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The control and organization of parental feeding and its relationships to the food supply for the glaucous… Henderson, Bryan Alexander 1972

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c\ THE CONTROL AND ORGANIZATION OF PARENTAL FEEDING AND ITS RELATIONSHIPS TO THE FOOD SUPPLY FOR .THE GLAUCOUS WINGED GULL, Larus glaucescens by BRYAN ALEXANDER HENDERSON B.Sc. (H) , Univers i ty of B r i t i s h Columbia, 1970 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Zoology We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF May, BRITISH 1972 COLUMBIA In p resen t ing t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree tha t the L i b r a r y sha l l make i t f r e e l y a v a i l a b l e f o r reference and s tudy. I f u r t h e r agree t h a t permiss ion f o r ex tens ive copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head o f my Department or by h is r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l ga in s h a l l not be a l lowed w i thou t my w r i t t e n permiss ion . Department o f The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada Date Abstract From work previously done by John Ward, i t was shown that the p roduc t i v i t y on Cleland Island was greater than that on Mandarte Is land. The mean weight and f ledging rate fo r a l l brood sizes from one to s i x , was higher on Cleland Is land. I t was thought that an analysis of the parental feeding system and a comparison of the feeding biology would provide a p a r t i a l explanation f o r the d i f ference in p roduc t i v i t y between Mandarte and Cleland Is lands. The main object ives of th i s comparative study were to d is t ingu ish between the ra t ion requested by the ch icks, i . e . , parental feeding system, and the ra t ion received by the ch icks, i . e . , how the feeding biology modifies the r a t i o n . Both the qua l i t y and quant i ty o f foods given to the chicks d i f f e r between the two colonies. Mandarte adults eat a greater var ie ty o f foods than the Cleland adu l ts . The observations of courtship feed-ing and the co l lec t ions of stomach contents, show tha t Mandarte adults eat garbage, i n t e r t i d a l organisms, and f i s h , whereas Cleland gul ls eat f i s h and i n t e r t i d a l organisms. I f courtship feedings do s i g n i f i c a n t l y contr ibute to the requirements fo r egg product ion, the superior qua l i t y of food given to the Cleland females may be correlated wi th the higher hatching success on Cleland. In addi t ion to the e f fec t on the hatching success, the qua l i t y of food may also inf luence the f ledging success. On both Mandarte and Cleland, the adu l t ' s food d i f f e r s from the ch ick 's food. I n i t i a l l y , the Mandarte chick predominantly receives f i s h but as the chick stage progresses, less f i s h and more garbage is given to the i i i chicks. This de ter io ra t ion in the qua l i t y of the foods may be correlated wi th the low f ledging success of chicks raised la te in the season. Cle-land' s chicks receive only f i s h throughout the parental period so that chick mor ta l i t y could not be associated wi th a change in the qua l i t y of the food. The lower f ledging rate recorded fo r Mandarte chicks may also be associated wi th the smaller quant i t ies of food given to the chicks. Although no s i g n i f i c a n t d i f ference e x i s t s , the weight of food given to the Cleland chicks exceeds that given to the Mandarte chicks. Some o f the di f ferences of qua l i t y and quant i ty of food may be explained by the temporal and spat ia l patterns of food a v a i l a b i l i t y . The parental feeding frequency i s thought to be dependent on the needs of the chicks but is modified by prey a v a i l a b i l i t y . Parental feeding frequency reaches a peak in the ear ly morning, af ternoon, and evening on Mandarte but is r e l a t i v e l y constant u n t i l j u s t before dusk on Cleland Is land. The sandlance, obtainable in the immediate v i c i n i t y of Cleland, i s avai lable throughout the day; the he r r ing , rare ly caught near Mandarte Is land, has a per iodic a v a i l a b i l i t y during the day. A Mandarte adu l t , seeking an a l ternate food during a period of low herr ing a v a i l a b i l i t y , may have to make a long t r i p to some garbage dump or i n t e r t i d a l zone. Not only the patterns of feeding a c t i v i t y but also the to ta l feeding a c t i v i t y d i f f e r between the colonies. The frequency of feeding a f t e r a t r i p is s im i la r fo r both colonies but the foraging frequency is s i g n i f i c a n t l y greater f o r Mandarte parents. This may mean that the Mandarte parent presents fewer feedings than Cleland adults from the catch of one foraging t r i p . Although parents from i v both colonies increased the foraging and feeding frequency when chicks were added to normal broods, nei ther Mandarte nor Cleland adults i n -creased the feeding frequency in d i r e c t proport ion to the brood s ize . The adjustments made in the foraging and feeding frequency are made possible through a series of signals cons t i tu t ing the parental feeding control system. The control of parental feeding is achieved through the use of two s igna ls , c a l l i n g and pecking. A funct ional re la t ionsh ip ex is ts between the hunger level and the ca l l frequency, peck frequency, and ca l l i n t e n s i t y . For any hunger level the frequency and amplitude of the ca l l change wi th age but the pecking response does not change wi th the ch ick 's age. From the f i e l d observations of feeding, i t i s evident that a pa r t i cu la r cumlative pecking value and ca l l frequency e l i c i t s a feeding and the change in ca l l amplitude associated wi th sa t i a t i on is instrumental in terminat ing a feeding. The foraging frequency, propor-t iona l to the brood s i z e , may be modulated by the cumulative c a l l i n g o f the chicks. A simple experiment did show that the a c t i v i t y of the parent is inf luenced by the hunger of the brood; th i s fac t was demonstrated by an experiment in which I sat ia ted the gu l l chicks thus increasing the time that the parents remained on the t e r r i t o r y . In summary, the feeding frequency is adjusted by the pecking and c a l l i n g frequency whi le the foraging frequency of the parents is adjusted by the cumulative c a l l i n g of the chicks. A feeding i s i n i t i a t e d by a spec i f i c c a l l i n g frequency and cumulative pecking value and terminated when the food is consumed or when the ca l l amplitude indicates s a t i a t i o n . V The a b i l i t y of the parents to adjust t h e i r feeding a c t i v i t y raises some in te res t ing questions. The re la t ionsh ip of both the feeding biology and parental feeding system to the p roduc t i v i t y of an is land is discussed from a comparative viewpoint. The importance of the feeding system and other factors is discussed in re la t i on to the evolut ion of c lu tch s i ze . vi TABLE OF CONTENTS Page LIST OF TABLES ix LIST OF FIGURES x i I GENERAL INTRODUCTION 1 Chapter 1 FEEDING BIOLOGY OF THE GULLS ON MANDARTE AND CLELAND ISLANDS 5 A. In t roduct ion 5 B. Study Areas 5 1 . Mandarte Island 5 2. Cleland Island 6 C. Courtship Feeding Behaviour 9 D. Volume of Food in the Stomach of Incubating Adults . 13 E. Volume of Food in the Stomach of Chicks 17 F. Feeding and Foraging Frequency wi th Respect to Brood Size 24 G. Feeding and Foraging A c t i v i t y in Relation to the Time of Day 28 H. Ef fect of Brood Size on the Length of the Foraging Absence 30 I . Growth Rate and Ecological Growth Ef f ic iency . . . . 32 J . Digestive Rate of the Chicks 38 K. Summary and Discussion 40 2. THE CONTROL AND ORGANIZATION OF PARENTAL FEEDING BEHAVIOUR 42 A. In t roduct ion 42 v i i Chapter Page B. Ethogram of Parental Feeding Behaviour 43 1 . Adult Routine 43 a. Return 43 b. Presentation 44 (1) Prel iminary 44 (a) Feeding Call 44 (b) Oblique Extension 44 (c) Lead 44 (2) Regurgitat ion 44 (a) Swell ing 44 (b) Hold 44 (c) Point 47 (d) Manipulate 47 2. Chick Routine 47 a. Begging Call 47 b. Pecking Response 47 c. Bobbing 48 d . Following 48 C. Control Signals 48 1. General Methods 48 2. Pecking of the B i l l o f a Model 50 3. Frequency of Calls given by the Chicks 50 4. In tens i ty of Indiv idual Begging Calls 52 5. Qua l i ta t i ve Aspects of Hunger Calls 55 v i i i Chapter Page D. The Influence of the Control Signals on the Parent . 56 1 . General Methods 56 2. Pecking Behaviour 56 3. Cal l ing Behaviour 58 a. Call Frequency Before and A f te r a Feeding . 58 b. Ef fect of Hunger on the Departure Time fo r the Adult 60 c. Call Character is t ics f o r D i f fe ren t Brood Sizes 62 4. In tens i ty of Indiv idual Begging Calls 65 E. Summary and Discussion 66 3. THE MODEL 70 A. In t roduct ion 70 B. Descript ion of the Model 70 I I GENERAL DISCUSSION 77 SUMMARY 88 LITERATURE CITED 90 i x LIST OF TABLES Table Page 1. Figures re fer to the estimated number of grams of food given to the female during courtship feeding on Cleland. These are conservative estimates 9 2. These data show the percentage of food types which were given to the female by the male on Cleland. The sample s i z e , (n) days, refers to the number of observation days from which the data were col lected . 10 3. The upper par t o f the table shows that a courtship feeding occurs, on the average, every 10 hours. The lower por t ion of the table shows that feeding frequency reaches a peak in the early afternoon and then the a c t i v i t y subsides * 14 4. Incubating adults were caught in the c love r - lea f traps used on both Mandarte and Cleland. Although the Cleland birds are s l i g h t l y heavier and contained more food, the di f ferences are not s i g n i f i c a n t ( t - t e s t P = 0.05) . I t is obvious from the second par t of th is t ab le , that Mandarte adults u t i l i z e a greater var ie ty of food sources 16 5. This table shows the percentage of times that various items were present in the stomachs of the chicks. There i s a trend f o r more va r ia t i on in food as the chicks get o lder . On Cleland no such trend ex is t 19 6. The f igures wi th the brackets are percentages. The occurrence of garbage is coincidental with the peak of hatching on the i s land . Supernormal broods did not appear to receive more garbage than the normal broods. The numbers under the heading of 'week' re fe r to consecutive weeks of observations beginning on 5 July 19 7. No s i g n i f i c a n t d i f ference exis ts between the foraging absences fo r normal and supernormal broods on both Mandarte and Cleland. There is an obvious trend f o r the parents of supernormal broods to decrease the foraging absence 31 X Table Page 8. The mean da i l y growth, ecological growth e f f i c i ency and weight loss over a 10 hour night is presented in t h i s tab le . The growth rate of these hand-reared chicks (Y = 26X + 47.1 r = .98, d . f . = 225) is lower than that of the f i e l d chicks. *RA = running average . . . . 34 9. An average of 18.4 pecks preceded a successful feeding whereas a lower number of pecks did not e l i c i t a feeding from the parent (unsuccessful) 57 10. These f i e l d data demonstrate that some threshold ca l l frequency precedes a parental feeding and tha t the ca l l frequency subsides a f t e r a feeding 59 11 . The flow diagram explains the terms 'Pre' and ' P o s t ' . Al and A2 re fe r to the two parental adu l ts . Obviously the departure times are increased i f the chicks are s a t i a t -ed. The aster isk indicates the point in the sequence where I force fed the chicks 61 12. The "% attendance" describes the percentage of times tha t two parents were on the t e r r i t o r y (%=b/Nxl00). The greatest nest attendance is shown by the commonest natural brood size 63 13. The din measurement refers to the number o f ca l l s heard from a brood over a period of t ime. A f i v e brood ca l l s 5 x as f requent ly as a s ingle brood 63 14. This simple f low chart summarizes the information d i s -cussed in th i s thes is . The 43 steps describes the sequence of events, commands, and decisions which must be made during the parental feeding act 71 15. These data, taken from J . Ward's t hes i s , show not only that the variance of the growth rate increases wi th brood size but also that the variance is greater on Mandarte. The trend does not pe rs is t in to the supernormal broods which are experimentally enlarged broods 79 16. I n i t i a l l y i t was thought that the time spent in prefeeding behaviour would be some funct ion of the hunger and size of the brood. The BEG-REG in te rva l is the time measured from the f i r s t ca l l or peck to the parent 's regu rg i ta -t i o n . PRES-REG is the time between the presentat ion of the parent 's b i l l and the subsequent r e g u r g i t a t i o n . The REG-ET in te rva l is a measure of the time that the parent of fers food to the chicks. I t i s obvious that no con-s is ten t trends ex is t in these data 85 x i LIST OF FIGURES Figure Page 1. Mandarte Island is s i tuated in the Haro S t r a i t which l i e s 6 miles east of Sydney on Vancouver Island . . . 