n t h i s s t u d y I have not e x c l u d e d t h e p o s s i b i l i t y t h a t t h e w e i g h t - r e l a t e d c o s t s t o i n c r e a s i n g f a t r e s e r v e s a r e due t o t r a n s p o r t o r maintenance c o s t s . Rogers (1987) a l s o r e c o g n i z e s t h a t h i s r e s u l t s a r e e q u a l l y c o n s i s t e n t w i t h o t h e r models t h a t i n v o l v e c o s t s t o w i n t e r f a t t e n i n g . M e a s u r i n g t h e r e l a t i v e i m p o r t a n c e o f p r e d a t i o n r i s k and t r a n s p o r t o r maintenance c o s t s w i l l be a u s e f u l n e x t s t e p i n u n d e r s t a n d i n g body mass r e g u l a t i o n i n w i n t e r i n g b i r d s . Trends i n Body Condition A l t h o u g h t h e F a l l 1993 t r a p p i n g s e s s i o n i n c l u d e d o n l y f o u r c o n t r o l b i r d s , t h e t r e n d s o f i n d i v i d u a l b i r d s matched the p o p u l a t i o n c o n d i t i o n t r e n d s i m p l y i n g r e l i a b i l i t y i n t h e p a t t e r n ( F i g u r e s 2, 3a, and 3 b ) . W h i l e b i r d s w i t h a c c e s s t o fo o d a d d i t i o n were a b l e t o m a i n t a i n o r even i n c r e a s e t h e i r w e i g h t s between t h e Summer and F a l l 1993 t r a p p i n g s e s s i o n , t h e c o n t r o l b i r d s ' w e i g h t s d e c l i n e d . D e s p i t e t h i s d e c l i n e , 31 c o n t r o l b i r d s w e r e a b l e t o r a i s e t h e i r w e i g h t s i n t h e W i n t e r s e s s i o n s u c h t h a t t h e w e i g h t s w e r e c l o s e r t o f o o d a d d i t i o n b i r d s t h a n i n a n y o t h e r t r a p p i n g s e s s i o n . B e t w e e n t h e s e t w o s e s s i o n s c a c h e s w o u l d l i k e l y b e t h e o n l y f o o d a v a i l a b l e t o c o n t r o l b i r d s . I n g e n e r a l , t h e r e a r e f e w s t u d i e s o f b i r d s t h a t h a v e r e c o r d e d s e r i e s o f w e i g h t s ; m o r e i n f o r m a t i o n i s a v a i l a b l e f o r m a m m a l s . F a t t e n i n g a t t h e o n s e t o f w i n t e r a n d g r a d u a l l y d e c l i n i n g u n t i l s p r i n g h a s b e e n o b s e r v e d i n c h i p m u n k s (Tamias striatus; P a n u s k a 1 9 5 9 ) a n d i n b e a v e r (Castor canadensis; A l e k s i u k 1 9 7 0 ) . My r e s u l t s a n d t h o s e i n W a i t e ( 1 9 9 2 ) s u g g e s t t h a t g r e y j a y s , l i k e o t h e r s m a l l b i r d s ( B l e m 1 9 7 6 , B l e m a n d P a g e l s 1 9 8 4 ) , i n c r e a s e m a s s i n m i d - w i n t e r r a t h e r t h a n b u i l d i n g u p r e s e r v e s f o r t h e o n s e t o f w i n t e r . F u r t h e r m o r e , t h e d i f f e r e n c e s I f o u n d i n w e i g h t a n d c o n d i t i o n t r e n d s b e t w e e n b i r d s w i t h a n d w i t h o u t a c c e s s t o f o o d a d d i t i o n s u g g e s t t h a t f o o d a d d i t i o n m a y a l t e r t h e s t r a t e g y u s e d . T h e r e a r e c u r r e n t l y n o m o d e l s o f o v e r w i n t e r c a c h e u s e t h a t h a v e c o n s i d e r e d a b i r d s u b s i s t i n g e n t i r e l y o f f c a c h e s a s g r e y j a y s s e e m t o d o , b u t t h e r e a r e a t l e a s t t w o m o d e l s t h a t c o n s i d e r c a c h i n g a n d c a c h e u s e . T h e a c o r n w o o d p e c k e r (Melanerpes formicivorus) c a c h e s a c o r n s f o r u s e i n t h e w i n t e r w h e n o t h e r f o o d s o u r c e s a r e l e s s a b u n d a n t . I n a m o d e l o f c a c h i n g i n t h i s w o o d p e c k e r , H i t c h c o c k a n d H o u s t o n ( 1 9 9 4 ) p r e d i c t a c o n s t a n t r a t e o f c a c h e u s e r e s u l t i n g i n s t a b l e b o d y w e i g h t s ( F i g u r e 7 a ) . I n a m o d e l o f c a c h i n g 32 a) CO 1— cu cr TOO Time (days) CD CD C UJ b) Dawn Dusk Figure 7. Predicted weight trends according to two models of cache use during periods of food scarcity: a) a model of a small b i r d i n winter from McNamara et a l (1990) ; b) a model of acorn woodpecker cache use from Hitchcock and Houston (1994). S o l i d l i n e s represent body reserves, dashed l i n e s are caches. 33 b e h a v i o u r , McNamara e t a l (1990) c o n s i d e r a b i r d t h a t f o r a g e s and makes s h o r t - t e r m caches d u r i n g t h e n o n - b r e e d i n g s e a s o n . The p r e d i c t e d d a i l y p a t t e r n of f a t s t o r a g e ( F i g u r e 7b) r e s e m b l e s t h e s e a s o n a l p a t t e r n of c o n t r o l j a y s from Summer 1993 t h r o u g h W i n t e r 1994. A m o d i f i c a t i o n o f one o f t h e s e models c o u l d be used t o model o v e r w i n t e r cache use by g r e y j a y s . By i n c o r p o r a t i n g b o t h the c o n t r o l c o n d i t i o n s and t h e f o o d a d d i t i o n i n t o such a model, a p r e l i m i n a r y t e s t o f t h e model e x i s t s . The weight s e r i e s p r e d i c t e d by the model under c o n t r o l and su p p l e m e n t a l f o o d c o n d i t i o n s s h o u l d r e s e m b l e t h e p a t t e r n s I o b s e r v e d f o r t r e a t m e n t and c o n t r o l b i r d s ( F i g u r e 2) . Perhaps more i m p o r t a n t t h a n a n o t h e r model of c a c h i n g , i s t h a t experiments c a t c h up w i t h t h e o r y and b e g i n t o t e s t t h e models and assumptions o f how a n i m a l s use t h e i r c a ches d u r i n g p e r i o d s of f o o d s h o r t a g e . One way t h i s c o u l d be done i s by m o n i t o r i n g the caches of i n d i v i d u a l s w i t h and w i t h o u t s u p p l e m e n t a l f o o d . N i l s s o n e t al. (1993) have o b s e r v e d t h a t European nuthatches ( S i t t a europaea) u s e d caches o f supplemented f o o d more o f t e n i n p o o r weather, b u t t h e y d i d n o t c o l l e c t d a t a on cache use by c o n t r o l b i r d s . T h i s s t u d y has o b s e r v e d v e r y d i f f e r e n t w e i g h t t r e n d s between f e d b i r d s and c o n t r o l s , but t h i s f a l l s f a r s h o r t of u n d e r s t a n d i n g how d i f f e r e n c e s i n cache use has r e s u l t e d i n t h e s e p a t t e r n s . T h i s i s l i k e l y a f r u i t f u l a r e a f o r f u t u r e r e s e a r c h . 34 C h a p t e r 3: E f f e c t s o f F ood A d d i t i o n on Grey J a y T e r r i t o r i e s Introduction I n t e r e s t i n t e r r i t o r i a l i t y has l e d t o a g r e a t d e a l o f r e s e a r c h i n t o t h e economics o f m a i n t a i n i n g an e x c l u s i v e t e r r i t o r y and t h e o p t i m a l s i z e o f such a t e r r i t o r y (e.g. Brown 1964, D a v i e s 1978, McNab 1963, E b e r s o l e 1980, H i x o n 1980, Schoener 1983). Theory s u g g e s t s t h a t t e r r i t o r y s i z e o f a n i m a l s h o l d i n g f o o d t e r r i t o r i e s (sensu Hinde 1956) i s r e g u l a t e d by f o o d a v a i l a b i l i t y and i n t r u d e r p r e s s u r e ( E b e r s o l e 1980, H i x o n 1980, Schoener 1983). Food a d d i t i o n s have been the most common e x p e r i m e n t a l m a n i p u l a t i o n t o t e s t t h i s model of t e r r i t o r i a l i t y ( f o r a r e v i e w see B o u t i n 1990), a l t h o u g h t h e r e have been a few a t t e m p t s t o l o o k more e x p l i c i t l y a t the e f f e c t of i n t r u d e r p r e s s u r e (Mares et al. 1982, P r i c e e t al. 1986). R e s u l t s f r om f o o d a d d i t i o n s t u d i e s have p r o v e d t o be v a r i a b l e d e p e n d i n g on t h e s p e c i e s s t u d i e d . Enoksson and N i l s s o n (1983) f o u n d t h a t t h e European n u t h a t c h (Sitta europaea) d e c r e a s e d t e r r i t o r y s i z e i n r e sponse t o f o o d a d d i t i o n , w h i l e t h e r u f o u s - s i d e d towhee (Pipilo erythrophthalmus) d i d not change home range s i z e ( F r a n z b l a u and C o l l i n s 1980), and t h e r e e f f i s h {Eupomacentrus leucostictus) i n c r e a s e d . t e r r i t o r y s i z e ( E b e r s o l e 1 9 8 0). • • Schoener (1983) examined a number of models of t e r r i t o r y s i z e and, by c l a r i f y i n g t h e u n d e r l y i n g