7 2. Cleland Island is a small is land located 12 miles west of Tofino which is on the outer coast of Vancouver Island 8 3. I t i s evident that a uniform d ie t of sandlance in the courtship feeding i s coincident w i th the commencement of egg- laying. Pr io r to 4 June both gooseneck barna-cles and sandlance are given to the female but a f t e r 4 June the feedings consist e n t i r e l y of sandlance . . 12 4. The trap is constructed from 1 inch chicken wire cut in to 3' x 6' sect ions. The wire i s formed in to a bi- lobed s t ructure wi th a wire top. Wire handles are attached to the bottom so that the traps can be secured wi th rocks 15 5. This f i gu re demonstrates that the food ra t ion is propor-t ionate to the age of the chick. Although no s i g n i f i -cant d i f ference exis ts between Mandarte and Cleland in t h i s respect (covariance t e s t ) , Cleland chicks appear to receive more food than Mandarte chicks . . . 18 6. The re la t ionsh ip between the hatching d i s t r i b u t i o n and the var ie ty of foods given to the chicks is demonstrat-ed in t h i s f i g u r e . The increase in the amount of garbage fed to the chicks is corre lated wi th the number of chicks on the i s land . These data are derived from observations of parental feeding 20 7. The composition of the ch ick 's d ie t changes wi th age. The older chicks receive a greater var ie ty of food than the young chicks. There are two possible explanations f o r th is t rend: (1) i n a b i l i t y of parents to f u l f i l l needs of the large chicks so tha t addi t ional food sources are exp lo i ted ; (2) the ex is t ing natural food supply is de te r io ra t ing as the population demand i n -creases 21 x i i Figure Page 8. This f i g u r e , courtesy of John Ward, shows the marked super io r i t y of growth fo r Cleland chicks. The growth and survival of a l l brood sizes is greater on Cleland than on Mandarte. 9 = Cleland; 0 = Mandarte. The maximum weight achieved by a chick p r i o r to f ledging is the weight asymptote. A ch ick 's mean da i l y weight gain is the da i l y weight increment 23 9. The number of foraging t r i p s (T) and feedings (F) associa-ted with a brood size (1 to 6) is displayed in t h i s f i g u r e . Mandarte (M) adults make s i g n i f i c a n t l y more foraging t r i p s f o r the same number of feeds per forag-ing t r i p (means test -analys is of covariance). The data points represent the da i l y mean a c t i v i t y through-out the chick stage 25 10. The frequency of foraging t r i p s and parental feedings throughout the day i s shown fo r both Cleland and Mandarte. The Mandarte periodgram is characterized by three peaks of a c t i v i t y whereas the a c t i v i t y on Cleland shows no predominate peaks 29 11 . Both the regression l i n e and the l i n e through a three point running average are shown on th i s f i g u r e . The three point running average gives a be t te r b io log ica l descr ip t ion of the abrupt changes in the e f f i c iency t rends. The regression equation is y = 52 - 1.3x. The dotted l i n e passing through the 3-point running averages i s f i t t e d by eye 35 12. This f igure shows a regression of the volume of food eaten/ body weight on depr ivat ion t ime. This regression gives a good approximation of the d igest ive rate fo r any weight of chicks. The f igure beside the so l id c i r c l e is the sample size f o r that mean 39 13. The parent is shown regurg i ta t ing some f i s h to a 40 day o ld chick. While begging f o r food, an older chick always assumes th i s crouched submissive posture 45 14. No obvious di f ferences can be seen between the ca l ls given before and a f t e r a feeding. A more thorough inves t iga-t ion may show that these ca l l s d i f f e r in p i tch and durat ion 46 15. The Gull Grance, a 9' x 3' x 4 ' wire cage, provided an adequate shel ter f o r the s ix gu l l chicks 49 xi i i Figure Page 16. The frequency of pecks, which a chick d i rected toward a model gu l l head, was recorded fo r various periods of depr ivat ion 51 17. The frequency o f ca l l s given by a l l the Gull Grange chicks was recorded continuously throughout the depr ivat ion per iod. The regression l i ne f o r the f i r s t week old chicks is s i g n i f i c a n t l y d i f f e r e n t from the remaining weeks which do not d i f f e r s i g n i f i c a n t l y (mean-s t e s t -analysis of covariance) 53 18. The amplitude of ind iv idual ca l l s given a f t e r a sa t i a t i on feeding is shown f o r three age groups of chicks. The loudness was measured wi th a small sound in tens i t y meter 54 19. While in the h ide, I would make spot chicks to determine which of the range of 6 brood sizes were c a l l i n g . This simply shows that the cumulative c a l l i n g heard from a brood is a funct ion of the brood size 64 20. The flow diagram summarizes the sequence of events and decisions which const i tu te the control system . . . . 72 ACKNOWLEDGMENTS I am g r a t e f u l to Dr. R. H. Drent f o r h i s a s s i s t a n c e both w i th the f i e l d work and the manusc r ip t . Dr. J . R. Krebs , Dr. G. J . Krebs , and I . Robertson a l l c r i t i c a l l y read the manusc r ip t . For a s s i s t a n c e w i th the f i e l d work, I must thank K. C l a r k , C. Henderson, I . Rober tson, and T. Koe l i n k . Both Mrs. Mathews and "The McLorys" helped to make the t r i p s to town en j oyab l e . I . GENERAL INTRODUCTION Diverse factors could have influenced the evolut ion of c lutch size in nidicolous and nidifugous b i rds . There have been many attempts to explain the ul t imate determination of c lutch size (Klomp 1970). Competit ion, predat ion, food supply and brooding capacity could a l l inf luence the evolut ion of c lutch size but no unequivocal explanation has been achieved. Cody's; (1966) general theory o f c lutch s i z e , attempt-ing to impl icate a number of se lect ive f a c t o r s , is not read i ly testable because some of the assumptions, e . g . , s t a b i l i t y , are untenable. One testable hypothesis which has received considerable a t t e n t i o n , is that nidicolous birds have evolved a c lutch size corresponding to the maxi-mum number of chicks which the parents can normally ra ise successful ly (Lack 1966). Only f o r the Glaucous-winged gu l l (Larus glaucescens) is there evidence that the post - f ledging mor ta l i t y is not higher f o r super-normal broods (Vermeer 1963). Unfortunately th i s information applies only to one year which may not r e f l e c t the condit ions under which c lutch size evolved. This present study is par t of a la rger program i n v e s t i -gating the re la t ionsh ip between the foraging capacity of a parent and the maximum number of young which can be reared successfu l ly . The a b i l i t y to br ing enough food to the chicks is inf luenced by: a) A v a i l a b i l i t y of food The amount of prey taken by a predator does not increase in a l i nea r fashion with increasing prey density (Tinbergen 1960; Hoi 1ing 1966). 1 2 S e a r c h i m a g e s , h a n d l i n g t i m e , degree o f p r e y c a m o u f l a g e , and p r e y a c t i v i t y a r e some o f t h e f a c t o r s w h i c h i n f l u e n c e t h e c a p t u r e r a t e o f t h e p r e d a t o r . In s e a b i r d s , w a t e r c o n d i t i o n s and w e a t h e r may w e l l m o d i f y t h e a v a i l a b i l i t y o f t h e p r e y (Dunn 1 9 7 2 ) . b) F o r a g i n g e f f i c i e n c y F o r t h e f i r s t f o u r months a f t e r f l e d g i n g , young g u l l s may n o t f o r a g e as s u c c e s s f u l l y as o l d e r g u l l s . E x p e r i e n c e w i t h the h a b i t a t and p r e y w i l l u n d o u b t e d l y improve t h e h u n t i n g e f f i c i e n c y o f t h e e s t a b l i s h e d g u l l ; t h i s has been d e m o n s t r a t e d f o r t h e Tawny Owl ( S o u t h e r n and Lowe 1 9 6 8 ) . The number o f g u l l s f e e d i n g on a s c h o o l o f f i s h may a l s o d e t e r -m i n e , t o some e x t e n t , t h e e f f i c i e n c y o f f o r a g i n g ( S a l t and W i l l a r d 1971) . A p o i n t may be r e a c h e d where mutual i n t e r f e r e n c e w i l l d e c r e a s e f o r a g i n g e f f i c i e n c y . c ) M o r p h o l o g i c a l a d a p t a t i o n s f o r c a r r y i n g f o o d The G r e a t T i t b r i n g s s i n g l e i t e m s o f f o o d t o t h e n e s t l i n g ( T i n b e r g e n 1960) whereas t h e G l a u c o u s - w i n g e d g u l l r e t u r n s from f o r a g i n g w i t h a l a r g e q u a n t i t y o f f i s h c o n t a i n e d i n an e x p a n d a b l e p r o v e n t r i c u l u s . d) C o n t r o l s y s t e m The r e s p o n s e o f a p a r e n t t o t h e s t i m u l i p r o v i d e d by t h e c h i c k s w i l l p r o b a b l y be a f f e c t e d t o some e x t e n t by d i f f e r e n c e s i n t h e i n d i v i d u a l p a r e n t s ' b e h a v i o r . F o r e x a m p l e , e v e r y p a r e n t w i l l n o t respond e q u a l l y t o t h e same i n t e n s i t y o f b e g g i n g . The amount o f f o o d b r o u g h t t o t h e c h i c k s may depend on the a c c u r a c y w i t h w h i c h t h e p a r e n t " i n t e r p r e t s " t h e s t i m u l i p r o v i d e d by t h e c h i c k s . The d i s t i n c t i o n between t h r e e and s i x b e g g i n g 3 c h i c k s may be more n o t i c e a b l e than between f i v e and s i x c h i c k s . The c o n t r o l o f p a r e n t a l f e e d i n g i n b i r d s i s n o t a w e l l documented s u b j e c t ; a few p e o p l e , h o w e v e r , have d e s c r i b e d t h e c o n t r o l and o r g a n i z a -t i o n o f p a r e n t a l f e e d i n g i n b i r d s ( v . H a a r t m a n 1 9 5 3 ; E i s n e r 1 9 6 3 ; N o r t o n -G r i f f i t h s 1 9 6 9 ) . E c o l o g i s t s have d e s c r i b e d " b e g g i n g b e h a v i o r " i n r e l a t i o n t o t h e a b i l i t y o f p a r e n t s t o p r o v i d e f o o d f o r e n l a r g e d broods o f c h i c k s ( M a r l e r 1957; K l u y v e r 1 9 6 1 ) . The p e c k i n g b e h a v i o r o f young g u l l s and t e r n s i s a w e l l documented s u b j e c t b u t t h e emphasis i s on p e c k i n g b e h a v i o r as an example o f " i n s t i n c t i v e b e h a v i o r " ( T i n b e r g e n & P e r d e c k 1 9 5 0 ; Weidman & Weidman 1 9 6 8 ; Quine & ^ u l l e n 1964; Hailman 1967; Dawkins & Impekoven 1 9 6 9 ) . G e n e r a l c o n c e p t s o f s t i m u l u s c o n t r a s t (Andrew 1964) and s t i m u l u s p r e f e r e n c e (Impekoven 1969) a r e d i s c u s s e d i n r e l a t i o n t o c h i c k v o c a l i z a t i o n s . A l t h o u g h t h e i m p o r t a n t c o n t r o l s i g n a l s o r " r e l e a s e r s " ( M a r l e r 1957) have been d i s c u s s e d by a u t h o r s , no one has d i s c u s s e d how t h e s e r e l e a s e r s may be p a r t o f a s y s t e m w h i c h c o n t r o l s p a r e n t a l f e e d i n g b e h a v i o r i n a q u a n t i t a t i v e way. The main o b j e c t i v e s o f t h i s c o m p a r a t i v e s t u d y were t o d i s t i n g u i s h between t h e r a t i o n r e q u e s t e d by t h e c h i c k s , i . e . , p a r e n t a l f e e d i n g s y s t e m , and the r a t i o n r e c e i v e d by t h e c h i c k s , i . e . , how t h e f e e d i n g b i o l o g y m o d i f i e s t h e r a t i o n . I t was t h o u g h t t h a t an a n a l y s i s o f t h e p a r e n t a l f e e d i n g s y s t e m and a c o m p a r i s o n o f t h e f e e d i n g b i o l o g y w o u l d p r o v i d e a p a r t i a l e x p l a n a t i o n f o r t h e d i f f e r e n c e i n p r o d u c t i v i t y between Mandarte and C l e l a n d I s l a n d s . C h a p t e r 1 d e a l s w i t h some a s p e c t s o f the g u l l ' s f e e d i n g b i o l o g y on b o t h Mandarte and C l e l a n d I s l a n d s . Most o f the s e c t i o n s a r e o n l y 4 p e r i p h e r a l l y r e l a t e d t o t h e d e s c r i p t i v e model b u t do c o n t r i b u t e t o t h e e x p l a n a t i o n f o r the d i f f e r e n c e i n p r o d u c t i v i t y between Mandarte and C l e l a n d I s l a n d s . C h a p t e r 2 i n c l u d e s a g e n e r a l and d e t a i l e d d e s c r i p t i o n o f t h e components o f t h e p a r e n t a l f e e d i n g b e h a v i o r . The g e n e r a l c h a r a c t e r i s -t i c s e v o l v e d from o b s e r v a t i o n s o f p a r e n t a l b e h a v i o r made d u r i n g t h e 1970 f i e l d s e a s o n . F o l l o w i n g t h i s s e a s o n , a q u a l i t a t i v e model was c o n s t r u c