a s s u m p t i o n s , was a b l e t o u n i f y them i n t o a comprehensive 35 m o d e l . B y d e f i n i n g t h r e e t y p e s o f t e r r i t o r y h o l d e r s ( t i m e m i n i m i z e r s , a n d t w o t y p e s o f e n e r g y m a x i m i z e r s ) a n d f o u r t y p e s o f e n v i r o n m e n t a l v a r i a t i o n ( i . e . . t h e r e l a t i o n s h i p b e t w e e n f o o d a v a i l a b i l i t y a n d i n t r u d e r p r e s s u r e ) S c h o e n e r ' s m o d e l s h o w s how t h e s e f a c t o r s d e t e r m i n e t h e o p t i m a l t e r r i t o r y s i z e . A m o r e d e t a i l e d d e s c r i p t i o n o f t h e m o d e l a s i t a p p l i e s t o t h i s s t u d y i s g i v e n i n t h e d i s c u s s i o n b e l o w . F e w e r s t u d i e s h a v e e x a m i n e d t h e e f f e c t o f f o o d a d d i t i o n o n o v e r l a p b e t w e e n t e r r i t o r i e s ( s e e B o u t i n 1 9 9 0 ) . O v e r l a p s h o u l d i n c r e a s e a s t h e n e t b e n e f i t o f d e f e n d i n g a n e x c l u s i v e a r e a d e c r e a s e s . T e r r i t o r i a l e c o n o m i c s p r e d i c t s t h a t a n a n i m a l s h o u l d d e f e n d a n e x c l u s i v e f o r a g i n g a r e a w h e n t h e b e n e f i t s o f e x c l u s i v e n e s s o u t w e i g h t h e c o s t s o f d e f e n s e ( B r o w n 1 9 6 4 ) . W h e n f o o d i s s c a r c e , t h e e n e r g y s p e n t g u a r d i n g i t e x c e e d s t h e e n e r g y g a i n e d f r o m t h e t e r r i t o r y , a n d w h e n c o m p e t i t i o n f o r f o o d i s r e d u c e d , t h e b e n e f i t s o f e x c l u s i v e a c c e s s a r e n e g a t e d . C a r p e n t e r a n d M a c M i l l e n ( 1 9 7 6 ) f o u n d t h a t a s p e c i e s o f H a w a i i a n h o n e y c r e e p e r s ( V e s t i a r i a coccinea) a l t e r e d t h e i r l e v e l o f d e f e n s e s u c h t h a t i n t e r m e d i a t e d e n s i t i e s o f f l o w e r s w e r e d e f e n d e d m o r e t h a n l o w o r h i g h d e n s i t i e s . O t h e r s t u d i e s s h o w t h a t s ome a n i m a l s w i l l m a i n t a i n t e r r i t o r i e s o r n o t d e p e n d i n g o n t h e l e v e l o f f o o d r e s o u r c e s ( s e e r e v i e w i n D a v i e s 1 9 7 8 ) . T h i s s t u d y u s e d t h e l o n g t e r m , y e a r - r o u n d f o o d a d d i t i o n o f t h e K l u a n e B o r e a l F o r e s t E c o s y s t e m P r o j e c t ( K r e b s e t al. 1 9 9 2 ) t o e x a m i n e t h e e f f e c t o f f o o d a v a i l a b i l i t y o n t h e t e r r i t o r y c h a r a c t e r i s t i c s o f g r e y j a y s {Perisoreus 3 6 canadensis) . My o b j e c t i v e s were t o examine t h e e f f e c t s of f o o d a d d i t i o n on t e r r i t o r y s i z e , o v e r l a p , and s p a t i a l arrangement. Methods Study S i t e The s t u d y was c o n d u c t e d near Kluane Lake i n southwest Yukon (61°N, 138°W) May t h r o u g h August, 1993 and 1994. As p a r t o f t h e Kl u a n e B o r e a l Ecosystem P r o j e c t , t h r e e 36-ha g r i d s have had comm e r c i a l r a b b i t chow (16% cru d e p r o t e i n , 2% f a t , 18% f i b r e ) added y e a r - r o u n d s i n c e 1987. Among t h e s p e c i e s t h a t use t h e f o o d a r e g r e y j a y s - y e a r - r o u n d r e s i d e n t s o f t h e b o r e a l f o r e s t , w h i c h m a i n t a i n a l l - p u r p o s e group t e r r i t o r i e s ( R u t t e r 1969). T e r r i t o r y Mapping To map t e r r i t o r i e s my a s s i s t a n t and I f o u n d groups o f c o l o u r banded j a y s (see c h a p t e r 2) on f o o d a d d i t i o n g r i d s and on o t h e r g r i d s o f s i m i l a r h a b i t a t b u t w i t h o u t a c c e s s t o t h e f o o d a d d i t i o n ( F i g u r e 1 ) . We f o l l o w e d t h e b i r d s between 0600h and 1400h from June 9, 1993 t o August 13, 1993, and from J u l y 10, 1994 t o August 15, 1994. We r e c o r d e d t h e i r l o c a t i o n t o t h e n e a r e s t 15 m on g r i d s w i t h s t a k e s spaced a t 30 m. When t h e b i r d s t r a v e l l e d ' o f f t h e g r i d we used 1:10 000 a e r i a l p h o t o s o f t h e a r e a t o e s t i m a t e t h e i r l o c a t i o n s u s i n g i d e n t i f i a b l e landmarks. A c c u r a c y and p r e c i s i o n o f l o c a t i o n s o f f t h e g r i d v a r i e d d e p ending on t h e 37 n a t u r e o f the landmarks i n t h e a r e a . A t b e s t , we were a b l e t o l o c a t e the e x a c t t r e e on t h e photo w h i l e a t w o r s t we c o u l d o n l y e s t i m a t e t h e l o c a t i o n t o t h e n e a r e s t 30 m. I n t h e t e r r i t o r y maps we i n c l u d e d l o c a t i o n s where b i r d s made r e p e a t e d v o c a l i z a t i o n s , f o r a g e d , cached f o o d , p e r c h e d f o r more th a n one minute, o r i n t e r a c t e d w i t h n e i g h b o u r s a t a b o r d e r . I t soon became c l e a r t h a t i n t r u s i o n s onto n e i g h b o u r i n g t e r r i t o r i e s were o c c u r r i n g , but t h e s e were n o t seen t o extended more t h a n 100 m i n t o t h e n e i g h b o u r i n g t e r r i t o r y . A t t h e s e t i m e s t h e b e h a v i o u r o f i n t r u d i n g b i r d s changed from b e i n g h i g h l y v i s i b l e and a u d i b l e , t o b e i n g v e r y q u i e t and i n c o n s p i c u o u s . The f o c a l b i r d s were sometimes chased away by o t h e r b i r d s ( e i t h e r t h e owners o r assumed owners of the n e i g h b o u r i n g t e r r i t o r y ) . We n o t e d t h i s b e h a v i o u r and d i d n o t i n c l u d e t h e s e a r e a s i n the t e r r i t o r y maps. Ar e a s where two groups t o l e r a t e d each o t h e r ( i . e . were p l a i n l y v i s i b l e t o one a n o t h e r , b u t were not a g g r e s s i v e ) were r e c o r d e d as a r e a s o f o v e r l a p ( F i g u r e 8 ) . My a s s i s t a n t and I mapped a t o t a l o f n i n e t e r r i t o r i e s o v e r t h e two summers ( F i g u r e 1 ) . Mapping was l a b o u r i n t e n s i v e ; i t r e q u i r e d about one month, mapping t h r e e t e r r i t o r i e s a t a t i m e , t o be s a t i s f i e d t h a t v e r y l i t t l e a r e a w a s . m i s s i n g o r w r o n g l y i n c l u d e d . T h i s i s l a r g e l y a f u n c t i o n o f t e r r i t o r y s i z e : t h e b i g g e r t h e t e r r i t o r y , t h e more ti m e t h e b i r d s spend i n t h e i n t e r i o r o f t h e t e r r i t o r y , and t h e l o n g e r t h e p e r i m e t e r t h a t i s needed t o d e f i n e t h e t e r r i t o r y 38 Control Food Addition • • **\ y * \ * . \^/^*^ *\ * * * — • • • T \ . " * * " A • • « • , - \ • * • * - • \ x • • * « » • . i V X « • • • / * . • .1 * ^ N t - • * « • , \ 60 m . , mJ Figure 8 . Examples of t e r r i t o r y maps for control (a) and food addition (b) groups. Each point represents at lea s t one observation of one or more birds from the group. Locations where birds were judged to be intruding on other t e r r i t o r i e s were not included i n the maps. Dots represent members of the owner group, crosses represent neighbouring groups. Areas where there are crosses inside the t e r r i t o r y borders drawn were used to calculate overlap. 