t e d and a number o f h y p o t h e s e s were f o r m u l a t e d . D u r i n g the 1971 f i e l d s e a s o n , t h e r e q u i s i t e t e s t s and c o l l e c t i o n s were c o n d u c t e d . C h a p t e r 3 c o n s i s t s o f the model i n t e g r a t i n g t h e f e e d i n g b i o l o g y and p a r e n t a l f e e d i n g c o n t r o l s y s t e m . Only t h o s e components f o r w h i c h d a t a e x i s t , have been i n c l u d e d i n t h i s d e s c r i p t i v e m o d e l . CHAPTER 1 FEEDING BIOLOGY OF THE GULLS ON MANDARTE AND CLELAND ISLANDS A. Introduction This chapter includes the common and comparative aspects of the gu l l s from the two colonies. From Ward (1972) i t i s apparent that Cleland gu l l s can successfu l ly ra i se more and heavier young than Mandarte adults. A comparison of the food and feeding habits of gu l l s from both colonies may demonstrate that the d i f f e ren t p roduct i v i t i e s are a function of the feeding biology. The f i r s t 8 sections (B to I) deal with the comparative aspects of the feeding biology fo r Mandarte and Cleland Islands. In the l a s t 2 sections (J and K), the common phys io log ical charac te r i s t i c s of d igest ive r a te , growth ra te , and growth e f f i c i ency are quant i f ied so that the e f fect s of the control system could be expressed in terms of the brood's growth. B. Study Areas 1. Mandarte Island (48 38' N, 123 17' W) During the 1970 f i e l d season, from May to August, I l i v ed on th i s small i s l and , approximately 100 m. wide and 700 m. long, located in the Georgia s t r a i t which separates Vancouver Island from the mainland of 5 6 B r i t i s h C o l u m b i a . A p a r t from t h e 2200 b r e e d i n g p a i r s o f G l a u c o u s - w i n g e d g u l l s , t h e r e a r e a l s o D o u b l e - c r e s t e d c o r m o r a n t s ( v . T e t s 1959; R o b e r t -son 1 9 7 1 ) , Song Sparrows (Tompa 1 9 6 4 ) , P i g e o n G u i l l e m o t s ( D r e n t 1 9 6 5 ) , T u f t e d p u f f i n s , B l a c k o y s t e r c a t c h e r s , and Western c r o w s , a l l o f w h i c h b r e e d on the i s l a n d . A d e s c r i p t i o n o f t h e s e b i r d s i s g i v e n i n D r e n t , v . T e t s , Tompa, and Vermeer ( 1 9 6 4 ) . The g u l l s n e s t i n t h e g r a s s y meadows w h i c h l i e w i t h i n b o u n d a r i e s o f S a l m o n b e r r y and W i l d r o s e ( F i g . 1 ) . An average o f 2 . 8 eggs p e r c l u t c h were l a y e d between 12 May and 20 June o f 1970. From 1958 t o 1 9 6 1 , t h e a v e r a g e h a t c h i n g s u c c e s s was 60% and the m i n i m a l f l e d g i n g r a t e ( i . e . , t h e number o f banded c h i c k s i n a g i v e n a r e a ) was 0 . 8 c h i c k s p e r p a i r . F o r an e x c e p t i o n a l y e a r ( 1 9 6 2 ) , the average h a t c h i n g and f l e d g i n g r a t e was 83% and 1 .7 p e r p a i r , r e s p e c t i v e l y . 2 . C l e l a n d I s l a n d (49 10' N , 126 5' W) From May u n t i l August i n 1971, I l i v e d on C l e l a n d I s l a n d . T h i s 660 by 440 met re o u t c r o p p i n g o f r o c k s u p p o r t s a v a r i e t y o f i n s h o r e and o f f s h o r e m a r i n e b i r d s w h i c h a r e d e s c r i b e d i n Campbell and S t i r l i n g ( 1 9 6 7 ) . As i t i s one o f t h e most exposed i s l a n d s on t h e w e s t c o a s t o f Vancouver I s l a n d , t h e d i s t r i b u t i o n o f v e g e t a t i o n i s l i m i t e d by t h e a c t i o n o f t h e s u r f . The 1500 b r e e d i n g p a i r s o f G l a u c o u s - w i n g e d g u l l s a r e r e s t r i c t e d t o the r o c k y p e r i p h e r y o f t h e i s l a n d ( F i g . 2 ) . An a v e r a g e o f 2 . 8 eggs p e r c l u t c h were l a y e d between 29 May and 21 June o f 1971. The h a t c h i n g and f l e d g i n g r a t e were 72% and 2 . 2 p e r p a i r , r e s p e c t i v e l y . 7 • G u l l m e a d o w s Figure 1 Mandarte Island is s i tuated in the Haro S t r a i t which l ies 6 miles east of Sydney on Vancouver Is land. 9 C. C o u r t s h i p F e e d i n g B e h a v i o r M e t h o d s : D u r i n g t h e f o u r hour i n t e r v a l s on 14 days p r i o r t o egg l a y i n g , t h e p r e p a r e n t a l o r " c o u r t s h i p " f e e d i n g b e h a v i o r was r e c o r d e d w h i l e the o b s e r v e r s a t i n a h i d e l o c a t e d i n t h e c o l o n y on C l e l a n d I s l a n d . A l t h o u g h b i n o c u l a r s were n o t n e c e s s a r y f o r o b s e r v i n g c o l o n y a c t i v i t y , t h e y d i d a i d i n d e t e r m i n i n g t h e amount and t y p e o f f o o d b e i n g f e d t o t h e f e m a l e . The d a t a d e s c r i b i n g t h e amounts o f f o o d p a s s e d between mates a r e e s t i m a t e s s i n c e t h e r e was no method f o r o b t a i n i n g a c t u a l volumes o r w e i g h t s . Having measured volumes o f f o o d f o r two f i e l d s e a s o n s , I c o u l d f a i r l y a c c u r a t e l y e s t i m a t e v o l u m e s . The volumes c o u l d e a s i l y be t r a n s f o r m e d i n t o w e i g h t s because I had c o n s t r u c t e d a graph o f volume v e r s u s w e i g h t ( t h e r e l a t i o n s h i p was n e a r l y one t o o n e ) . R e s u l t s : The f r e q u e n c y o f f e e d i n g r e a c h e s a peak a t 1800 h r s . and then d e c r e a s e s u n t i l d a r k a f t e r w h i c h f e e d i n g c e a s e s . The a v e r a g e amount o f f o o d g i v e n t o t h e f e m a l e was 1 7 . 9 ± 1 . 4 gms. ( T a b l e 1) w h i c h c o n s i s t e d o f s a n d l a n c e and gooseneck b a r n a c l e s u n t i l 4 J u n e , a f t e r w h i c h T a b l e 1 F i g u r e s R e f e r t o t h e E s t i m a t e d Number o f Grams o f Food g i v e n t o t h e Female d u r i n g C o u r t s h i p F e e d i n g on C l e l a n d . These a r e C o n s e r v a t i v e E s t i m a t e s E s t i m a t e s o f t h e Q u a n t i t y o f t h e C o u r t s h i p F e e d i n g Sample s i z e = 92 SD = 1 3 . 3 Mean = 1 7 . 9 SE = 1 . 4 V a r i a n c e = 177.1 10 t i m e t h e f o o d c o n s i s t e d l a r g e l y o f s a n d l a n c e ( T a b l e 2 ) . No f e m a l e s were f e d a f t e r t h e c o m p l e t e c l u t c h o f eggs had been l a y e d . T a b l e 2 These Data Show t h e P e r c e n t a g e o f Food Types w h i c h were g i v e n t o t h e Female by t h e Male on C l e l a n d . The Sample S i z e , (n) D a y s , R e f e r s t o the Number o f O b s e r v a t i o n Days f r o m w h i c h t h e Data were C o l l e c t e d C o u r t s h i p Foods n=days Date S a n d l a n c e B a r n a c l e s F i s h T o t a l 4 May 20 - May 3 0 , 1971 14(48%) 9(31%) 6(21%) 29(100%) 3 May 31 - J u n e 3 , 1 9 7 1 12(57%) 6(29%) 3(14%) 21(100%) 8 June 4 - June 1 3 , 1971 66(82%) 0 14(18%) 80(100%) D i s c u s s i o n : The b r e e d i n g season o f a l l b i r d s i s c o r r e l a t e d w i t h t h e s e a s o n a l abundance o f f o o d ( M a r s h a l l 1951; N e l s o n 1 9 6 4 ; H a r r i s 1969; P e r r i n s 1 9 7 0 ) . The p r e d o m i n a n t f o o d o f t h e young g u l l c h i c k s on C l e l a n d i s s a n d l a n c e so t h e s e a s o n a l abundance o f s a n d l a n c e i s t h e u l t i m a t e f a c t o r d e t e r m i n i n g t h e b r e e d i n g s e a s o n o f t h e G l a u c o u s - w i n g e d g u l l . The movement o f s a n d l a n c e i n t o t h e n e i g h b o u r i n g w a t e r s may be one o f t h e p r o x i m a t e f a c t o r s w h i c h s t i m u l a t e s egg l a y i n g (BCihler 1 9 6 4 ) . A l t h o u g h t h e e c o l o g y o f t h e s a n d l a n c e i n t h e B r i t i s h C o l u m b i a w a t e r s i s n o t d o c u -mented many o f t h e f i s h e r m e n l i v i n g i n T o f i n o have some knowledge o f t h e s a n d l a n c e movements n e a r C l e l a n d I s l a n d . G e n e r a l l y , t h e numbers o f s a n d -l a n c e i n c r e a s e i n t h e i n l a n d w a t e r s f r o m A p r i l t o J u n e . Not o n l y t h e a v a i l a b i l i t y o f f o o d b u t t h e q u a l i t y o f f o o d may be a p r o x i m a t e f a c t o r i n i t i a t i n g egg l a y i n g ( R i d p a t h 1971) . P r i o r t o c l u t c h 11 commencement, goose neck b a r n a c l e s c o n t r i b u t e t o a l a r g e p e r c e n t a g e o f t h e f o o d ( T a b l e 2 , p . 10) t h a t t h e f e m a l e r e c e i v e s from t h e m a l e . A f t e r 4 J u n e , t h e f e m a l e s w i t h an i n c o m p l e t e c l u t c h a r e f e d o n l y f i s h ( F i g . 3 ) . The a d d i t i o n a l c a l o r i e s , r e c e i v e d t h r o u g h t h e s u b s t i t u t i o n o f i n t e r t i d a l f o o d f o r s a n d l a n c e , may p r o v i d e t h e r e q u i s i t e c a l o r i e s f o r egg p r o d u c t i o n . A p p r o x i m a t e l y 5% o f t h e f e m a l e s d i d l a y eggs b e f o r e 4 June when t h e t r a n -s i t i o n from a mixed ( f i s h and i n t e r t i d a l f o o d ) t o a u n i f o r m d i e t ( f i s h ) o c c u r r e d , b u t t h e s e f e m a l e s may have r e c e i v e d a u n i f o r m d i e t b e f o r e t h e m a j o r i t y o f t h e f e m a l e s . The e a r l y p a i r s c o u l d p o s s i b l y have been o l d e r b i r d s who were more f a m i l i a r w i t h t h e f o r a g i n g a r e a s . T h i s i s i n d i r e c t e v i d e n c e f o r t h e h y p o t h e s i s t h a t t h e s t a r t o f t h e b r e e d i n g season o f t h e G l a u c o u s - w i n g e d g u l l on C l e l a n d I s l a n d i s d e t e r m i n e d by t h e s e a s o n a l abundance o f s a n d l a n c e i n t h e i n l a n d w a t e r s . C o u r t s h i p f e e d i n g c o u l d have two f u n c t i o n s : 1) enhance t h e p a i d bond ( i . e . , t r a d i t i o n a l l y c o n s i d e r e d t o be an i m p o r t a n t f u n c t i o n , 2) s u p p l y e n e r g y f o r t h e p r o d u c t i o n o f t h e e g g s . The enhancement o f t h e p a i r bond w o u l d be i m p o r t a n t f o r a s p e c i e s i n w h i c h b o t h s e x e s p a r t i c i p a t e i n p a r e n t a l f e e d i n g (Lack 1 9 4 0 ) . C o p u l a -t i o n u s u a l l y f o l l o w s a c o u r t s h i p f e e d i n g i n t h e G l a u c o u s - w i n g e d g u l l s o t h a t t h e c o u r t s h i p f e e d i n g i s p r o b a b l y an i m p o r t a n t i n f l u e n c e on t h e s t a b i l i t y o f t h e p a i r b o n d . The amount o f f o o d g i v e n t o t h e f e m a l e w h i l e she i s l a y i n g h e r eggs ( a p p r o x i m a t e l y 2 days between s u c c e s s i v e eggs) i s a s i g n i f i c a n t p r o p o r t i o n o f t h e c a l o r i e s n e c e s s a r y t o p r o d u c e each e g g . In a H e r r i n g GOOSENECK BARNACLES SANDLANCE SANDLANCE 4 70 DAYS IN JUNE 15 Figure 3 I t is evident that a uniform d ie t o f sandlance in the courtship feeding is coincident wi th the commencement of egg-laying. Pr io r to 4 June both gooseneck barna-cles and sandlance are given to the female but a f t e r 4 June the feedings consist e n t i r e l y of sandlance. 13 g u l l ' s eggs t h e r e a r e 1 . 4 K c a l . / g r a m ( D r e n t 1967) so t h a t the t o t a l c a l o r i e s i n one egg may be 94 grams x 1 . 4 K c a l . / g r a m = 1 4 1 . 6 K c a l . I f one assumes t h a t the f e m a l e i s f e d t w i c e a day ( T a b l e 3) i n t h e two-day i n t e r v a l between s u c c e s s i v e e g g s , t h e t o t a l c a l o r i c c o n t e n t o f t h e e x t r a f e e d i n g w o u l d be 108 K c a l . , (2 f e e d i n g s p e r day x 17 grams p e r f e e d i n g x 1 . 6 K c a l . / g r a m o f f i s h x 2 day i n t e r v a l = 108 K c a l . ) . Royama (1966a) and Krebs (1970) have s u g g e s t e d t h a t c o u r t s h i p f e e d i n g may f u l f i l t h i s need i n a d d i t i o n t o t h e enhancement o f t h e p a i r b o n d . D. Volume o f Food i n t h e Stomach o f I n c u b a t i n g A d u l t s M e t h o d s : On Mandarte and C l e l a n d I s l a n d s I c a u g h t i n c u b a t i n g a d u l t s i n c l o v e r l e a f t r a p s made f r o m one i n c h c h i c k e n w i r e ( F i g . 4 ) . The t r a p was p l a c e d o v e r t h e eggs so t h a t t h e e n t r a n c e l a y on t h e runway o f t h e i n c u b a t i n g a d u l t ( t h e runway i s t h e r o u t e t a k e n by t h e a d u l t as i t a p p r o a c h e s t h e n e s t ) . A f t e r t h e b i r d had e n t e r e d t h e t r a p and s t a r t e d t o i n c u b a t e , I would l e a p from t h e c o v e r and t h e t r a p p e d a d u l t w o u l d be t o o s t a r t l e d t o e s c a p e f r o m t h e t r a p . I f t h e b i r d was f u l l o f f o o d , t h e f o o d w a s , i n most c a s e s , i m m e d i a t e l y r e g u r g i t a t e d . I w o u l d then i n s e r t a s p e c i a l l y d e s i g n e d d e s s e r t spoon i n t o t h e t h r o a t o f t h e g u l l so t h a t any u n r e g u r g i t a t e d f o o d c o u l d be removed from t h e s t o m a c h . The f o o d samples were p l a c e d i n p l a s t i c bags w h i c h were t a k e n back t o t h e camp where w e i g h t , v o l u m e , and c h a r a c t e r i s t i c s o f t h e f o o d were r e c o r d e d f o r each g u l l ' s r e g u r g i t a t i o n . 