39 boundary. L i m i t e d v i s i b i l i t y i n many of the h a b i t a t t y p e s a l s o c o n t r i b u t e d t o t h e d i f f i c u l t y of t e r r i t o r y mapping. From t r a p p i n g r e c o r d s and o b s e r v a t i o n s , my a s s i s t a n t and I n o t e d t h e a p p r o x i m a t e b o r d e r s of o t h e r t e r r i t o r i e s on the g r i d s where we worked. Most o f the groups t h a t i n c l u d e d p a r t o f a g r i d i n t h e i r t e r r i t o r i e s had a t l e a s t some members c o l o u r banded, so we a r e c o n f i d e n t t h a t we were a b l e t o a c c u r a t e l y d i s t i n g u i s h between groups. T h i s a l l o w e d us t o d e t e r m i n e t h e number of t e r r i t o r i e s t h a t were e n t i r e l y o r p a r t i a l l y w i t h i n t h e b o u n d a r i e s o f t h r e e f o o d a d d i t i o n and t h r e e c o n t r o l g r i d s . Results T e r r i t o r y maps c o n s i s t e d o f an average o f o v e r 220 p o i n t s as w e l l as p o i n t s from n e i g h b o u r i n g groups t h a t h e l p e d t o d e f i n e b o r d e r s ( F i g u r e 8 ) . These p o i n t s were o f t e n c o n c e n t r a t e d n e a r t h e edges of the t e r r i t o r i e s s i n c e we were i n t e r e s t e d i n d e f i n i n g t h e t e r r i t o r y b o r d e r s r a t h e r t h a n r e c o r d i n g p r o p o r t i o n a l usage o f t h e a r e a . The mean s i z e o f t e r r i t o r i e s on f o o d a d d i t i o n g r i d s was a p p r o x i m a t e l y 30% s m a l l e r t h a n c o n t r o l t e r r i t o r i e s . The mean s i z e o f f i v e c o n t r o l t e r r i t o r i e s was 23.2 ha (SE = 1.4) compared t o a mean a r e a o f 15.8 ha (SE = 1.9) f o r the f o u r t e r r i t o r i e s on f o o d a d d i t i o n g r i d s ( t - t e s t , t=3.19, p=0.02). Based on t h e s e r e s u l t s , i f t h e number o f t e r r i t o r i e s p e r g r i d i s s t r i c t l y p r o p o r t i o n a l t o mean t e r r i t o r y s i z e (which assumes t h e r e i s no u n o c c u p i e d s p a c e ) , t h e r e s h o u l d be more t e r r i t o r i e s on 40 f o o d a d d i t i o n g r i d s . On t h e t h r e e c o n t r o l g r i d s t h e r e were an average of 2.67 t e r r i t o r i e s p e r g r i d , and on the t h r e e f o o d a d d i t i o n g r i d s s t u d i e d , t h e r e was an average of 4.67 t e r r i t o r i e s ( F i g u r e 9 ) . To d e t e r m i n e whether t h i s i n c r e a s e i s more o r l e s s t h a n e x p e c t e d based o n l y on t e r r i t o r y s i z e , I u s e d a s i m p l e Monte C a r l o s i m u l a t i o n . I n t h e s i m u l a t i o n s , t e r r i t o r i e s were approximated as s q u a r e s w i t h an a r e a e q u a l t o t h e mean t e r r i t o r y s i z e I o b s e r v e d f o r t h e r e s p e c t i v e t r e a t m e n t . A 36-ha g r i d was p l a c e d a t random on t h e l a n d s c a p e of c o n t i g u o u s t e r r i t o r i e s 1000 t i m e s , and t h e mean number of t e r r i t o r i e s p e r g r i d c a l c u l a t e d . There was no s i g n i f i c a n t d i f f e r e n c e between the mean number of t e r r i t o r i e s on c o n t r o l g r i d s and the mean number t h e model p r e d i c t e d ( T able 6 ) . There were 1.7 times more t e r r i t o r i e s on f o o d a d d i t i o n g r i d s t h a n e x p e c t e d based on t h e model (Table 6 ) . Because some of t h e n i n e mapped t e r r i t o r i e s were a d j a c e n t t o each o t h e r , o n l y s i x t e r r i t o r i e s were used i n t h e o v e r l a p a n a l y s i s t o keep each measurement independent. A l l b u t one of t h e s i x t e r r i t o r i e s u s e d i n t h i s a n a l y s i s had a t l e a s t some o v e r l a p w i t h o t h e r t e r r i t o r i e s . The amount of o v e r l a p between t e r r i t o r i e s on c o n t r o l g r i d s was c o n s i s t e n t l y low, w h i l e t h a t on f o o d g r i d s was 4.7 times h i g h e r compared t o c o n t r o l g r i d s . The l a c k o f a s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e ( F i g u r e 10) d e s p i t e t h e magnitude of t h e d i f f e r e n c e i s due t o t h e low power of t h e t e s t ( e s t i m a t e d t o be 0.3). 41 Control Grids Food Addition Grids Figure 9. Diagrams showing approximate boundaries between a l l the t e r r i t o r i e s that had access to three food addition and three control g r i d s . Each square i s 600 m by 600 m, representing the borders of the g r i d . 42 Table 6. Observed and predicted number of territories per 36-ha grid. Predicted numbers are from a Monte Carlo simulation as described in chapter two. The p-values shown are from t-tests between observed and predicted mean number of territories per grid. Observed Model Control mean (SE) 2.67 (0.66) 2.35 (0.047) 3 1000 p>0.5 n Food Addition mean (SE) 4.67 (0.33) 2.77 (0.043) 3 1000 p=0.016 n 43 14 C O N T R O L F O O D Figure 10. Percent overlap between territories with 9 5 % confidence intervals. The data were arcsine transformed; the values presented are back-transformed. T-test: t=2.16, df=5, two-tailed p=0.084. 44 The mean number of a d u l t b i r d s p e r group was i d e n t i c a l f o r f o o d a d d i t i o n and c o n t r o l a r e a s . We were c o n f i d e n t o f our e s t i m a t e s o f t h e number of members, i n each group f o r f i v e c o n t r o l and f i v e f o o d a d d i t i o n groups. Four groups had f o u r members, and one had t h r e e on b o t h supplemented and c o n t r o l g r i d s (mean=3.8, SE=0.2). To be c o n f i d e n t of o u r c o u n t s , a l l o r a l l but one b i r d had t o be c o l o u r banded, and we had t o see them r e g u l a r l y enough t h a t we were a b l e t o v e r i f y o u r c o u n t s . Discussion Me thodo1ogy I n g e n e r a l t h e methods used t o map t e r r i t o r i e s were s u c c e s s f u l , b u t some d i f f i c u l t i e s a r o s e . Because th e t e r r i t o r i e s a r e l a r g e , n e i g h b o u r i n g groups a r e not seen t o g e t h e r f r e q u e n t l y a t t e r r i t o r i a l b o r d e r s . Woolfenden and F i t z p a t r i c k (1984) were a b l e t o use v e r y c o n s p i c u o u s t e r r i t o r i a l d i s p l a y s a t b o r d e r s t o map F l o r i d a s c r u b j a y t e r r i t o r i e s . The s c r u b j a y s they s t u d i e d a r e a l s o s t r o n g l y t e r r i t o r i a l . A l t h o u g h g r e y j a y s do not g e n e r a l l y t o l e r a t e o t h e r j a y s on t h e i r t e r r i t o r i e s , b o r d e r d i s p l a y s a r e not as f r e q u e n t o r as v i s i b l e as d e s c r i b e d i n Woolfenden and F i t z p a t r i c k (1984) . A n o t h e r f a c t o r t h a t makes t e r r i t o r y mapping e a s i e r i s a r e a s - t h a t a r e not used by t h e a n i m a l s . • I n s c r u b j a y s , Woolfenden and F i t z p a t r i c k (1984) found t h a t t h e b i r d s n e v e r u s e d l o w - l y i n g and open a r e a s ; c o n s e q u e n t l y 45 t h e s e a r e a s o f t e n formed a l a r g e p a r t o f the t e r r i t o r i a l b o u n d a r i e s . Grey j a y s use such a d i v e r s i t y o f . h a b i t a t s , t h a t no such n a t u r a l f e a t u r e s were c o n s i s t e n t l y u s e f u l i n mapping t e r r i t o r i e s . The use of r a d i o t e l e m e t r y might be u s e f u l f o r q u i c k l y l o c a t i n g groups, but t r i a n g u l a t i o n i s u n l i k e l y t o be p r e c i s e enough t o map the r e l a t i v e l y complex b o r d e r s of g r e y j a y t e r r i t o r i e s . N o n e t h e l e s s , the use of g r i d s t a k e s and a e r i a l p h o t o s appears t o be a r e l i a b l e method f o r mapping t e r r i t o r i e s . T e r r i t o r y Size To a p p l y Schoener's (1983) model, we f i r s t must be a b l e t o c l a s s i f y t h e t e r r i t o r y h o l d e r as e i t h e r a time m i n i m i z e r , o r an energy m a x i m i z e r (Schoener 1971). A time m i n i m i z e r meets a minimum f e e d i n g r e q u i r e m e n t t o meet i t s needs, whereas an energy m a x i m i z e r t r i e s t o g a i n as much energy as p o s s i b l e . Energy m a x i m i z e r s can be c o n s t r a i n e d by e i t h e r t h e t i m e a v a i l a b l e t o f o r a g e , o r by a p r o c e s s i n g c o n s t r a i n t . Because j a y s s t o r e f o o d i n t h e summer f o r use o v e r w i n t e r and i n t o t h e b r e e d i n g season ( R u t t e r 1969, p e r s o n a l o b s e r v a t i o n ) i t seems l i k e l y t h a t t hey s h o u l d be energy m a x i m i z e r s ( P i a n k a 1976) . Waite and Ydenberg (1994) found t h a t c a c h i n g by j a y s was a c c u r a t e l y modeled as net r a t e m a x i m i z i n g (where t h e r a t i o o f energy s t o r e d , l e s s t h e c o s t o f t r a n s p o r t , t o t i m e spent c a c h i n g i s maximized) . Ydenberg e t a l . (1994) show t h a t t h