14 Table 3 The Upper Part o f the Table Shows that a Courtship Feeding Occurs, on the Average, every 10 Hours. The Lower Portion o f the Table shows that Feeding Frequency Reaches a Peak in the Early Afternoon and then the A c t i v i t y Subsides Courtship Feeding Frequency Before Clutch Commencement No. Feedings Pairs Observed Hours of Observation Time Between Feedings 20 9 24 3 . 5 9 . 4 28 18 39 4 8 . 6 29 10 39 4 1 5 . 6 29 29 40 4 1 5 . 5 31 21 26 3 4 . 3 1 9 26 3 8 . 7 3 13 26 4 8 . 0 4 17 33 4 7 . 8 5 21 33 4 6 . 3 6 14 33 4 9 . 4 6 6 33 2 1 1 . 0 7 25 40 3 . 5 5 . 6 8 12 40 4 1 3 . 3 10 13 26 4 8 . 0 10 7 26 4 1 4 . 9 12 10 33 4 1 3 . 2 13 9 26 4 11 .6 Courtship Feeding Frequency During the Day  Time Sample Size Mean Feeding Freq. /Pai r 0800 2 . 0 5 5 0900 6 . 0 8 3 1000 6 .115 1100 9 .115 1200 7 .120 1300 6 . 1 3 0 1400 7 . 1 6 0 1500 8 .120 1600 6 . 1 3 0 1700 2 .170 15 Figure 4 The t rap is constructed from 1 inch chicken wire cut in to 3' x 6' sect ions. The wire is formed in to a bi- lobed s t ructure with a wire top. Wire handles are attached to the bottom so that the traps can be secured wi th rocks. 16 Results: Mandarte gu l ls contained a greater var ie ty of foods than Cleland gu l ls which contained only sandlance. Both Herring and garbage contr ibuted a large proport ion of food volumes on Mandarte (Table 4 ) . Both the mean weight of the stomach contents and the g u l l ' s body were greater on Cleland Island than on Mandarte but the di f ferences were not s i g n i f i c a n t (Table 4 ) . Table 4 Incubating Adults were Caught in the Clover- leaf Traps Used on Both Mandarte and Cleland. Although the Cleland Birds are S l i g h t l y Heavier and Contained More Food, the Differences Are Not S ign i f i can t ( t - t e s t P=.05). I t i s Obvious, From the Second Part of This Table, that Mandarte Adults U t i l i z e a Greater Var iety of Food Sources Food of Incubating Adults ,Mandarte Cleland weight (gms) t o t a l 1372 n= 37 mean 37 t o t a l 540 n= 12 mean 45 volume (cc) 1191 37 32 470 13 36 adul t weight (gms) 54975 50 1099 2716 24 1131 Occurrence o f Food Types Found in the Incubating Adults Mandarte % Cleland % Herring Garbage 65 29 I n t e r t i d a l 6 0 Sandlance 0 100 17 D i s c u s s i o n : The Mandarte g u l l s u t i l i z e a g r e a t e r number o f f o o d s o u r c e s than t h e C l e l a n d g u l l s : t h i s may i n d i c a t e t h a t Mandarte g u l l s a r e b e i n g f o r c e d t o e x p l o i t a d d i t i o n a l f o o d r e s o u r c e s because t h e r e i s a g r e a t burden on the e x i s t i n g n a t u r a l f o o d s u p p l y . U n f o r t u n a t e l y I am unknowledgeable r e g a r d i n g t h e f o o d a v a i l a b i l i t y t h r o u g h o u t t h e n e s t i n g phase o f t h e c h i c k s . E. Volume o f Food i n t h e Stomach o f C h i c k s M e t h o d : U s i n g a spoon o r my f i n g e r , I scooped t h e f o o d from t h e stomach o f the c h i c k s i n t h e meadows. None o f t h e c h i c k s were i n j u r e d because o f t h i s t r e a t m e n t ; I have removed f o o d f r o m t h e same c h i c k s t h r o u g h o u t t h e i r n e s t l i n g l i f e . The f o o d samples were c o n t a i n e d i n egg c a r t o n s u n t i l they were t a k e n t o t h e camp where t h e v o l u m e , w e i g h t , and c h a r a c t e r i s t i c s o f t h e f o o d were r e c o r d e d f o r each c h i c k w e i g h e d . R e s u l t s : The w e i g h t o f f o o d i n t h e stomach i n c r e a s e s w i t h t h e w e i g h t o f t h e c h i c k ( F i g . 5). A l t h o u g h C l e l a n d b i r d s appeared b e t t e r f e d , n e i t h e r t h e s l o p e n o r t h e mean o f t h e r e g r e s s i o n l i n e s d i f f e r between Mandarte and C l e l a n d I s l a n d s . A l t h o u g h t h e C l e l a n d c h i c k s r e c e i v e o n l y s a n d l a n c e t h r o u g h o u t t h e i r growth p e r i o d , the c o m p o s i t i o n o f f o o d s g i v e n t o t h e Mandarte c h i c k s changes d u r i n g t h e c h i c k s t a g e ( T a b l e 5, 6; F i g . 6, 7 ) . D i s c u s s i o n : The l a r g e v a r i a n c e i n t h e d a t a f r o m t h e two i s l a n d s masks t h e d i f f e r e n c e between t h e amounts o f food e a t e n by t h e c h i c k s on 18 5 I CO •8 e o ci 50 40 30" .20: 10 0 MANDARTE Y= 0.016 X «• 5.305 0 CLELAND Y= 0.027X-1.647 o o - t : -o_-~- r=.28 d.f- 105 o o o o « ^ « / ~ v"> ,. o ,o o oo -f-CrO o f-Q>0 O O O O 8 200 400. 600 . $00 WEIGHT OF CHICKS (gm) 1000 1200 Figure 5 This f igure demonstrates that the food ra t ion is proport ionate to the age of the chick. Although no s i g n i f i c a n t d i f ference exis ts between Mandarte and Cleland in th i s respect (covariance t e s t ) , Cleland chicks appear to receive more food than Mandarte chicks, e = Cleland, o = Mandarte. 19 Table 5 This Table Shows the Percentage o f Times t h a t Var ious Items Were Present i n the Stomachs o f Chicks . There i s a Trend f o r More V a r i a t i o n i n Food as the Chicks get O lder . On Cle land No Such Trend E x i s t s Stomach Contents o f Chicks which were Caught on the Mandarte Colony  Weight Class o f Chicks Mean Volumes Fish I n t e r t i d a l Garbage 0-200 grams 7.2 cc 89% 11% 0% 201-300 grams 9.3 cc 91% 9% 0% 301-525 grams 11.4 cc 71% 20% 9% 525-625 grams 15.7 cc 65% 30% 6% >625 grams 21.4 54% 32% 14% Table 6 The F igure w i t h the Brackets are Percentages. The Occurrence o f Garbage i s Co inc iden ta l w i t h the Peak o f Hatch ing on the I s l a n d . Supernormal Broods Did Not Appear t o Receive More Garbage Than the Normal Broods. The Numbers Under the Heading o f 'Week' Refer t o Consecut ive Weeks o f Observat ions Beginning on 5 J u l y Food Brought to the Chicks on Mandarte I s l a n d (From Observa t ions) Week Her r ing Other F ish Blenny Sandlance Garbage I n t e r t i d a l 1 23(58) 11(28) 1 5 0 0 2 21(84) 3(12) 0 0 0 1 3 6(22) 7(26) 0 0 13(50) 1 4 13(32) 3( 8) 0 0 24(60) 0 6 10(31) 4(13) 0 2 16(50) 0 20 T 1 i 1 r July 5 13 19 26 August 7 Figure 6 The re lat ionsh ip between the hatching d i s t r i bu t i on and the var iety of foods given to the chicks is demonstrated in th i s f i gure. The increase in the amount of garbage fed to the chicks i s correlated with the number of chicks on the i s land. These data are derived from observations of parental feeding. 21 Figure 7 The composition of the ch ick 's d ie t changes with age. The older chicks receive a greater var ie ty of food than the young chicks. There are two possible explanations f o r t h i s t rend: (1) i n a b i l i t y of parents to f u l f i l l needs of the large chicks so that addi t ional food sources are exp lo i ted ; (2) the ex is t ing natural food supply is de te r io ra t ing as the population demand increases. 22 t h e two i s l a n d s . With a l a r g e r sample s i z e , t h e means may have been s i g n i f i c a n t l y d i f f e r e n t ( F i g . 5 , p . 1 8 ) . The g r e a t e r s l o p e o f t h e r e g r e s s i o n l i n e f o r C l e l a n d than f o r Mandarte may e x p l a i n t h e s l o w e r growth r a t e and l o w e r f l e d g i n g w e i g h t o f the Mandarte c h i c k s ( F i g . 8 ) . In a d d i t i o n t o t h e i n a b i l i t y o f t h e Mandarte a d u l t s t o s a t i s f y t h e needs o f the c h i c k s as t h e y a g e , t h e l o w e r c a l o r i c v a l u e o f t h e f o o d g i v e n t o Mandarte c h i c k s would c o n t r i b u t e t o t h e g e n e r a l l y l o w e r growth r a t e s and a s y m p t o t i c w e i g h t s r e c o r d e d f o r Mandarte I s l a n d ' s c h i c k s ( F i g . 8 ) . I d i d n o t measure t h e c a l o r i c c o n t e n t o f the f o o d t a k e n from t h e c h i c k s b u t a g e n e r a l s t a t e m e n t about t h e c a l o r i c c o n t e n t s o f t h e f o o d s can be o f f e r e d . As can be seen from F i g u r e 7 ( p . 2 1 ) , t h e Mandarte c h i c k s ( w e i g h i n g 301 t o 425 grams) s t a r t e d t o r e c e i v e l e s s f i s h and more i n t e r t i d a l and garbage f o o d , whereas t h e C l e l a n d c h i c k s c o n t i n u e d t o r e c e i v e f i s h . . The g a r b a g e , w h i c h c o n s i s t e d o f f r e n c h f r i e s , b r e a d and meat w o u l d u n d o u b t e d l y l o w e r t h e c a l o r i c c o n t e n t o f the f o o d g i v e n t o t h e Mandarte c h i c k s . In a d d i t i o n , the q u a l i t y and q u a n t i t y o f p r o t e i n s and i n o r g a n i c compounds would d i f f e r between h e r r i n g and r e f u s e t i p s . Whether o r not the q u a l i t y o f t h e c h i c k ' s f o o d a d v e r s e l y a f f e c t s t h e c h i c k ' s growth and s u r v i v a l i s p u r e l y s p e c u l a t i v e . The o c c u r r e n c e o f garbage i n the d i e t o f Mandarte c h i c k s i s c o i n c i d e n t a l w i t h t h e peak i n h a t c h i n g . Perhaps t h e a d u l t s a r e f o r c e d t o seek garbage when t h e a v a i l -a b l e h e r r i n g s t o c k i s not s u f f i c i e n t l y l a r g e t o m a i n t a i n a b l o s s o m i n g p o p u l a t i o n o f ravenous c h i c k s . W h i l e i n t h e h i d e , I n o t i c e d t h a t c h i c k s o f any age were p r e s e n t e d w i t h garbage a f t e r t h i s d a t e ( F i g . 6 , p . 20) so t h a t the p r e s e n c e o f garbage i s p e c u l i a r t o a d a t e and n o t t o t h e age o f W5a 950-850-750-e o Weight Asymptote <0 £ 35 Q : CD 30] o o o o 4 25-20 Daily weight increment 7 2 3 BROOD SIZE Figure 8 This f i g u r e , courtesy o f John Ward, shows the marked super io r i t y of growth f o r Cleland chicks. The growth and surv ival of a l l brood sizes is greater on Cleland than on Mandarte. e = Cleland; o = Mandarte. The maximum weight achieved by a chick p r i o r to f ledging is the weight asymptote. A ch ick 's mean da i l y weight gain is the da i l y weight increment. 24 t h e c h i c k s . I would e x p e c t t h a t t h e growth and s u r v i v a l o f t h e e a r l y broods would be g r e a t e r than t h a t o f t h e l a t e r b r o o d s . Vermeer (1963) found t h a t 1 . 7 3 young r e a c h e d f l e d g i n g age f r o m c l u t c h e s o f 3 l a i d a f t e r 2 June whereas 1 .84 young s u r v i v e d t o f l e d g i n g from broods o f t h r e e l a i d b e f o r e 2 J u n e . T h i s d e c r e a s e i n c h i c k ' s s u r v i v a l on Mandarte may be i n f l u e n c e d by a d e t e r i o r a t i o n i n t h e f o o d s u p p l y . A number o f s t u d i e s d e m o n s t r a t e t h a t t h e f o o d o f t h e a d u l t s d i f f e r s from t h a t o f t h e c h i c k s (Newton 1967; S o u t h e r n 1969; Spaans 1 9 7 0 ) . Royama (1971) found t h a t the p r o p o r t i o n o f s p i d e r s w h i c h a p p e a r e d i n t h e d i e t o f t h e n e s t l i n g G r e a t T i t s d i d n o t c o r r e s p o n d t o t h e p r e y ' s d e n s i t y i n t h e h a b i t a t . He c o n c l u d e d t h a t t h e a d u l t G r e a t T i t was b r i n g i n g s p i d e r s i n d i s p r o p o r t i o n a t e l y l a r g e amounts because t h e y f u l f i l l e d some n u t r i t i v e r e q u i r e m e n t o f t h e n e s t l i n g s . F. F e e d i n g and F o r a g i n g Frequency w i t h R e s p e c t t o Brood S i z e M e t h o d s : From h i d e s on b o t h Mandarte and C l e l a n d I s l a n d s , I r e c o r d e d t h e f e e d i n g and f o r a g i n g a c t i v i t y o f s i x i n d i v i d u a l l y marked broods r a n g i n g from one t o s i x c h i c k s i n s i z e . O b s e r v a t i o n s were made w e e k l y from 0600 t o 2100 hours f o r f i v e c o n s e c u t i v e w e e k s . R e s u l t s : Mandarte p a r e n t s make more f o r a g i n g t r i p s than t h e C l e l a n d a d u l t s ( F i g . 9) f o r t h e same number o f f e e d i n g s . The r e s p o n s e o f t h e a d u l t s t o the a d d i t i o n a l c h i c k s does n o t d i f f e r between Mandarte and C l e l a n d ( i . e . , t h e s l o p e s a r e n o t d i f f e r e n t ) . The f r e q u e n c y o f f e e d s d e c r e a s e s w i t h t h e age o f t h e c h i c k . A l t h o u g h s u p e r n o r m a l broods 25 20 18 161 ( M.R) Y--2.03X+ 3.40 (° ) ( C . F ) Y=1.16X+ 4.84(A) 3 4 BROOD SIZE Figure 9 The number of foraging t r i p s (T) and feedings (F) associated with a brood s i ze (1 to 6) i s displayed in th is f i gu re . Mandarte (M) adults make s i gn i f i c an t l y more foraging t r i p s for the same number of feeds per foraging t r i p (Means tes t -analysis of covariance). The data points represent the da i l y mean a c t i v i t y throughout the chick stage. 26 r e c e i v e more f e e d i n g s t h a n normal b r o o d s , the number o f f e e d i n g s p e r c h i c k d e c r e a s e s as t h e b r o o d s i z e i n c r e a s e s . D i s c u s s i o n : The f o r a g i n g and f e e d i n g f r e q u e n c y w i l l n o t a c c u r a t e l y d e m o n s t r a t e the f e e d i n g c a p a c i t y o f the a d u l t because t h e c a l o r i c c o n t e n t o f each f e e d i n g i n r e l a t i o n t o t h e e n e r g y needs o f t h e c h i c k s i s the c r i t i c a l measure o f the f e e d i n g c a p a c i t y o f t h e a d u l t . U n t i l t h e energy r e q u i r e m e n t s f o r v a r i o u s brood s i z e s i s c a l c u l a t e d , one c a n n o t say t h a t t h e p a r e n t i s n o t b r i n g i n g s u f f i c i e n t f o o d because t h e number o f f e e d i n g s p e r c h i c k d e c r e a s e s as t h e brood s i z e i n c r e a s e s . A s i n g l e c h i c k may need more f o o d ( e n e r g y ) t o keep warm t h a n each c h i c k i n a b r o o d o f s i x c h i c k s a l l o f w h i c h c o n t r i b u t e t o brood warmth i f t h e y a r e h u d d l e d t o g e t h e r (Royama, 1 9 6 6 b ) . Mertens (1969) measured t h e h e a t p r o d u c t i o n a t 12 C. o f n e s t l i n g s from d i f f e r e n t l y s i z e d broods and found t h a t each n e s t l i n g from a 2 brood p r o d u c e d 0 . 2 8 7 3 K c a l . / h o u r w h i l e an i n d i v i d u a l f r o m a 12 brood produced 0.1765 K c a l . / h o u r . The i n d i v i d u a l s f r o m a 12 b r o o d w o u l d l o s e l e s s w e i g h t o v e r n i g h t so t h a t t h e n e s t l i n g s f r o m a l a r g e brood need fewer grams o f f o o d t o m a i n t a i n t h e same w e i g h t o f a c h i c k f r o m a s m a l l b r o o d . T h i s work i s more a p p r o p r i a t e t o n i d i c o l o u s s p e c i e s , s u c h as t h e G r e a t T i t , b u t n o t f o r t h e G l a u c o u s - w i n g e d g u l l c h i c k w h i c h w i l l o f t e n be s e p a r a t e d from i t s p e e r s a f t e r t h r e e days i n t h e n e s t . The c h i c k s o c c a s i o n a l l y h u d d l e b u t n o t t o t h e e x t e n t t h a t a n i d i c o l o u s s p e c i e s d o e s . The f e e d i n g f r e q u e n c y changes n o t o n l y w i t h brood s i z e b u t a l s o w i t h b r o o d a g e . The d e c r e a s e i n f e e d i n g f r e q u e n c y w i t h t h e i n c r e a s e i n t h e age o f the c h i c k means t h a t l a r g e r amounts o f f o o d a r e g i v e n t o each c h i c k 27 < a t a f e e d i n g . The young c h i c k demands a s m a l l p r e d i g e s t e d p o r t i o n o f f o o d o r i t w i l l not a c c e p t the f o o d , so t h a t many s m a l l f e e d i n g s a r e w i t n e s s e d d u r i n g the f i r s t few days o f t h e c h i c k ' s l i f e . The f o o d c a p a c i t y o f the c h i c k s i n c r e a s e s w i t h age so t h a t f e w e r i n d i v i d u a l f e e d i n g s a r e n e c e s s a r y . One s a t i a t i o n f e e d i n g o f a s m a l l c h i c k does n o t l a s t as l o n g as a s a t i a t i o n f e e d i n g o f an o l d c h i c k so t h a t f e e d i n g f r e q u e n c y would d e c r e a s e w i t h a g e . A l t h o u g h t h e f o r a g i n g and f e e d i n g f r e q u e n c i e s do not u n e q u i v o c a l l y r e f l e c t the f e e d i n g c a p a c i t y o f a p a r e n t , f o r c o m p a r a t i v e purposes t h e s e d a t a p r o v i d e some i n s i g h t i n t o t h e f e e d i n g b i o l o g y o f t h e two c o l o n i e s . The a v a i l a b i l i t y o f f o o d i n the e n v i r o n m e n t s o f t h e s e two c o l o n i e s i s r e f l e c t e d i n t h e r e l a t i o n s h i p between f e e d i n g f r e q u e n c y and f o r a g i n g t r i p f r e q u e n c y . The C l e l a n d g u l l s p r e s e n t more f e e d i n g s t o the c h i c k s a f t e r one f o r a g i n g t r i p t h a n the Mandarte a d u l t s . T h i s i n d i c a t e s t h a t t h e C l e l a n d g u l l o b t a i n s more f o o d p e r f o r a g i n g t r i p t h a n t h e Mandarte g u l l . S u p p o s e d l y , t h i s i s n o t a r e f l e c t i o n o f the g u l l ' s phenotype b u t o f t h e low f o o d a v a i l a b i l i t y n e a r M a n d a r t e . The e v i d e n c e f o r l o w e r f o o d a v a i l a b i l i t y n e a r Mandarte i s p r e s e n t e d i n s e c t i o n s D, E , and G. E s s e n t i a l l y t h e q u a l i t y and q u a n t i t y o f f o o d found i n s p e c i f i c s p a t i a l and temporal p a t t e r n s may i n f l u e n c e t h e a v a i l a b i l i t y o f f o o d . I t w o u l d seem l o g i c a l f o r t h e Mandarte g u l l t o s t a y o u t l o n g e r and o b t a i n more f o o d r a t h e r t h a n t o r e t u r n w i t h an i n a d e q u a t e amount o f f o o d . The maximum amount o f t i m e t h a t a p a r e n t spends f o r a g i n g i s dependent on t h e s i z e o f i t s brood r a t h e r t h a n t h e a v a i l a b i l i t y o f t h e f o o d so t h a t a p a r e n t c o u l d r e t u r n w i t h o u t an adequate c a t c h ( S e c t i o n H). 28 G. Feeding and Foraging A c t i v i t y in Relation to the Time of Day Methods: The data were col lected while I observed the feeding a c t i v i t y of various brood s izes. Results: The Mandarte gul ls feed the chicks ac t i ve l y in the ear ly morning, af ternoon, and evening whi le the Cleland gu l ls feed t h e i r chicks regular ly throughout the day u n t i l the ear ly evening when the a c t i v i t y subsides (F ig . 10 ,a ,b ) . The feeding frequency curve has the same general shape as the foraging t r i p frequency curve. Discussion: The di f ference in the feeding a c t i v i t y periodograms between Mandarte and Cleland Island gu l ls is probably a r e f l e c t i o n of the a c t i v i t y of the p r inc ip le prey species; herr ing i s the predominate food f o r Mandarte chicks and sandlance is the p r i nc ip le food f o r the Cleland chicks. I t i s known that herr ing schools ascend at dusk and descend at dawn (Outram 1965); supposedly the herr ing move in response to the v e r t i c a l migration o f zooplankton which are the her r ing 's food. Herring do eat during the day but the feeding a c t i v i t y i s r e s t r i c t e d to the periods of a moving t i d e (B. Humphrey pers. comm.). The f i r s t and l a s t peaks of a c t i v i t y f o r the Mandarte periodogram could be explained i f the gu l ls forage as the herr ing descend and ascend, respect ive ly ; the central peak could be explained i f the gul ls forage on herr ing feed-ing in a moving t ide which occurred during the ear ly afternoon f o r the days that I observed the broods. Unfortunately I do not know exact ly where most of the gu l ls forage f o r her r ing . The sandlance are usual ly 28 6 * ^ — i 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1-5 6 7 8 9 TO 11 12 13 14 15 15 17 18 19 20 21 TIME OF. DAY (hrs.) Q. 40 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 -TIME OF DAY (hrs.) Figure 10 The frequency of foraging t r i p s and parental feedings throughout the day is shown f o r both Cleland and Mandarte. The Mandarte periodogram is characterized by three peaks of a c t i v i t y whereas the a c t i v i t y on Cleland shows no predominate peaks. . 30 found near the surface of the water during the day (N. Seymour pers. comm.) so that Cleland gul ls would not have a r e s t r i c t e d foraging time during the day. H. Ef fect o f Brood Size on the Length of the Foraging Absence Methods: Indiv idual adults were not colour banded but f ac ia l and other morphological charac ter is t i cs were used to d is t ingu ish between the mates. I recorded the time when a parent departed and returned to feed the chicks. I f the return ing adul t did not feed the ch icks, the absence was not c l a s s i f i e d as a foraging t r i p . The foraging t r i p i s , there fore , the maximum amount of time needed to obtain food f o r the chicks. Results: On both colonies, the foraging in te rva l decreased wi th increasing brood size (Table 7). The absences are of s im i la r length on Mandarte and Cleland Is lands. Discussion: The di f ferences in foraging absences between succes-sive brood sizes is 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 because the va r ia t ion i s large and the sample size i s smal l . There are, however, obvious di f ferences between normal and supernormal broods. A parent of a s ix brood may be more motivated to return to the t e r r i t o r y than a parent of three chicks. Table 7 No S ign i f i can t Difference Exists Between the Foraging Absences For Normal and Supernormal Broods on Both Mandarte and Cleland. There is an Obvious Trend f o r the Parents of Supernormal Broods to Decrease the Foraging Absence Foraging In terva ls Brood Size Normal (1-3) Supernormal (4-6)  Cleland N = 42 N = 86 AV = 122 AV = 93 VR = 7353 VR = 4162 SD = 85.7 SD = 64.5 SE = 13.2 SE = 6.9 95% C L . = 96-149 95% C L . = 79-106 Mandarte N = 22 N = 35 AV = 156 AV = 91 VR =26097 VR = 907 SD = 161.5 SD = 30.1 SE = 34.4 SE = 5.1 95% C L . = 89-213 95% C L . = 81-101 32 Both on Mandarte and C l e l a n d , the p a r e n t s f o r a g e f o r s i m i l a r l e n g t h s o f t i m e . There a r e two p o s s i b l e e x p l a n a t i o n s f o r t h e s i m i l a r i t y i n f o r a g i n g a b s e n c e s : 1) e q u a l a v a i l a b i l i t y o f f o o d 2) s i m i l a r r e s p o n s e o f p a r e n t t o b e g g i n g c h i c k s . The p a r e n t ' s absence f r o m the c h i c k s i s c o n t r o l l e d by c e r t a i n a s p e c t s o f t h e c h i c k s ' b e h a v i o u r b u t i s l i t t l e m o d i f i e d by t h e a v a i l a b i l i t y o f f o o d i n t h e e n v i r o n m e n t . When o u t on a f o r a g i n g t r i p p a r e n t s o f s u p e r n o r m a l broods a r e more drawn t o t h e t e r r i t o r i e s than a r e p a r e n t s o f normal b r o o d s . The maximum absence i s , t h e r e f o r e , a f u n c t i o n o f the b r o o d ' s b e h a v i o u r r a t h e r t h a n a f u n c t i o n o f f o o d a v a i l a b i l i t y , t h u s Mandarte and C l e l a n d a d u l t s have a s i m i l a r f o r a g i n g a b s e n c e . I f both Mandarte and C l e l a n d g u l l s a r e e q u a l l y m o t i v a t e d t o r e t u r n t o t h e t e r r i t o r y , and i f more f o o d i s a v a i l a b l e t o the C l e l a n d g u l l , t h e Mandarte g u l l must make more t r i p s t o o b t a i n t h e same amount o f f o o d as t h e C l e l a n d g u l l d o e s . The d a t a i n d i c a t e t h a t Mandarte g u l l s do make more f o r a g i n g t r i p s p e r f e e d i n g t h a n do C l e l a n d g u l l s . I . Growth Rate and E c o l o g i c a l Growth E f f i c i e n c y M e t h o d s : From s i x d i f f e r e n t n e s t s s i x c h i c k s were removed when t h e young were one t o f o u r days o l d . For t h e f i r s t w e e k , t h e c h i c k s were m a i n t a i n e d a t n i g h t i n a c a r d b o a r d box p l a c e d i n s i d e t h e "Green c a b i n " and o u t s i d e i n a c a g e , t h e " G u l l G r a n g e " , d u r i n g t h e d a y . A f t e r t h e f i r s t w e e k , t h e c h i c k s were l e f t o u t s i d e i n t h e G u l l G r a n g e . I n d i v i d u a l l y marked 33 chicks were weighed each morning p r i o r to a feeding and each night a f t e r a feeding. The t ime, when a recorded weight of a spec i f ied food was given to the chicks, was also recorded. They refused to eat cod f i l l e t s a f t e r nine days so that f o r the remaining 25 days the chicks received port ions o f herr ing purchased at the f i s h camp. Ecological grov/th e f f i c iency is the ca lo r i c value o f the biomass increment per un i t o f time divided by the ca lo r ic value of the food i n -gested by a given organism. This measure re f l ec ts the : 1) cost of metabolic maintenance 2) e f f i c iency of food u t i l i z a t i o n . The ca lo r i c value of cod is 0.84 Kcal./gram of f resh weight and the ca lo r i c value of Pac i f i c herr ing i s 1.6 Kcal./gram of f resh weight (Barley pers. comm.). The ca lo r i c value of a maturing gu l l changes s l i g h t l y during the growth (Br isbin 1962) so the mean ca lo r i c content of 1.94 Kcal./gram of l i v e weight was used in the ca lcu la t ions . These values were used to determine the ecological growth e f f i c iency f o r the s ix chicks. Results: The regression l ines f o r growth and E.G.E. (ecological growth e f f i c i ency ) are shown on Table 8 and Figure 1 1 , respect ive ly . The regression on growth e f f i c iency has a negative slope and the average e f f i c iency i s 26%. The chicks lose 1% of t h e i r body weight per hour over a 10 hour n ight (Table 8 ) . Discussion: Br isb in (1962) determined the growth e f f i c i ency f o r the Herring gu l l (Larus argentatus). My data f o r the Glaucous-winged gu l l (Larus glaucescens) d i f f e r from his data in two main ways: 34 Table 8 The Mean Daily Growth, Ecological Growth Ef f ic iency and Weight Loss Over a 10 Hour Night is Presented in This Table. The Growth Rate of These Hand-reared Chicks (Y=26X=47.1 r=.98 d.f .