i s c u r r e n c y i s e x p e c t e d 46 when f o r a g i n g i s l i m i t e d by t i m e . T h i s i s c o n s i s t e n t w i t h g r e y j a y s b e i n g an example o f Schoener's f i r s t t y p e of energy m a x i m i z e r , w h i c h i s c o n s t r a i n e d , by time a v a i l a b l e f o r f o r a g i n g r a t h e r t h a n by a c a p a c i t y c o n s t r a i n t (Schoener 1983) . Moreover, i n summer t h e r e was no s i g n i f i c a n t d i f f e r e n c e i n t h e p r o p o r t i o n o f t i m e spent f o r a g i n g by b i r d s w i t h added f o o d and t h o s e on c o n t r o l a r e a s . As was d i s c u s s e d i n c h a p t e r 2, t h i s s u g g e s t s t h a t b o t h groups of b i r d s were s p e n d i n g as much t i m e as p o s s i b l e i n f o r a g i n g a c t i v i t i e s . T h i s , and t h e f a c t t h a t j a y s a r e f o r a g i n g t o make caches and not j u s t f o r immediate consumption, i s good e v i d e n c e t h a t g r e y j a y s a r e c o n s t r a i n e d by t i m e r a t h e r t h a n p r o c e s s i n g c o n s t r a i n t s i n summer. I n summary, of the t h r e e t y p e s of t e r r i t o r y h o l d e r d e f i n e d by Schoener, g r e y j a y s meet the c r i t e r i a f o r b e i n g c l a s s e d as energy m a x i m i z e r s w i t h time c o n s t r a i n t s . The n e x t c o n s i d e r a t i o n i n a p p l y i n g Schoener's model of o p t i m a l t e r r i t o r y s i z e i s what he d e s c r i b e s as t h e t y p e of e n v i r o n m e n t a l v a r i a t i o n . More e x p l i c i t l y , t h i s r e f e r s t o t h e r e l a t i o n s h i p between f o o d d e n s i t y and i n t r u d e r p r e s s u r e . The f o u r t y p e s a r e as f o l l o w s : 1. a change i n f o o d d e n s i t y does not change i n t r u d e r p r e s s u r e ; . • . • 2. a change i n i n t r u d e r p r e s s u r e independent of changes i n f o o d abundance; 4 7 3. an i n c r e a s e i n f o o d d e n s i t y i n c r e a s e s i n t r u d e r p r e s s u r e ; and 4. an i n c r e a s e i n i n t r u d e r p r e s s u r e d e c r e a s e s f o o d d e n s i t y i n t h e t e r r i t o r y . F o r g r e y j a y s , i n t r u d e r s can have two e f f e c t s : i n t r u d e r s can s t e a l caches f o o d r e s o u r c e s , o r t h e y can s t e a l uncached f o o d . There i s a l s o t h e c o s t o f e x p e l l i n g i n t r u d e r s . A l t h o u g h t h e r e may be arguments f o r not s i m p l y d i s m i s s i n g t h i s l a t t e r c o s t , I b e l i e v e t h a t t h e energy spent i n f l i g h t d u r i n g chases and v o c a l i z a t i o n s i s m i n i m a l r e l a t i v e t o the energy l o s t when f o o d o r caches a r e s t o l e n ( p e r s o n a l o b s e r v a t i o n s ) . A l t h o u g h t h i s a s s u m p t i o n seems r e a s o n a b l e , i t remains t o be c o n f i r m e d . F i r s t I c o n s i d e r t h e r e l a t i o n s h i p between cache d e n s i t y and i n t r u d e r p r e s s u r e . Waite (1988) found t h e r a t e of l o s s of s i m u l a t e d g r e y j a y caches was d e n s i t y dependent. D e n s i t y dependent s u r v i v a l o f caches seems t o be common i n s c a t t e r h o a r d e r s : S t a p a n i a n and S m i t h (1984) found such a r e l a t i o n s h i p f o r s i m u l a t e d f o x s q u i r r e l {Sciurus niger) caches, and C l a r k s o n e t a l . (1986) found t h a t the d i s t r i b u t i o n of magpie (Pica pica) caches was c o n s i s t e n t w i t h d e n s i t y dependent cache l o s s . Thus, f o o d a d d i t i o n , by i n c r e a s i n g the number o f caches made ( c h a p t e r 2, F i g u r e 4) s h o u l d r e s u l t i n p r o p o r t i o n a l l y g r e a t e r l o s s e s due t o i n t r u d e r s . Second, I c o n s i d e r t h e r e l a t i o n s h i p between uncached f o o d d e n s i t y and i n t r u d e r p r e s s u r e . T h i s i n t e r a c t i o n i s 48 l e s s c l e a r . Presumably t h e same mechanism t h a t a l l o w s t h i e v e s t o s t e a l a g r e a t e r p r o p o r t i o n o f caches as cache d e n s i t y i n c r e a s e s , would r e s u l t i n a s i m i l a r d e n s i t y dependent r a t e o f l o s s of uncached f o o d . I f t h i s a s s u m p t i o n i s v a l i d , t h e n f o o d a d d i t i o n s h o u l d r e s u l t i n an i n c r e a s e d e f f e c t o f i n t r u d e r s . I n summary, t h e r e i s e v i d e n c e (Waite 1988) t h a t i n c r e a s i n g f o o d d e n s i t y i n g r e y j a y t e r r i t o r i e s r e s u l t s i n i n c r e a s e d c o s t s o f i n t r u d e r s . The f i n a l p i e c e of i n f o r m a t i o n needed i n Schoener's (1983) model i s how t h e c o s t s of i n t r u d e r s change w i t h the a r e a o f t h e t e r r i t o r y . As g r e y j a y t e r r i t o r y s i z e i n c r e a s e s cache d e n s i t y s h o u l d d e c r e a s e and t h e c o s t o f t h e f t by i n t r u d e r s s h o u l d t h e r e f o r e d e c r e a s e . As t e r r i t o r y s i z e i n c r e a s e s t h e d e n s i t y of uncached f o o d w i l l n o t change, but th e p r o p o r t i o n o f t o t a l f o o d r e s o u r c e s s t o l e n by i n t r u d e r s w i l l d e c r e a s e . Thus the net c o s t o f i n t r u d e r s s h o u l d d e c r e a s e w i t h i n c r e a s i n g t e r r i t o r y s i z e . I n summary, g r e y j a y s a r e t i m e c o n s t r a i n e d e n e rgy m a x i m i z e r s , i n c r e a s i n g f o o d d e n s i t y r e s u l t s i n an i n c r e a s e s i n i n t r u d e r p r e s s u r e ( i . e . an i n c r e a s e d e f f e c t o f i n t r u d e r s i f n o t an i n c r e a s e i n the number o f i n t r u d e r s ) , and t h e c o s t of i n t r u d e r s i s l i k e l y t o d e c r e a s e as t e r r i t o r y s i z e i n c r e a s e s . W i t h t h i s i n f o r m a t i o n , t h e o p t i m a l t e r r i t o r y s i z e f o r j a y s on f o o d a d d i t i o n g r i d s i s p r e d i c t e d t o be s m a l l e r t h a n on c o n t r o l g r i d s by Schoener's (1983) model. My r e s u l t s show t h a t g r e y j a y t e r r i t o r i e s do r e s p o n d i n t h i s way t o f o o d a d d i t i o n . 49 T e r r i t o r y Overlap As f o o d d e n s i t y i n c r e a s e s , d e f e n s e i s e x p e c t e d t o a t f i r s t i n c r e a s e , and t h e n t o d e c r e a s e as f o o d becomes v e r y abundant (e.g. C a r p e n t e r and M a c M i l l e n 1976). I n t h i s s t u d y t h e p o s i t i o n of g r e y j a y s on c o n t r o l g r i d s a l o n g t h i s continuum was unknown, w h i c h i s why t h e t - t e s t comparing o v e r l a p was t w o - t a i l e d . Food a d d i t i o n c o u l d e i t h e r i n c r e a s e t h e v a l u e of e x c l u s i v e n e s s and t h e r e f o r e reduce the amount of o v e r l a p , o r i t c o u l d i n c r e a s e t h e f o o d d e n s i t y t o t h e p o i n t where the b e n e f i t s of e x c l u s i v e n e s s b e g i n t o f a d e , w h i c h would l e a d t o more o v e r l a p on f o o d a d d i t i o n g r i d s . I n t h e extreme case, i f t h e Kluane f o o d a d d i t i o n i n c r e a s e s f o o d d e n s i t y t o the p o i n t where defense i s no l o n g e r e c o n o m i c a l , t e r r i t o r i a l i t y s h o u l d b r e a k down. However, because j a y s a r e dependent on t h e i r caches i n w i n t e r , and w i n t e r i s such an e n e r g e t i c a l l y s t r e s s f u l p e r i o d (Waite 1991), such an extreme case would be u n l i k e l y . These re s p o n s e s depend on t h e t e r r i t o r i a l b e h a v i o u r o f j a y s b e i n g p l a s t i c enough t o respond t o t h e f o o d a d d i t i o n . A l t h o u g h my sample s i z e of t e r r i t o r i e s was s m a l l , t h e r e appears t o be a s l i g h t i n c r e a s e i n o v e r l a p between g r e y j a y t e r r i t o r i e s on f o o d a d d i t i o n g r i d s compared t o c o n t r o l g r i d s . ; The d i f f e r e n c e was n o t . s t a t i s t i c a l l y s i g n i f i c a n t , b u t t h e power of t h e t - t e s t was e s t i m a t e d t o be 0.3, s i g n i f y i n g a 70% chance o f making a t y p e I I e r r o r . The a v a i l a b l e d a t a i s n o t s u f f i c i e n t t o draw c o n c l u s i o n s , b u t 50 does s u g g e s t t h a t more r e s e a r c h i s w a r r a n t e d . Stamps and Tanaka (1981) measured p e r c e n t o v e r l a p i n j u v e n i l e Anolis aeneus home r a n g e s . These j u v e n i l e s a r e n o r m a l l y t e r r i t o r i a l , b u t t h e s e t e r r i t o r i e s sometimes o v e r l a p w i t h t h o s e o f n e i g h b o u r s . Stamps and Tanaka (1981) found t h a t o v e r l a p i n c r e a s e d when f o o d was added u n i f o r m l y t h r o u g h t h e s t u d y a r e a d u r i n g t h e d r y (reduced food) season. T h i s s i t u a t i o n i s s i m i l a r t o the food a d d i t i o n t o g r e y j a y s i n th e summer. I n b o t h cases the a n i m a l s a r e e x p e r i e n c i n g f o o d s h o r t a g e s : t h e d e n s i t y of food f o r the l i z a r d s i s r e d u c e d i n th e d r y season (Stamps and Tanaka 1981) , and t h e j a y s a r e f o o d l i m i t e d by t i m e c o n s t r a i n t s . I n b o t h cases t h e r e a ppears t o be an i n c r e a s e i n the amount of o v e r l a p . I n t h i s s t u d y I d i d not measure t e r r i t o r y s i z e and o v e r l a p i n w i n t e r . A l s o , t h i s s t u d y t o o k p l a c e d u r i n g t h e low o f t h e "10 y e a r c y c l e " of snowshoe hare (Lepus americanus) when t h e r e i s l i t t l e c a r r i o n a v a i l a b l e t o supplement g r e y j a y caches. Measuring t e r r i t o r y s i z e and o v e r l a p i n w i n t e r and when n a t u r a l f o o d i s more abundant may l e a d t o more i n s i g h t i n t o how these f e a t u r e s o f t e r r i t o r i e s v a r y i n r e s p o n s e t o e n v i r o n m e n t a l c o n d i t i o n s . Stamps and Tanaka (1981) found home range o v e r l a p i n A. aeneus d i d n o t i n c r e a s e when f o o d was added t o an a l r e a d y f o o d - r i c h e n v i r o n m e n t i n t h e wet - season,, and i n c o n t r o l a r e a s , t h e r e was more o v e r l a p i n t h e d r y season t h a n i n t h e wet se a s o n . An a l t e r n a t i v e e x p l a n a t i o n f o r t h e o v e r l a p o b s e r v e d i s t h a t t h e n e i g h b o u r i n g groups t h a t had t h e g r e a t e s t o v e r l a p 51 had f a m i l i a l t i e s , and so g r e a t e r o v e r l a p was p e r m i t t e d . I n F l o r i d a s c r u b j a y s , new t e r r i t o r i e s a r e sometimes formed by t h e h e l p e r s o f one group ( u s u a l l y o f f s p r i n g of the mated p a i r ) d e f e n d i n g p a r t o f t h e home and/or n e i g h b o u r i n g t e r r i t o r i e s (Woolfenden and F i t z p a t r i c k 1984). I f a s i m i l a r p r o c e s s o c c u r s i n g r e y j a y s , the r e l a t e d n e s s e x p l a n a t i o n may be v a l i d ( a l t h o u g h no such o v e r l a p was n o t e d i n s c r u b j a y t e r r i t o r i e s ) . More w i l l have t o be l e a r n e d about how g r e y j a y t e r r i t o r i e s a r e formed and how j u v e n i l e s d i s p e r s e b e f o r e t h e s e e x p l a n a t i o n s can be d i s t i n g u i s h e d . Number of T e r r i t o r i e s per Grid There were more t e r r i t o r i e s on food a d d i t i o n g r i d s t h a n e x p e c t e d b a s e d on a s i m p l e s i m u l a t i o n model. The model u s e d a s i m p l i f i e d l a n d s c a p e of t e r r i t o r i e s and g e n e r a t e d the e x p e c t e d d i s t r i b u t i o n by l a y i n g 1000 36-ha g r i d s a t random on t h i s l a n d s c a p e and c o u n t i n g the number of t e r r i t o r i e s on each g r i d . When s i m u l a t i n g c o n t r o l and f o o d a d d i t i o n g r i d s , t h e model u s e d t h e o b s e r v e d mean t e r r i t o r y s i z e f o r t h e r e s p e c t i v e t r e a t m e n t s (23.2 ha f o r c o n t r o l , and 15.8 ha f o r f o o d a d d i t i o n g r i d s ) . The observed mean number of t e r r i t o r i e s p e r 36-ha c o n t r o l g r i d was not s i g n i f i c a n t l y d i f f e r e n t f r om t h e model's e x p e c t e d mean. T h i s s u g g e s t s t h a t c o n t r o l t e r r i t o r i e s a r e randomly d i s t r i b u t e d w i t h r e s p e c t t o t h e K l u a n e g r i d s as t h e model assumes. When t h e model was r u n t o s i m u l a t e f o o d a d d i t i o n g r i d s t h e e x p e c t e d mean number of t e r r i t o r i e s was l e s s t h a n t h e 52 o b s e r v e d mean. T h i s s u g g e s t s t h a t the i n c r e a s e i n number of t e r r i t o r i e s on f o o d a d d i t i o n g r i d s i s not s o l e l y due t o the s m a l l e r mean s i z e o f t e r r i t o r i e s on t h e s e g r i d s , and t h a t t h e f o o d a d d i t i o n has a t t r a c t e d t e r r i t o r i e s . S i m i l a r r e s p o n s e s have been found i n o t h e r s t u d i e s . B o u t i n (1984) f o u n d t h a t some snowshoe hares t h a t were a t t r a c t e d t o a f o o d a d d i t i o n a r e a became r e s i d e n t s , but o t h e r s commuted t o and from t h e i r home ranges o f f the s i t e . I n the t e r r i t o r i a l r e d s q u i r r e l {Tamiasciurus hudsonicus) , K l e n n e r and Krebs (1991) f o u n d i n c r e a s e d d e n s i t i e s on f o o d a d d i t i o n s i t e s , and some of t h e i m m i g r a n t s seemed t o become r e s i d e n t . I n t h e i r s t u d y t h e p u l s e f o o d a d d i t i o n ended i n the f a l l , a t which time d e n s i t i e s on supplemented s i t e s f e l l t o c o n t r o l l e v e l s o v e r t h e n e x t s i x months ( K l e n n e r and Krebs 1991). B o u t i n (1990) r a i s e s t h e p o s s i b i l i t y t h a t apparent i n c r e a s e s i n p o p u l a t i o n d e n s i t i e s may be p a r t l y due t o an i n c r e a s e i n the e f f e c t i v e c e n s u s i n g a r e a caused by the a t t r a c t i o n o f a n i m a l s t o t h e s u p p l e m e n t a l f o o d . My r e s u l t s support t h i s h y p o t h e s i s i n t h e c a s e o f g r e y j a y s . B a sed on t h e number of t e r r i t o r i e s p e r g r i d and average group s i z e , 75% more groups had a c c e s s t o the f o o d a d d i t i o n g r i d s compared t o c o n t r o l g r i d s . S i n c e f o o d a d d i t i o n had no e f f e c t on t h e number o f b i r d s p e r group, the number of b i r d s p e r g r i d . i s a l s o 75% h i g h e r . I n a census- o f the p o p u l a t i o n on t h e g r i d w i t h o u t knowledge about t e r r i t o r y s i z e s , t h i s w o u l d l i k e l y t r a n s l a t e i n t o an e s t i m a t e o f d e n s i t i e s b e i n g 75% h i g h e r on f o o d a d d i t i o n g r i d s . Based on mean t e r r i t o r y 53 s i z e , d e n s i t i e s on f o o d a d d i t i o n g r i d s were o n l y 47% h i g h e r t h a n on c o n t r o l g r i d s , r e p r e s e n t i n g a c o n s i d e r a b l y s m a l l e r r e s p o n s e t o f o o d a d d i t i o n t h a n would be e s t i m a t e d w i t h l e s s l a b o u r i n t e n s i v e methods. I n t h e case o f g r e y j a y s and o t h e r t e r r i t o r i a l a n i m a l s , comparing t e r r i t o r y s i z e s and o v e r l a p s w i l l a v o i d t h e pro b l e m o f a t t r a c t i n g i n d i v i d u a l s t o the t r e a t m e n t a r e a s . I t i s more d i f f i c u l t when a p o p u l a t i o n i n c l u d e s f l o a t e r s ( n o n - t e r r i t o r i a l i n d i v i d u a l s ) o r when the a n i m a l i s non-t e r r i t o r i a l . I n such c a s e s s e v e r a l i n d i v i d u a l s may be u s i n g t h e same a r e a s , making i t h a r d e r t o d i s t i n g u i s h commuters from r e s i d e n t s . As B o u t i n (1990) s u g g e s t s , i n c r e a s i n g t h e s p a t i a l and t e m p o r a l s c a l e s o f f o o d s u p p l e m e n t a t i o n e x p e r i m e n t s (which t e n d t o be v e r y s m a l l i n most s t u d i e s ) c o u l d r e s o l v e t h e s e p r o b l e m s . Food a d d i t i o n s s h o u l d be made t o a r e a s l a r g e enough t h a t t h e census a r e a i n the i n t e r i o r i s not a f f e c t e d by commuting i n d i v i d u a l s , and t h e y s h o u l d be l o n g enough t h a t t e r r i t o r i a l b o r d e r s , and home range c o n f l i c t s a r e r e s o l v e d b e f o r e the c e n s u s i n g i s done. The p r o b l e m o f a t t r a c t i n g i n d i v i d u a l s , a l t h o u g h i t may nev e r change t h e q u a l i t a t i v e r e s u l t s o f a s t u d y , w i l l make comparing t h e r e s p o n s e s o f d i f f e r e n t organisms and d i f f e r e n t e n v i r o n m e n t s u n r e l i a b l e . E x p l i c i t s t u d y o f t h i s phenomenon i s needed t o u n d e r s t a n d how- i t may b i a s s t u d i e s u s i n g f o o d a d d i t i o n s . 