=225) is Lower Than That of the Field Chicks. RA=Running Average Mean Growth, Growth E f f i c iency , and Weight Loss Over Night Age Mean Weight Mean Growth 3-Point RA Mean Weight Loss Ef f ic iency % % Over Night % 1 83 42 13 2 95 59 46 8 3 101 37 53 9 4 108 63 45 12 5 124 36 52 8 6 134 57 44 10 7 160 38 56 7 8 152 74 56 10 9 162 57 58 10 10 181 44 50 10 11 203 48 44 7 12 227 41 38 11 13 254 25 30 8 14 282 25 24 8 15 311 21 24 11 16 329 25 23 11 17 355 22 23 11 18 390 21 21 13 19 422 20 19 13 20 457 17 18 13 21 484 16 18 10 22 512 22 17 13 23 555 14 19 12 24 586 22 16 11 25 628 11 16 11 26 645 15 12 11 27 678 10 12 11 28 698 12 11 12 29 732 11 13 12 30 758 16 13 11 31 788 11 15 10 32 810 19 17 9 33 853 20 17 9 34 876 12 15 11 35 883 14 13 10 36 888 14 15 10 37 917 16 10 12 38 946 9 12 12 39 927 12 10 9 40 925 9 9 Mean 26 10 35 o 0 10 20 30 40 DAYS SINCE HATCHING Figure 11 Both the regression l i n e and the l i ne through a three point running average are shown on th is f i g u r e . The three point running average gives a bet ter b io log ica l descr ip t ion of the abrupt changes in the e f f i c iency trends. The regression equation is y = 52 - 1.3x. The dotted l i ne passing through the 3-point running average is f i t t e d by eye. 36 1) Peak e f f i c iency of L. glaucescens, approximately 55%, i s higher than that of L. argentatus whose peak is 40%. 2) Peak e f f i c iency occurred a f t e r approximately 18% o f the growth period f o r the Glaucous-winged gul l but occurred a f t e r 30% of the growth period f o r the Herring g u l l . Behle and Goates (1957) recorded increases in the rec ta l temper-atures of maturing Ca l i f o rn ia gu l l chicks (Larus c a l i f o r n i c u s ) up to 21 days. Br isbin c i ted th i s study as corroborat ion f o r his explanation f o r the increase in the growth e f f i c iency f o r the f i r s t 15 days of the chick 's l i f e . Br isbin says tha t the L. ca l i fo rn icus ch ick 's l a b i l e condi t ion of thermoregulation s t a b i l i z e d a f t e r 12-13 days (the time when the peak e f f i c i ency occurs) but Behle and Coates s ta te that the L. ca l i f o rn icus chicks thermoregulatory a b i l i t y increased up to 21 days a f t e r which they have no data. Apparently Br isb in misread the statement by Behle and Goates; Br isb in has not s a t i s f a c t o r i l y explained the increase in growth e f f i c iency wi th respect to increasing thermoregulatory capaci ty. On Figure 1 1 , I have shown a regression l i n e and a l i n e f i t t e d through the points of a three point running average. The l i n e f i t t e d through the three point running average provides a be t te r b io log ica l descr ip t ion of the data than the negative regression l i n e . The s l i g h t increase in growth e f f i c iency during the f i r s t week may be corre lated wi th changes in the surface to volume r a t i o ; i t may be d i f f i c u l t to maintain body temperature i f the surface area is large i n r e l a t i o n to body volume. As the chick grows, the r a t i o decreases so that more energy is transformed in to t issue rather than used f o r thermoregulat ion. 37 The benef i t of a decreasing surface to volume r a t i o is outweighed by the changes in morphology, physiology, and behavior a l l of which contr ibute to a decrease in growth e f f i c iency a f t e r the f i r s t week. The period during which the growth e f f i c iency declined dramat-i c a l l y may be accompanied by gross changes in physiology, behaviour and morphology, a l l of which require energy which is not avai lable f o r t issue format ion. Behaviour: The chicks were moved in to the Gull Grange where they were more act ive than in the Green cabin. In a natural s e t t i n g , chicks are more mobile a f t e r the f i r s t week; the abrupt decl ine in growth e f f i c i ency may be coincident wi th increased mob i l i t y which would require extra energy. Morphology: The development of feathers also begins a f t e r the f i r s t week; plumage development may u t i l i z e energy which previously w a s transformed in to body t i ssue . Physiology: As discussed e a r l i e r , Behle and Goates found tha t rec ta l temperatures, perhaps a r e f l e c t i o n of thermogenesis, increased u n t i l they reached an asymptote at 21 days. This is the point where my chicks ' growth e f f i c iency begins to s t a b i l i z e at a low l e v e l . With increasing thermoregulatory a b i l i t y , the chicks d i v e r t less energy in to body t i ssue . 38 J . Digestive Rate of the Chicks Methods: The weight o f food eaten a f t e r periods of depr ivat ion were recorded f o r chicks of d i f f e r e n t ages. The weight of food div ided by the weight of the chick corre lated wi th depr ivat ion time gives a good estimate of the d igest ive ra te . Results: Between one and ten hours of depr iva t ion , the d igest ive rate i s the same so tha t 1.8% of the food is removed from the stomach each hour (F ig . 12). Discussion: A f te r some period of depr i va t ion , chicks w i l l not eat any more than the previous shorter depr ivat ion per iod ; t h i s threshold would indicate the maximum capacity of the stomach fo r tha t p a r t i c u l a r food. Hol l ing (1966), Neish (1970), and Ware (1971) a l l have used th i s measurement of hunger to explain changes which occur in the predator 's behaviour a t d i f f e r e n t depr ivat ion l e v e l s . In the fo l lowing sections I w i l l describe how the hunger level modifies the begging behaviour o f young chicks. I t must be remembered, however, that t h i s d e f i n i t i o n o f hunger applies only to short term regulat ion of feeding. In view o f the extensive l i t e r a t u r e describing the physiology of feeding (Booth and P i t t 1968; Rodgers 1967; de Ruiter 1963; Anand, Chhina & Singh 1962; Morgane 1961) th is measure may seem rather s i m p l i s t i c but considering the d i f f i c u l t i e s associated wi th measuring hunger " i n the f i e l d " , depr i -vat ion time is a workable measure of hunger. 39 O o X x: O m . c cn % Q O e 23-20-75-10-0 ©18 ©17 33© © 23 0 ©23 29 ©23 Y=2.1X + 1.92 © 38 07 o 22 -i 1 1 1 1 i — — i 1 h 1 2 3 4 5 6 7 c? 9 10 11 12 DEPRIVATION TIME (hrs) Figure 12 This f igure shows a regression o f the volume of food eaten/ body weight on depr ivat ion t ime. This regression gives a good approximation of the d igest ive rate f o r any weight of ch i cks . The f igure beside the so l i d c i r c l e i s the sample size f o r that mean. 40 K. Summary and Discussion In sections C to H of th i s chapter, the s t r i k i n g di f ferences between the feeding biology o f Mandarte and Cleland were shown. From section C i t i s apparent that the courtship feeding could contr ibute to both the energy requirements of egg production and the energy requirements of the lay ing female. The qua l i t y of food given to the female f o r egg production may be re lated to the hatching success. The lower hatching success of the Mandarte colony may be re lated to the diverse low qua l i t y foods presented to the female f o r egg product ion. A comparison of the information presented in sections D and E shows that the type of food eaten by the adults is d i f f e r e n t from the food given to the young chicks. For the Mandarte g u l l s , t h i s comparison is more s t r i k i n g . Pr io r to the chick stage, the Mandarte adul t consumed a var ie ty of foods such as garbage, i n t e r t i d a l organisms, and f i s h , whereas f i s h predominated in the ear ly stages of the chick stage. The proport ion of garbage and i n t e r t i d a l organisms increased and the propor-t i on of herr ing decreased as the chick stage progressed in t ime. On Cleland, f i s h was given to the chicks throughout the chick stage. This d i f ference in the qua l i t y and quant i ty o f foods given to the chicks may provide a p a r t i a l explanation fo r both the i n f e r i o r growth and lower surv ival rate of the Mandarte chicks. Section E provided addi t ional information showing that the amount of food given to a chick is propor-t iona l to the ch ick 's weight. This information was necessary f o r the growth model. 41 In section F, i t was shown that Mandarte parents present fewer feedings than Cleland adults f o r the same number of t r i p s . The d i f f e r -ence in the frequency of foraging is thought to be re la ted to the q u a l i t y and a v a i l a b i l i t y of food. The sandlance are more l o c a l l y and consis tent ly avai lable than the her r ing . The frequency of feedings and foraging t r i p s throughout the day on Mandarte and Cleland is described in sect ion G. The bimodal periodogram fo r Mandarte parental feeding a c t i v i t y may be re la ted to the ascent and descent of the herr ing schools. The sandlance schools are seen more regular ly throughout the day so the Cleland per iod-ogram has a regular shape. The per iodic a v a i l a b i l i t y of herr ing f o r the Mandarte parent may in par t be responsible fo r the i n f e r i o r growth of the Mandarte chicks. The foraging absence tends to be shorter f o r larger broods. In section H i t was suggested tha t i f the a v a i l a b i l i t y of food was less near Mandarte then the adul t would re turn to the is land even i f the foraging t r i p was not maximally successful . The growth r a t e , growth e f f i c i e n c y , and d igest ive rate were studied so that a more r e a l i s t i c model of growth in r e l a t i o n to behaviour could be constructed. In conclusion, the d i f f e r e n t p roduc t i v i t i es f o r Mandarte and Cleland may be related to the feeding biology of the two colonies. CHAPTER 2 THE CONTROL AND ORGANIZATION OF PARENTAL FEEDING BEHAVIOUR A. In t roduct ion The inf luence of brood size on the foraging absences and on the foraging-feeding frequency was quan t i t a t i ve l y described in Chapter 1. In add i t i on , i t was shown that old chicks received larger feedings than young chicks. A comparison of the foraging-feeding periodograms f o r Mandarte and Cleland Island suggests tha t the foraging and feeding t r i p frequency is adjusted by the needs of the chicks but modified by prey a v a i l a b i l i t y . This chapter deals p r imar i l y wi th the questions of how the chicks communicate t h e i r hunger to t h e i r parents and how the parents respond to the signals given by the chicks. In add i t i on , th i s chapter addresses i t s e l f to the question o f how the parental control system could exacerbate the problems associated wi th brood size and food a v a i l -a b i l i t y . Two approaches were used to answer these questions. F i r s t l y , the pecking frequency, c a l l frequency, and ca l l i n tens i t y were recorded from caged chicks whose hunger level was known. Secondly, the cor re la t ion between the pecking and c a l l i n g behaviour of chicks and the associated behaviour of the adul t was quan t i f i ed . In addi t ion to these observations, 42 43 an experiment was conducted so that the re la t ionsh ip between the chick 's hunger and the parent 's a c t i v i t y was confirmed. Section B is an ethogram which describes a l l the behaviour relevant to the parental feeding. A thorough understanding of these behaviour patterns was necessary before hypotheses could be formulated. The e f f e c t of hunger on the pecking and c a l l i n g behaviour i s quant i f ied in section C. The resul ts from the f i e l d observations and experiments are described in section D. B. Ethogram of Feeding Behaviour A generalized account of parental feeding behaviour f o r the Herring gu l l i s presented by Tinbergen (1953). For the most p a r t , parental feeding in L . glaucescens is s im i la r to that of L. argentatus. 1 . Adult Routine a. Return The incoming parent f requent ly emits a long ca l l which is answered accordingly by the partner and sometimes by the other adults in the v i c i n i t y . The pai r w i l l occasional ly engage in courtship behaviour, t y p i f i e d by head f lagging and mewing, then the adul t who has not been foraging w i l l depart. Depending on the brood s i ze , there is usual ly a quick change of partners on the t e r r i t o r y . Upon the return o f an adu l t , the ravenous, peering, and bobbing chicks appear from the cover of res t -ing s i t e s . 44 b. Presentation (1) Prel iminary: (a) Feeding c a ! 1 : Nearly every feeding is preceeded by a feed-ing c a l l . This sustained low p i tch sound is charac te r i s t i c f o r ind iv idual parents so that chicks learn the ca l l o f t h e i r parents (Evans 1970). This c a l l , ca l led a "mew c a l l " by Tinbergen (1953), is given by an adul t in the oblique extension posture. (b) Obiique extension: The mew ca l l is given while the adul t is extending i t s head toward the ground (F ig . 13). (c) Lead: Concomitant wi th the feeding cal l -cum-obl ique-extension, the adul t walks quick ly back and fo r th w i th in the t e r r i t o r y whi le the chicks are en thus ias t i ca l l y fo l l ow ing . (2) Regurgi tat ion: (a) Swel l ing: With the body in the oblique extension, the adul t twis ts i t s neck from side to side as the food moves up the eosophagus. This grotesque posture i s maintained while the chicks peck at the parent 's face and b i l l . (b) Hold: The regurgi tated food is held in the b i l l f o r varying lengths of time depending on the age of the chicks. Young chicks ( i . e . , less than one week old) are rou t ine ly given food in t h i s manner but older chicks eat the food from the moment that i t i s v i s i b l e in the parent 's mouth. The food usually reaches the ground before i t i s consumed so that o ld chicks eat the food from the ground. 45 Figure 13 The parent is shown regurg i ta t ing some f i s h to a 40 day old chick. While begging fo r food, an older chick always assumes th i s crouched submissive posture. HUNGRY CALLS 2500 1500 , / .» ' ' • • " • ' . V —\ Q _ t « 500- (a) • (b) >— »-1 i i i ) 2 3 4 5 6 7 8 9 10 11 12 1 8 e 2500-TIME (SEC.) 1500 - 1 ^.^ /-/-.—-500- (c) i _ 1 1 1 1 •(d) — i 1 \ 1 1 1 1 • 0 1 2 3 4 5 6 7 8 9 10 11 12 TIME (SEC.) I O 2500 1500 500 CALLS AFTER A FEEDING (a) (b) 2500 1500 500 \ 2 3 4 5 TIME (SEC.) 8 ~9 10 Tl 12 (c) (d) 2 3 4 5 TIME (SEC.) 8 — t — 9 — i 1 1— 10 11 12 Figure 14 No obvious di f ferences can be seen between the ca l l s given before and a f t e r a feeding. A more thorough inves t iga t ion may show that these ca l l s d i f f e r in p i tch and dura t ion . 47 (c) Point : A parent wi th young or old chicks holds the oblique extension posture over the regurgi tated food u n t i l i t i s eaten or ignored by the chicks. (d) Manipulate: I f the chicks do not eat a l l the food, the adul t w i l l pick up some food and emit a mew c a l l . The frequency o f manipulation fol lowed by ingest ion increases u n t i l a l l the remaining food is consumed. A large f i s h w i l l be broken apart by the parent i f the chicks cannot handle the f i s h . 2. Chick Routine a. Begging Call The charac te r i s t i c "peer" (F ig . 14) is found in many Larus gu l l chicks (Hailman 1967; Impekoven 1969). I f the chicks are s i t t i n g or standing qu ie t l y on the t e r r i t o r y , the return of a parent w i l l e l i c i t peers from the chicks. The hunger level a l te rs the charac te r is t i cs o f t h i s c a l l . Chicks w i l l ca l l to a parent which has been on the t e r r i t o r y f o r an extended per iod, i f the chicks are hungry, but these ca l l s are not usual ly accompanied by pecking. b. Pecking Behaviour The motor patterns of th is behaviour have been well documented by Hailman (1967). The pecking behaviour of L. glaucescens is not q u a l i t a t i v e l y d i f f e r e n t from Hailman's descr ipt ion of L. a r t r i c i l i a pecking behaviour. The chick pecks the b i l l , face, or body of the parent i f the chick is hungry and the frequency of pecks del ivered to the b i l l changes wi th the hunger l e v e l . 48 c. Bobbing During the f i r s t week, the chick is s ta t ionary whi le i t ca l l s and pecks but a f t e r t h i s time the ch ick 's body, p a r t i c u l a r l y the head and neck, moves up and down in a pumping fashion as the chick peers and pecks. Ful ly feathered chicks assume the submissive posture (Tinbergen 1953) (F ig . 13) and pump the head v e r t i c a l l y from a hor izontal plane. d. Following The patterns described above are a l l exercised as the chicks pursue the parent moving w i th in the t e r r i t o r y . C. Control Signals 1 . General Methods The only feas ib le way to regulate the a c t i v i t i e s of the chicks used f o r experiments was to ra ise the chicks in a cage, the "Gull Grange". The dimensions and charac ter is t i cs of t h i s cage are shown in Figure 15. The growth and behaviour o f s ix hand-reared gu l ls (Hermann, Ivan, Timmy, Gunter, Lenny, and Archy) was studied and recorded from hatching to the age of 6 weeks. These experiments were a l l based on the assumption that the amount of food in the stomach influenced the ch ick 's begging behaviour. Essen-t i a l l y , the experiments consisted of tests of behaviour f o r d i f f e r e n t depr ivat ion t imes, i . e . , the time since a s a t i a t i o n feeding. 49 Figure 15 The Gull Grange, a 9' x 3' x 4 ' wire cage, provided an adequate shel ter f o r the s ix gul l chicks. 50 2. Pecking at the B i l l of a Model Method: I s tu f fed a dead gu l l found on the gul l colony and presented the b i l l to the chicks which had a l l been deprived o f food f o r some speci f ied t ime. The number of pecks given in one minute by one chick was recorded. Results: Figure 16 gives the regression of pecks/minute on depr ivat ion t ime. General ly, the frequency o f pecks increases w i th depr ivat ion t ime. Discussion: Evans (1970), Hailman (1967), Woodgush (1968), and Weidman & Weidman (1958) have a l l reported increased pecking a f t e r dep r i -vat ion of food, but none of these people suggest tha t the frequency o f pecking might inf luence the adu l t ' s behaviour. The f i e l d evidence is presented l a t e r . 3. Frequency of Calls Given by the Chicks Methods: I recorded the number of c a l l s given wi th 39 successive f i v e minute in te rva ls a f t e r a sa t i a t i on of a l l the chicks. Three hours of depr ivat ion was chosen because t h i s time is normally the maximum period of depr ivat ion experienced by the f i e l d chicks. Usually the ca l l s given in a l ternate f i v e minute in te rva ls were recorded f o r the f i r s t run , then the ca l l s given in the intervening in te rva ls were recorded on the second run. For example, the ca l l s given w i th in the f i r s t , t h i r d , f i f t h and seventh, e t c . , f i v e minute in te rva ls a f t e r a sa t i a t i on feeding were t a l l i e d , 51 Figure 16 The frequency of pecks, which a chick di rected toward-a model gu l l head, was recorded f o r various periods of depr iva t ion. 52 then the ca l l s emitted during the second, f o u r t h , s i x t h , and e igh th , e t c . , f i v e minute periods were recorded a f t e r a second sa t i a t i on feeding. This experiment was done f o r f i r s t , t h i r d , and f i f t h week-old chicks. Results: Both the slopes and means of the regressions of ca l l frequency on depr ivat ion time d i f f e r between the three age groups (F ig . 17). Discussion: Beer (pers. comm. wi th Hailman '1967') thinks that the peer ca l l s are important in e l i c i t i n g a regurg i ta t ion from L. b u l l e r i parents but Hailman does not th ink tha t the peer e l i c i t s a feeding; nei ther Beer nor Hailman present data to substant iate t h e i r c la im. Eisner (1963) observed that her chicks cal led more f requent ly a f t e r depr iva t ion . The s ign i f icance of these data w i l l become more apparent a f t e r I have presented the corroborat ing f i e l d evidence. 4. In tens i t y of Indiv idual Begging Calls Methods: At in te rva ls a f t e r a sa t i a t i on feeding, I measured the decibel i n t e n s i t y o f the feeding ca l l given by the chicks. The in ten -2 s i t y o f ca l l s (measured in decibels (db) re . 0.0002 dynes/cm ) was measured wi th a Scott Type 450 sound level metre held one foot from the chick. Results: A d i f ference ex is ts between the asymptotic sound i n t e n -s i t y of one week and three or f i v e week old chicks but no d i f ference ex is ts between three and f i v e week old chicks. The asymptotic sound i n t e n s i t y i s reached w i th in one hour a f t e r a sa t i a t i on feeding (F ig . 18) . 300 9 9 0 10 20 30 40 FIVE MINUTE INTER VALS AFTER SATIATION Figure 17 The frequency of ca l l s given by a l l the Gull Grange chicks was recorded continuously throughout the depr ivat ion per iod. The regression l i n e fo r the f i r s t week old chicks is s i g n i -f i c a n t l y d i f f e r e n t from the remaining weeks which also d i f f e r s i g n i f i c a n t l y (means tes t -ana lys is of covariance). 54 00 —I Uj S3 o Ul Q THIRD & FIFTH WK. -.042 T Y = c?4.6-109e 5 $ . 9 % of the'variance explained n = 745 .FIRST WEEK 04ST 74.9 -9.7 e 81.$ % of the variance explained n=465 60 120 180 240 (T) TIME (min.) AFTER SATIATION FEEDING F i g u r e 18 The a m p l i t u d e o f i n d i v i d u a l c a l l s g i v e n a f t e r a s a t i a t i o n f e e d i n g i s shown f o r t h r e e a g e g r o u p s o f c h i c k s . The l o u d n e s s was m e a s u r e d w i t h a s m a l l s o u n d i n t e n s i t y m e t e r . 55 Discussion: I am not aware of anyone who has measured the i n tens i t y of begging c a l l s . The b io log ica l s ign i f icance of these f indings w i l l be discussed in the next section dealing wi th the f i e l d evidence f o r these Gull Grange experiments. 5. Qua l i ta t ive Aspects of Hunger Calls Impekoven (1969) recorded the ca l l s of chicks under d i f f e r e n t motivat ional states and found that cer ta in c a l l s , from a repetoi re of nine c a l l s , were emitted more o f t e n , depending on the mot ivat ional s tate of the chicks. One type of c a l l , i . e . , "pee" (Henty 1966), was given most f requent ly by the hungry and warm chicks whereas the "pee" was ra re ly recorded fo r a sat ia ted chick. My recordings include only the "peer" or "pee" ca l l s and I cannot readi ly detect di f ferences between the hungry and sat ia ted c a l l . Hutchison et a l . , (1968) analyzed the " f i s h c a l l " o f a tern return ing to the colony and concluded that the young terns can recognize minute di f ferences in the c a l l s d is t ingu ish ing d i f f e r -ent adu l ts . Very small d i f ferences in the durat ion and frequency of the mew ca l l may be used by the gul l chick to recognize t h e i r parent 's mew c a l l (Beer 1969; Evans 1966, 1970). Tschanz (1959) suggested that a chick could d is t ingu ish between parents on the basis of the parent 's ca l l (Pigeon gu i l lemots ) . A small but i n s i g n i f i c a n t d i f ference exis ts between the sat ia ted and hungry ca l l (F ig . 14) of the ca l l s which I recorded, but I am not j u s t i f i e d in concluding anything from these resul ts because my experimental procedure was crude. Instead of recording the ca l l s of ind iv idua l chicks before and a f t e r a feeding, I recorded the c a l l s of a l l 56 s ix chicks before and a f t e r a sa t i a t i on feeding so that the va r ia t i on between indiv iduals would probably obscure any di f ferences in durat ion and p i tch f o r sat ia ted and hungry c a l l s . I might have, the re fo re , compared the hungry and sat iated ca l l s of two d i f f e r e n t chicks so tha t ind iv idua l di f ferences in the charac ter is t i cs of the hungry and sat ia ted ca l l s may be more apparent than di f ferences caused by d i f f e r i n g motivat ional s ta tes . D. The Influence of the Control Signals on the Parent 1 . General Methods Throughout the parental period f o r the g u l l s , I observed the feeding a c t i v i t y on the meadows. The broods were s u f f i c i e n t l y close to the hide so that I could eas i ly hear and observe the parents and chicks. For the two control signals (pecking and c a l l i n g ) which I could measure from the h ide, I recorded data which would extend the f indings of the Gull Grange experiments. 2. Pecking Behaviour Method: Any b i l l pecking by the chick was recorded along wi th the descr ip t ion of the events fo l lowing the pecking bout. Results: A feeding was, on the average, preceded by 18.4 ± 2.24 pecks and no feeding fol lowed a pecking bout without at leas t 16 pecks del ivered by the en t i re brood, i r respect ive of brood s i ze . The d i f f e r -ence between a successful ( fol lowed by a feeding) and unsuccessful (not 57 fol lowed by a feeding) pecking bout is s i g n i f i c a n t (Table 9 ) . Table 9 An Average of 1 8 . 4 Pecks Preceded a Successful Feeding Whereas a Lower Number of Pecks Did Not E l i c i t a Feeding From the Parent (Unsuccessful) Field Observations of B i l l Pecking (Cleland Island)  Successful Unsuccessful N = 165 94 AV = 1 8 . 4 6 . 2 3 VR = 2 0 7 . 5 4 2 . 4 6 SD = 1 4 . 4 6 . 5 2 SE = 1 .12 0 . 6 7 95% CL = 1 6 . 1 4 - 2 0 . 5 7 4 . 9 - 7 . 