54 Summary I f o u n d t h a t g r e y j a y s on f o o d a d d i t i o n g r i d s had s m a l l e r t e r r i t o r i e s t h a n c o n t r o l b i r d s . T h i s r e s u l t i s c o n s i s t e n t w i t h Schoener's (1983) model o f o p t i m a l f e e d i n g t e r r i t o r y s i z e . I a l s o f o u n d t h a t t h e r e was a s l i g h t i n c r e a s e i n the amount of o v e r l a p between t e r r i t o r i e s , but a l a r g e r sample s i z e i s needed t o d e t e r m i n e i f t h e o b s e r v e d d i f f e r e n c e s a r e r e a l , o r a r e j u s t a r t e f a c t s o f s m a l l sample s i z e . F i n a l l y , t h e r e were more t e r r i t o r i e s on f o o d a d d i t i o n g r i d s t h a n on c o n t r o l g r i d s . T h i s d i f f e r e n c e was not s o l e l y a t t r i b u t a b l e t o s m a l l e r t e r r i t o r y s i z e s on f o o d a d d i t i o n g r i d s , and l i k e l y r e p r e s e n t s t h e i n c r e a s e d v a l u e o f t h e h a b i t a t due t o t h e f o o d a d d i t i o n . 55 Chapter 4 : General Discussion I n t h i s s t u d y I examined the r e s p o n s e o f g r e y j a y s {Perisoreus canadensis) t o a l o n g term f o o d a d d i t i o n . The two a r e a s I examined were w i n t e r body mass r e g u l a t i o n and t e r r i t o r i a l i t y . Overwinter Condition I n c h a p t e r two I t e s t e d f o u r h ypotheses about t h e f a c t o r s t h a t l i m i t o v e r w i n t e r body mass i n g r e y j a y s . I found e v i d e n c e t h a t w i n t e r w e i g h t s of g r e y j a y s were l i m i t e d by f o o d s u p p l y and by c o s t s a s s o c i a t e d w i t h e x t r a w e i g h t . B i r d s t h a t had s u p p l e m e n t a l f o o d y e a r - r o u n d made more caches i n t h e summer t h a n c o n t r o l b i r d s d i d . As a r e s u l t , t h e s e b i r d s m a i n t a i n e d h i g h e r body c o n d i t i o n t h r o u g h t h e w i n t e r and y e t were a b l e t o spend l e s s time f o r a g i n g f o r f o o d t h a n c o n t r o l b i r d s . From t h e s e r e s u l t s , t h r e e a r e a s f o r f u t u r e work become a p p a r e n t . F i r s t , t h e n a t u r e o f f o o d l i m i t a t i o n i s n o t c l e a r . The v a l u e o f e x t r a f o o d can be e i t h e r i n a i d i n g d a i l y s u r v i v a l t h r o u g h t h e w i n t e r , o r as e x t r a f u e l f o r b r e e d i n g i n e a r l y s p r i n g (Jansson e t a l . 1981) . The r e l a t i v e i m p o r t a n c e o f t h e s e components can be e x p l o r e d by s u p p l e m e n t i n g f o o d a t d i f f e r e n t t i m e s o f t h e y e a r and comparing t h e responses o f j a y s . A second a r e a f o r f u t u r e r e s e a r c h i s t h e n a t u r e o f t h e c o s t s o f w e i g h t . Two p r e v i o u s c o r r e l a t i o n a l s t u d i e s (Rogers 56 1987, Waite 1992) found p a t t e r n s c o n s i s t e n t w i t h c o s t s t o w e i g h t as i n Lima's (1986) and McNamara and Houston's (1990) models of w i n t e r body mass. My r e s u l t s p r o v i d e e x p e r i m e n t a l e v i d e n c e f o r t h e s e c o s t s . Lima (1986) assumed the c o s t was t h e r i s k o f p r e d a t i o n , and t h a t p r e d a t i o n r i s k i n c r e a s e d a t an i n c r e a s i n g r a t e w i t h mass. McNamara and Houston (1990) modeled b o t h mass-independent and mass-dependent p r e d a t i o n r i s k as t h e c o s t s of f o r a g i n g . The a s s u m p t i o n t h a t p r e d a t i o n r i s k i s the u l t i m a t e c o s t o f added w e i g h t remains u n t e s t e d ; t h e c o s t may s i m p l y be t h e w e i g h t i n and o f i t s e l f . The h e a v i e r the b i r d , the g r e a t e r i t s t r a n s p o r t and body maintenance r e q u i r e m e n t s w i l l be (Blem 1975, 1976). A p h y s i o l o g i c a l s t u d y would r e v e a l the r e l a t i o n s h i p between w e i g h t and m e t a b o l i c expense. A s t u d y o f t h e impact of p r e d a t i o n on t h e b i r d s might a l s o l e a d t o some i n s i g h t s i n t o whether p r e d a t i o n r i s k i s a l i k e l y c o s t . The t h i r d a r e a r e q u i r i n g more s t u d y i s t h e p a t t e r n o f s e a s o n a l w e i ght t r e n d s . I found i n one y e a r o f s t u d y t h a t under n a t u r a l c o n d i t i o n s g r e y j a y w e i g h t s d e c l i n e d between summer and f a l l , and t h e n i n c r e a s e d from f a l l t o w i n t e r a g a i n . When p r o v i d e d w i t h s u p p l e m e n t a l f o o d , however, the j a y s I m o n i t o r e d i n c r e a s e d from summer t o f a l l and m a i n t a i n e d h i g h w e i g h t s t h r o u g h the w i n t e r . T h i s d i f f e r e n c e s u g g e s t s t h a t f o o d a d d i t i o n may change the way caches a r e made and used. The g r e y j a y s appeared t o r e l y e n t i r e l y on t h e caches t h e y made i n t h e summer and f a l l f o r o v e r w i n t e r f o o d . When 57 f o r a g i n g i n t h e summer, the b i r d s can e i t h e r consume the f o o d and s t o r e t h e energy as f a t r e s e r v e s , o r t h e y can cache the f o o d and s t o r e t h e energy i n t h a t way. As my r e s u l t s show, t h e r e i s some c o s t t o c a r r y i n g w e i ght, a l t h o u g h t h e e x a c t f o r m o r magnitude o f t h a t c o s t remains u n c e r t a i n . A t the same t i m e , s t o r i n g energy as caches can be r i s k y i f t h e y r o t , a r e f o r g o t t e n , o r a r e s t o l e n by o t h e r a n i m a l s o r c o n s p e c i f i c s . F u r t h e r m o r e , the r e l a t i v e i m p o r t a n c e of t h e s e c o s t s w i l l change thr o u g h o u t the y e a r , r e s u l t i n g i n a complex s y s t e m o f r e l a t i v e c o s t s and b e n e f i t s t o f a t s t o r a g e v e r s u s c a c h i n g . A c o m p a r i s o n o f time budgets between f o o d a d d i t i o n and c o n t r o l b i r d s t h r o u g h the f a l l when the we i g h t t r e n d s a r e so d i f f e r e n t i s needed t o c l a r i f y why the weight t r e n d s a r e so d i f f e r e n t ( i . e . i s t h i s a t r a n s i t i o n p e r i o d between making and consuming caches?) . I t s h o u l d then be p o s s i b l e t o model the t r a d e o f f s between making caches and p u t t i n g on f a t i n th e summer, and r e t r i e v i n g caches and k e e p i n g caches i n t h e w i n t e r . T e r r i t o r i a l i t y Based on t h e r e s u l t s o f o t h e r s t u d i e s , I was a b l e t o f i t g r e y j a y s i n t o Schoener's (1986) model o f t e r r i t o r y s i z e . To p r e d i c t how t e r r i t o r y s i z e w i l l r e s p o n d t o f o o d a d d i t i o n , one must know whether the an i m a l i s a time m i n i m i z e r o r e n e r g y m a x i m i z e r , and t h e n a t u r e o f i n t r u d e r p r e s s u r e and d e f e n s e c o s t s must be c l e a r . T h i s i n f o r m a t i o n 58 i s c r u c i a l i f t h e mechanisms b e h i n d the response a r e t o be u n d e r s t o o d . F o r g r e y j a y s , Schoener's model p r e d i c t s t h a t t e r r i t o r y s i z e s h o u l d d e c r e a s e as food, d e n s i t y i n c r e a s e s . I n c h a p t e r t h r e e I showed t h a t g r e y j a y s m a i n t a i n s m a l l e r t e r r i t o r i e s when p r o v i d e d w i t h e x t r a food, s u p p o r t i n g t h i s model. S c h o e n e r ' s model i l l u s t r a t e s t h a t s i m p l y o b s e r v i n g how t e r r i t o r y s i z e s r e s p o n d t o f o o d a d d i t i o n i s not i n t r i n s i c a l l y i n t e r e s t i n g . There a r e o n l y t h r e e p o s s i b l e r e s p o n s e s ( i n c r e a s e , d e c r e a s e , no change) but many s l i g h t l y d i f f e r e n t mechanisms can l e a d t o t h a t change. I n f u t u r e work on t e r r i t o r y s i z e i n response t o food a d d i t i o n , an u n d e r s t a n d i n g o f t h e s e mechanisms w i l l be c r u c i a l . A n o t h e r f i n d i n g of i n t e r e s t was t h a t the f o o d a d d i t i o n a p p e a r s t o have drawn i n t e r r i t o r i e s . The mean number of t e r r i t o r i a l g r oups t h a t were e n t i r e l y o r p a r t l y w i t h i n the b o r d e r s o f t h e 36-ha g r i d s I was u s i n g , i s not s i g n i f i c a n t l y d i f f e r e n t f r o m t h e e s t i m a t e d mean of a Monte C a r l o s i m u l a t i o n t h a t assumed t e r r i t o r i e s were d i s t r i b u t e d randomly w i t h r e s p e c t t o t h e g r i d s . There were more t e r r i t o r i e s on f o o d a d d i t i o n g r i d s t h a n p r e d i c t e d by the model, even when t h e s m a l l e r mean s i z e o f t e r r i t o r i e s was t a k e n i n t o a c c o u n t . T h i s i m p l i e s t h a t the t e r r i t o r i e s have changed shape i n a way t h a t more groups have a c c e s s t o a t l e a s t p a r t o f t h e f o o d a d d i t i o n . T h i s has some i m p l i c a t i o n s f o r t h e use o f f o o d a d d i t i o n s t o s t u d y p o p u l a t i o n d e n s i t y . One o f t h e most common g o a l s of f o o d a d d i t i o n e x p e r i m e n t s i s t o s t u d y p o p u l a t i o n dynamics, and the 59 m a j o r i t y o f t h e s e s t u d i e s have found i n c r e a s e d d e n s i t i e s of the s t u d y a n i m a l ( B o u t i n 1990) . A l t h o u g h t h e s e r e s u l t s p r o b a b l y r e p r e s e n t i n c r e a s e s i n d e n s i t y of t h e s t u d y a n i m a l s , t h e d a t a may be confounded by a n i m a l s t h a t a re a t t r a c t e d t o t h e f o o d , b u t w h i c h a r e not a c t u a l l y r e s i d e n t s of t h e f o o d a d d i t i o n a r e a . B o u t i n (1984) found t h a t non-t e r r i t o r i a l snowshoe h a r e s (Lepus americanus) commuted from t h e i r home ra n g e s o f f t h e supplemented g r i d t o f o r a g e on t h e added f o o d . I f o u n d s i m i l a r r e s u l t s f o r a t e r r i t o r i a l a n i m a l . The i n c r e a s e i n d e n s i t y as measured by the i n v e r s e o f t h e mean t e r r i t o r y s i z e i s l e s s than the i n c r e a s e i n t h e number o f j a y s t h a t have a c c e s s t o the food a d d i t i o n g r i d s . I f u s e f u l c o m p a r i s o n s a r e t o be made between s p e c i e s and e n v i r o n m e n t s , o r i f t h e r e s u l t s o f these s t u d i e s a r e t o be used i n m a t h e m a t i c a l models, the d i s t i n c t i o n between r e a l and a p p a r e n t e f f e c t s o f f o o d a d d i t i o n s on p o p u l a t i o n d e n s i t i e s s h o u l d be made. Conclusion T h i s s t u d y examined some o f the e f f e c t s a l o n g term f o o d a d d i t i o n has had on a p o p u l a t i o n of g r e y j a y s i n the southwest Yukon. The b i r d s a r e abundant, r e l a t i v e l y tame, and h a b i t u a t e t o human o b s e r v e r s . I n g e n e r a l t h e y a r e q u i t e t r a p p a b l e and e a s i l y observed-. These f a c t o r s combine t o • make g r e y j a y s good s u b j e c t s f o r s t u d y i n g a v a r i e t y of e c o l o g i c a l q u e s t i o n s . 60 C a c h i n g and cache use has r e c e i v e d much r e c e n t a t t e n t i o n (McNamara e t al. 1990, N i l s s o n e t a l . 1993, H i t c h c o c k and Houston 1994, Wa i t e and Ydenberg 1994) . By-comparing g r e y j a y s , w h i c h r e l y h e a v i l y on caches made i n t h e summer as o v e r w i n t e r f o o d , t o o t h e r b i r d s t h a t r e l y l e s s on l o n g t erm caches (e.g. European n u t h a t c h , S i t t a europaea; N i l s s o n e t al. 1993) o r b i r d s t h a t make s h o r t t erm caches (e.g. v a r i o u s t i t s , Parus spp.; McNamara e t a l . 1990) we may be a b l e t o g e t a more u n i f i e d u n d e r s t a n d i n g o f c a c h i n g and f o r a g i n g b e h a v i o u r i n b i r d s . 61 L i t e r a t u r e C i t e d A d d i c o t t , J.F., J.M. Aho, M.F. A n t o l i n , D.K. P a d i l l a , J.S. R i c h a r d s o n , and D.A. S o l u k . 1987. E c o l o g i c a l n e i g h b o r h o o d s : s c a l i n g e n v i r o n m e n t a l p a t t e r n s . O i k o s 49:340-346. A l e k s i u k , M. 1970. The f u n c t i o n of t h e t a i l as a f a t s t o r a g e depot i n t h e b e a v e r ( C a s t o r canadensis) . J . Mamm. 51:145-148. Blem, C R . 1975. G e o g r a p h i c v a r i a t i o n i n w i n g - l o a d i n g of t h e house sparrow. W i l s o n B u l l . 87:543-549. Blem, C R . 1976. P a t t e r n s of l i p i d s t o r a g e and u t i l i z a t i o n i n b i r d s . Amer. Z o o l . 16:671-684. Blem, C R . and J.F. P a g e l s . 1984. M i d - w i n t e r l i p i d r e s e r v e s of the golden-crowned k i n g l e t . Condor 86:491-492. B o u t i n , S.A. 1984. E f f e c t s of l a t e w i n t e r f o o d a d d i t i o n on numbers and movements of snowshoe h a r e s . O e c o l o g i a 62 : 393-400. B o u t i n , S.A. 1990. Food s u p p l e m e n t a t i o n e x p e r i m e n t s w i t h t e r r e s t r i a l v e r t e b r a t e s : p a t t e r n s , problems, and t h e f u t u r e . Can. J . Z o o l . 68:203-220. Brown, J . L . 19 64. The e v o l u t i o n of d i v e r s i t y i n a v i a n t e r r i t o r i a l systems. W i l s o n B u l l . 76:160-169. C a r p e n t e r , F.L., and R.E. M a c M i l l e n . 1976. T h r e s h o l d model of f e e d i n g t e r r i t o r i a l i t y and a t e s t w i t h a H a w a i i a n h o n e y - c r e e p e r . S c i e n c e 194:639-642. C l a r k s o n , K., S.F. Eden, W.J. S u t h e r l a n d , and A . I . Houston. 1986. D e n s i t y dependence and magpie f o o d h o a r d i n g . J . Anim. E c o l . 55:111-121. D a v i e s N.B. 1978. E c o l o g i c a l q u e s t i o n s about t e r r i t o r i a l b e h a v i o u r in 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. J.R. Krebs and N.B. D a v i e s eds. B l a c k w e l l : O x f o r d . E b e r s o l e , J.P. 1980. Food d e n s i t y and t e r r i t o r y s i z e : an a l t e r n a t i v e model and a t'est on t h e r e e f f i s h Eupomacentrus leucostictus. Am. Nat. 115:492-509. Enoksson, B., and S.G. N i l s s o n . 1983. T e r r i t o r y s i z e and p o p u l a t i o n d e n s i t y i n r e l a t i o n t o f o o d s u p p l y i n t h e n u t h a t c h Sitta europaea ( A v e s ) . J . Anim. E c o l . 52:927-935 . 62 E h r l i c h , P.R, D.S. Dobkin, and D. Wheye. 1988. The B i r d e r ' s Handbook. Simon and S c h u s t e r : New York. Evans, P.R. 1969. W i n t e r f a t d e p o s i t i o n and o v e r n i g h t s u r v i v a l of y e l l o w b u n t i n g s (Emberiza c i t r i n e l l a L . ) . J . Anim. E c o l . 38:415-423. Ewald, P.W., and F.L. C a r p e n t e r . 1978. T e r r i t o r i a l r e s p o n s e t o energy m a n i p u l a t i o n s i n t h e Anna hummingbird. O e c o l o g i a 31:277-292. Ewald, P.W., and S. Rohwer. 1982. E f f e c t s of s u p p l e m e n t a l f e e d i n g on t i m i n g of b r e e d i n g , c l u t c h s i z e , and p o l y g y n y i n red-winged b l a c k b i r d s Agelaius phoeniceus. J . Anim. E c o l . 51:429-450. H a i n s w o r t h , F.R., and L.L. Wolf. 1978. The economics o f te m p e r a t u r e r e g u l a t i o n and t o r p o r i n nonmammalian organisms in S t r a t e g i e s i n C o l d , L.C.H. Wang and J.W. Hudson eds. Academic P r e s s : New York. Hinde, R.A. 1956. The b i o l o g i c a l s i g n i f i c a n c e o f t h e t e r r i t o r i e s o f b i r d s . I b i s 98:340-369. H i t c h c o c k , C.L., and A . I . Houston. 1994. The v a l u e o f a ho a r d : not j u s t energy. Behav. E c o l . 5:202-205. H i x o n , M.A. 1980. Food p r o d u c t i o n and c o m p e t i t o r d e n s i t y as the d e t e r m i n a n t s of f e e d i n g t e r r i t o r y s i z e . Am. Nat. 115:510-530. H i x o n , M.A. 1982. Energy m a x i m i z e r s and t i m e m i n i m i z e r s : t h e o r y and r e a l i t y . Am. Nat. 119:596-599. J a n s s o n , C , J . Ekman, and A. von Bromssen. 1981. W i n t e r m o r t a l i t y and f o o d s u p p l y i n t i t s Parus spp. O i k o s 37:313-322. K i n g , J.R. 1972. A d a p t i v e p e r i o d i c f a t s t o r a g e i n b i r d s , pp. 200-217 in P r o c e e d i n g s o f t h e F i f t e e n t h I n t e r n a t i o n a l O r n i t h o l o g i c a l C ongress. K.H. Voous, ed. B r i l l , London, England. K l e n n e r , W., and C.J. Krebs. 1991. Red s q u i r r e l p o p u l a t i o n dynamics. I . The e f f e c t o f s u p p l e m e n t a l f o o d on demography. J . Anim. E c o l o g y 60:961-978. K o t l i a r , N.B., and J.A. Wiens. 1987. M u l t i p l e s c a l e s o f p a t c h i n e s s and p a t c h s t r u c t u r e : a h i e r a r c h i c a l framework f o r t h e s t u d y o f h e t e r o g e n e i t y . O i k o s 59:253-260 . K r e b s , C.J. 1989. E c o l o g i c a l Methodology. H a r p e r and Row: New York. 63 K r e b s , C.J., B.S. G i l b e r t , S. B o u t i n , A.R.E. S i n c l a i r , and J.N.M. Smith. 1986. P o p u l a t i o n b i o l o g y o f snowshoe h a r e s I : Demography of food supplemented p o p u l a t i o n s i n t h e s o u t h e r n Yukon. J . Anim. E c o l . 55:963-982. Krebs, C.J. and G.R. S i n g l e t o n . 1993. I n d i c e s o f c o n d i t i o n f o r s m a l l mammals. A u s t . J . Z o o l . 41:317-323. K r e b s , C.J., R. B o o n s t r a , S. B o u t i n , M. Da l e , S. Hannon, K. M a r t i n , A.R.E. S i n c l a i r , J.N.M. Smith, and R. T u r k i n g t o n . 1992. What d r i v e s t h e snowshoe h a r e c y c l e i n Canada's Yukon? i n W i l d l i f e 2001: P o p u l a t i o n s , ed. D.M. M c C u l l o u g h and R. B a r r e t t . 886-896. Lima, S.L. 1986. P r e d a t i o n r i s k and u n p r e d i c t a b l e f e e d i n g c o n d i t i o n s : d e t e r m i n a n t s of body mass i n b i r d s . E c o l o g y 67:377-385. Lucas, J.R. 1994. R e g u l a t i o n of cache s t o r e s and body mass i n C a r o l i n a c h i c k a d e e s {Parus c a r o l i n e n s i s ) . Behav. E c o l . 5:171-181. Mares, M.A., T.E. Lach e r , J r . , M.R. W i l l i g , and N.A. B i t a r . 1982. An e x p e r i m e n t a l a n a l y s i s o f s o c i a l s p a c i n g i n Tamias s t r i a t u s . E c o l o g y 63:267-273. McNab, B.K. 1963. B i o e n e r g e t i c s and t h e d e t e r m i n a t i o n o f home range s i z e . Am Nat 97:133-140. McNamara, J.M., and A . I . Houston. 1990. The v a l u e o f f a t r e s e r v e s and t h e t r a d e o f f between s t a r v a t i o n and p r e d a t i o n . A c t a B i o t h e o r e t i c a 38:37-61. McNamara, J.M., A . I . Houston, and J.R. Krebs. 1990. Why hoard? The economics of food s t o r i n g i n t i t s , Parus spp. Behav. E c o l . 1:12-23. Myers, J.P., P.G. Connors, and F.A. P i t e l k a . 1979. T e r r i t o r y s i z e i n w i n t e r i n g s a n d e r l i n g s : t h e e f f e c t s of p r e y abundance and i n t r u d e r d e n s i t y . Auk 96:551-561. N i l s s o n , J.-A., H. K a l l a n d e r , and 0. P e r s s o n . 1993. A p r u d e n t h o a r d e r : e f f e c t s of l o n g - t e r m h o a r d i n g i n t h e European n u t h a t c h , S i t t a europaea. Behav. E c o l . 4:369-373 . Novakowski, N.S. 1967. The w i n t e r b i o e n e r g e t i c s o f a b e a v e r p o p u l a t i o n i n n o r t h e r n l a t i t u d e s . Can. J . Z o o l . 45:1107-1118. Panuska, J.A. 1959. Weight p a t t e r n s and h i b e r n a t i o n i n Tamias s t r i a t u s . J . Mamm. 40:554-566. 64 P i a n k a , E.R. 1976. N a t u r a l s e l e c t i o n o f o p t i m a l r e p r o d u c t i v e t e c h n i q u e s . Am. Z o o l . 16:775-784. P r i c e , K., K. Broughton, S. B o u t i n , and A.R.E. S i n c l a i r . 1986. T e r r i t o r y s i z e and ownership i n r e d s q u i r r e l s : r e s p o n s e t o r e m o v a l s . Can. J . Z o o l . 64:1144-1147. Rogers, C M . 1987. P r e d a t i o n r i s k and f a s t i n g c a p a c i t y : do w i n t e r i n g b i r d s m a i n t a i n o p t i m a l body mass? E c o l o g y 68:1051-1061. Rogers, CM., and J.N.M. Smith. 1993. L i f e - h i s t o r y t h e o r y i n t h e n o n - b r e e d i n g p e r i o d : t r a d e - o f f s i n a v i a n f a t r e s e r v e s . E c o l o g y 74:419-426. R u t t e r , R.J. 1969. A c o n t r i b u t i o n t o t h e b i o l o g y of the Gray J a y {Perisoreus canadensis) . Can. F i e l d Nat. 83:300-316 . Schoener, T.W. 1971. Theory o f f e e d i n g s t r a t e g i e s . Annual Review o f E c o l o g y and S y s t e m a t i c s 2:370-404. Schoener, T.W. 1983. S i m p l e models of o p t i m a l f e e d i n g -t e r r i t o r y s i z e : a r e c o n c i l i a t i o n . Am. Nat. 121:608-629. S o k a l , R.R, and F . J . R o h l f . 1981. Bio m e t r y , 2nd ed. W.H. Freeman and Company: New York. Stamps, J.A., and S. Tanaka. 1981. The r e l a t i o n s h i p between f o o d and s o c i a l b e h a v i o r i n j u v e n i l e l i z a r d s (Anolis aeneus). C o p e i a 1981:422-434. S t a p a n i a n , M.A., and C C . Smith. 1984. D e n s i t y dependent s u r v i v a l o f s c a t t e r h o a r d e d n u t s : an e x p e r i m e n t a l a p p r o a c h . E c o l o g y 65:1387-1396. S t e n g e r , J.A. 1958. Food h a b i t s and a v a i l a b l e f o o d o f o v e n b i r d s i n r e l a t i o n t o t e r r i t o r y s i z e . Auk 75:335-346 . Vander W a l l , S. 1990. Food H o a r d i n g i n a n i m a l s . U n i v e r s i t y o f C h i c a g o P r e s s . W a i t e , T.A. 1988. A f i e l d t e s t of d e n s i t y - d e p e n d e n t s u r v i v a l o f s i m u l a t e d g r a y j a y caches. Condor 90:247-249. W a i t e , T.A. 1990. E f f e c t s of c a c h i n g s u p p l e m e n t a l f o o d on i n d u c e d f e a t h e r r e g e n e r a t i o n i n w i n t e r i n g Gray J a y s Perisoreus canadensis: a p t i l o c h r o n o l o g y s t u d y . O r n i s Scand. 21:122-128. W a i t e , T.A. 1991. N o c t u r n a l h y p o t h e r m i a i n g r a y j a y s Perisoreus canadensis w i n t e r i n g i n i n t e r i o r A l a s k a . O r n i s S c a n d i n a v i c a 22:107-110. 65 W a i t e , T.A. 1992. W i n t e r f a t t e n i n g i n Gray J a y s : s e a s o n a l , d i u r n a l and c l i m a t i c c o r r e l a t e s . O r n i s Scand. 23:499-503. W a i t e , T.A., and J.D. Reeve. 1992. C a c h i n g b e h a v i o u r i n the g r a y j a y and t h e s o u r c e - d e p a r t u r e d e c i s i o n f o r r a t e -m a x i m i z i n g s c a t t e r h o a r d e r s . B e h a v i o u r 120:51-67. Waite, T.A., and R.C. Ydenberg. 1994. What c u r r e n c y do s c a t t e r - h o a r d i n g g r a y j a y s maximize? Behav. E c o l . and S o c i o b i o l . 34:43-49. Woolfenden, G.E., and J.W. F i t z p a t r i c k . 1984. The F l o r i d a s c r u b j a y : Demography o f a c o o p e r a t i v e - b r e e d i n g b i r d . P r i n c e t o n U n i v e r s i t y P r e s s : P r i n c e t o n . W o l f f , J.O., and G.C. Bateman. 1978. E f f e c t s of f o o d a v a i l a b i l i t y and ambient temperature on t o r p o r c y c l e s of Perognathus flavus (heteromyidae) . J . Mamm. 59:707-716 . Wrazen, J.A., and L.A. Wrazen. 1982. Hoarding, body mass dynamics, and t o r p o r as components of the s u r v i v a l s t r a t e g y o f t h e e a s t e r n chipmunk. J . Mamm. 63:63-72. 66 

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