6 Discussion: The cumulative pecking va lue, which i s inf luenced by the brood size and hunger of the ind iv idual ch icks, is the sum o f the pecks del ivered by a l l the chicks. I f the chicks do not peck approx-imately 18 times during a pecking bout which normally las ts f o r 60 seconds, the parent w i l l not feed the brood. The parent reacts to a spec i f i c cumu-l a t i v e pecking value rather than to the pecking frequency o f each chick. One chick g iv ing . 3 4 pecks/sec f o r 60 seconds w i l l receive a feeding as w i l l 2 chicks each g iv ing .17 pecks/sec f o r 60 seconds. Although the members of the 2 brood may not be as hungry as the s ingle brood, both broods would receive a feeding because they have pecked 20 t imes. Assuming 58 tha t the parent returns wi th a standard quant i ty of food, one can see tha t less food would be eaten by each chick in a 2 brood. This mechanism, there fore , may ass is t in the d i s t r i b u t i o n of food in the brood. Because the cumulative pecking value which e l i c i t s a feeding is the same f o r a l l brood s izes , t h i s mechanism does not provide s t imu l i which would inf luence the foraging frequency. The pecking behaviour system does provide f o r an adjustment of feeding frequency. 3. Cal l ing Behaviour a. Call Frequency Before and Af te r a Feeding Methods: Whenever a chick c a l l e d , I recorded the number of ca l l s given by the chick in a u n i t o f time measured on a stop-watch. Voca l i -zations a f t e r a feeding were also recorded. Results: As wi th pecking bouts, ca l l frequency was higher p r i o r to a feeding than before an unsuccessful feeding attempt in which the parent ignored the chick. A f te r a feeding the ca l l frequency declined to a s i g n i f i c a n t l y lower level than before the feeding (Table 1 0 ) . Discussion: These f i e l d data and the Gull Grange data support my hypothesis that the hunger of the chicks controls the feeding frequency. In addi t ion to the pecking behaviour, c a l l i n g behaviour may cont r ibute s t imu l i which may provoke a feeding. Cal l ing behaviour is more important f u n c t i o n a l l y , before and a f t e r a feeding. The high frequency c a l l i n g may st imulate the parent to present i t s b i l l which is pecked by the chicks. I f the parent has regurg i ta ted a l l i t s food during a feeding, i t i s loathe 59 to present i t s b i l l . The c a l l i n g s t imu l i provide the means to communi-cate hunger when the p o s s i b i l i t y of pecking is l i m i t e d . Here is the important d i s t i n c t i o n between c a l l i n g and pecking behaviour which furnishes two mechanisms f o r the control of parental feeding behaviour. Pecking behaviour adjusts feeding frequency whereas c a l l i n g behaviour adjusts foraging frequency. Table 10 These Field Data Demonstrate That Some Threshold Call Frequency Precedes a Parental Feeding and That the Call Frequency Subsides A f te r a Feeding* Call Frequency Measured from the Hide (Cleland Island)  Resulted in Feeding A f te r Feeding No Feeding Ensued N = 70 35 64 AV = .8574 . 2 5 3 4 . 4 1 0 3 VR = . 0 9 5 8 . 0 3 7 8 . 0 8 0 4 SD = . 3 0 9 5 .1944 . 2 8 3 5 SE = . 0 3 7 0 . 0 3 2 9 . 0 3 5 4 95% CL = . 7 8 3 6 - . 9 3 1 2 . 1 8 6 7 - . 3 2 0 2 . 3 3 9 5 - . 4 8 1 1 * The variance of the c a l l s is greater before a feeding than a f t e r ; t h i s may mean that a range of hungers ex is ts between d i f f e r e n t brood sizes but that a feeding reduces a l l hunger to the same l e v e l . This supports my idea that the hunger required to e l i c i t a feeding decreases wi th an increase in brood s ize . 60 b. Effects of Hunger on the Departure Time f o r the Adult Methods: Immediately a f t e r a partner r e l i e f ( i . e . , f i r s t departure) and a subsequent feeding of the ch icks , I would move out of the hide and feed (sa t ia te ) a l l the chicks in that brood then move back in to the hide. A f te r the return of the parent which had departed f o r a feeding t r i p , I would measure the time f o r the other adul t to leave the t e r r i t o r y . This experiment was carr ied out ten t imes. Results: The second departure time is larger than the f i r s t (Table 11). Discussion: This mechanism determines the number of t r i p s which w i l l be taken in a day. Essen t ia l l y , a parent exposed to noisy chicks w i l l depart more quick ly upon return of the partner than a parent o f quiescient chicks. The chicks which were force fed by me, did not c a l l , so the parent was not motivated to go out and f i n d food f o r them. Upon the return of the Al parent, (Table 1 1 ) , the chicks were not fed unless the parent regurgi tated food immediately without a p r i o r pecking bout. In theory th i s parent reca l l s how hungery the chicks were before i t de-parted so th i s parent would feed the chicks i f the chicks ' begging i n t e n s i t y was above a given threshold value before the parent departed. This hypothesis would also pred ic t t h a t : 1) the general din produced by the chicks increases wi th brood s i z e , 2) departure time decreases as the brood size increases. 61 Unfortunately I did not have time to measure departure in te rva ls f o r the various brood sizes but I have i nd i rec t evidence in the next sect ion which corroborates these las t two po in ts . Table 11 The Flow Diagram Explains the Terms 'Pre* and ' P o s t ' . Al and A2 Refer to the Two Parental Adul ts . Obviously the Departure Times are Increased i f the Chicks are Sat iated. The Aster isk Indicates the Point in the Sequence when I Force Fed the Chicks Forced Feeding Experiment returns \ A2 A1A2 PRE • Al 1 eaves-; A1A2 POST • A2 leaves 1 feeds chicks A2 feeds feeds chicks chicks r i a l 'PRE' 'POST' Brood 1 13 sec. 120 jrii'iv 6 2 240 " 219 " 6 3 10 " 140 " 2 4 immediately 216 " 6 5 II 117 3 6 M 117 3 7 II 178 6 8 •I 136 6 9 II 85 2 10 •I 62 6 62 c. Call Character ist ics fo r D i f fe ren t Brood Sizes Methods: While in the h ide, I made spot checks to determine which broods (one to s i x ) were c a l l i n g and which parents were present on the t e r r i t o r y . Results: A s i g n i f i c a n t d i f ference in parental attendance exists between three and s ix broods (Table 12). There is a l i near re la t ionsh ip between brood size and " d i n " level (din level = the number of times that vocal izat ions are heard w i th in a period of time) (Table 13; F ig . 19) . Discussion: The din level f o r s ix chicks would be greater than f o r 2 chicks even i f a l l e ight chicks were equally hungry. The mean weight of large broods is s l i g h t l y less than f o r small broods (Ward 1972) so that i t may be assumed tha t the ind iv idua ls from a s ix brood are more hungry than two brood ind i v idua ls . The regression l i n e indicates tha t a supernormal brood ca l l s s l i g h t l y more than two times that of a normal brood so the increase in din is not qu i te proport ional to the increase in brood s i ze . Assuming tha t the parent can d is t ingu ish between varying levels o f " d i n " , one would expect tha t the parent would leave the t e r r i -to ry i f t h i s din level reached some threshold value of cumulative c a l l i n g . Considering three groups of data , namely: 1) din level is a funct ion of brood size 2) departure time is a l tered by modifying the ch ick 's hunger 3) parents of superbroods have a lower nest attendance than normal broods I suggest that the frequency of foraging t r i p s is inf luenced by the Table 12 The "% Attendance" Describes the Percentage of Times that Two Parents Were on the T e r r i t o r y (% = b/N x 100). The Greatest Nest Attendance i s Shown by the Commonest Natural Brood Size Attendance on the T e r r i t o r i e s (Cleland Island 1971) Brood Size 1 2 3 4 5 6 One Parent (a) 30 57 21 28 31 68 Two Parents (b) 28 68 38 24 25 34 No Parents (c) 0 0 0 0 1 2 N = a + b + c 58 125 59 52 57 104 % Attendance by Two Parents 48 54 65 46 44 33 Table 13 The Din Measurement Refers to the Number of Calls Heard From a Brood Over a Period of Time. A Five Brood Calls 5X as Frequently as a Single Brood Measurement of "Din" Brood Size 1 2 3 4 5_ 6 Total number o f times ca l l s were heard from brood 7 20 24 15 35 80 No. of broods observed 1 2 1 1 1 2 Mean 7 10 24 15 35 40 50 40-30 20-10-0 Y= 5.7X+3. t =3.53 d.f. = 4 2 3 4-BROOD SIZE Figure 19 While in the h ide, I would make spot checks to determine which of the range of 6 brood sizes were c a l l i n g . This simply shows that the cumulative c a l l i n g heard from a brood is a funct ion of the brood s ize . 65 cumulative ca l l i ng from the chicks on the t e r r i t o r y . The importance of chick voca l iza t ions , p a r t i c u l a r l y the i n t e n s i t y , to parental feeding a c t i v i t y was demonstrated by v. Haartman (1953) who showed that the parents respond to the in tens i t y of begging rather than to the number of nest l ings present. 4. In tens i ty of Indiv idual Begging Calls Discussion: Unfor tunately, I did not have the equipment necessary to measure the c a l l i n tens i t y of the chick ca l l s in the meadows so I can only suggest what the possible s ign i f icance of ca l l i n t e n s i t y is to the control of parental feeding. Two possible funct ions are: 1) enhancement Qf peer c a l l 2) termination of a feeding. The f i r s t funct ion simply implies that a loud ca l l i s received bet ter by the parent than a qu ie t c a l l . The second funct ion is of primary importance to the control o f feeding. I t has been pointed out in an e a r l i e r sect ion that ca l l frequency and pecking frequency vary w i th depr ivat ion time so that the hunger level is re f lec ted by these two behaviours. The changes i n ca l l i n t e n s i t y , p r imar i l y near the sa t i a t i on l e v e l , ind icate to the parent that the chick i s nearly sa t ia ted . The adul t would discontinue feeding at t h i s po in t . This s i g n a l , i . e . , the decl ine in the c a l l ' s amplitude associated wi th s a t i a t i o n , would possibly motivate the parent to re- ingest food lay ing on the ground or not to regurg i ta te more food f o r the chicks. This mechanism may ensure a s a t i a -t i on feeding each time that the chicks are fed. I f there are addi t ional 66 chicks which the parent could not sa t ia te at one feeding, th i s mechanism would be an addi t ional fac tor which motivates the parent to leave f o r a foraging t r i p and to return from the feeding t r i p more qu ick ly . E. Summary and Discussion In t h i s chapter the quant i ta t i ve changes in behaviour associated wi th increasing deprivat ion time are described in sect ion C. Both the pecking and c a l l i n g frequency increased as the depr ivat ion time length-ened. Although no age di f ferences ex is t f o r the pecking frequency-depr ivat ion re la t i onsh ip , three d i s t i n c t age class di f ferences f o r the c a l l i n g frequency do e x i s t . The amplitude of ind iv idual 'peer ' ca l l s increases and then reaches a plateau a f te r one hour of depr ivat ion f o r a l l ages. F i r s t week old chicks are s i g n i f i c a n t l y quieter than the t h i r d or f i f t h week chicks. No obvious d i f fe rences, wi th respect to p i tch and duration of the c a l l s , are apparent between the peers of hungry and sat ia ted chicks. Except f o r th i s l a s t fea tu re , a number of signals demonstrated a quant i ta t i ve re la t ionsh ip wi th hunger. The re la t ionsh ip between the ch ick 's and parent 's behaviour is discussed in section D. These f i e l d observations indicated tha t there are quan t i ta t i ve re lat ionships between the chick 's and parent 's behaviour. I r respect ive of the brood s i z e , a feeding is preceded by at least 16 pecks. I f these pecks are given w i th in a r e l a t i v e l y constant time per iod, th i s would mean that large broods would be fed at a lower depr ivat ion time (hunger leve l ) than small broods. Assuming th i s is t r u e , i t would 67 af ford a mechanism whereby more feedings would be given to larger broods each day. This model requires that the parent w i l l only return wi th a quant i ty of food proport ional to the age of the chicks rather than to the brood s ize . The pecking behaviour is only funct ional when the chicks can reach the parent which must, there fore , be receptive to some extent . Thus another mechanism must ex i s t which would communicate the hunger level to the parent. This mechanism, the c a l l i n g behaviour, is deal t wi th in subsection D.3. The c a l l i n g frequency per ind iv idua l is highest p r i o r to a feed-i n g , lowest a f t e r a feeding, and intermediate fo r the remaining t ime. In constrast to the pecking response, the ca l l frequency is the same f o r ind iv iduals from d i f f e r e n t brood sizes p r i o r to a feeding. This would allow f o r a cumulative c a l l i n g value to be greater f o r the larger broods. This is the case as seen in subsection D.3.c; the increased cumulative c a l l i n g would st imulate a parent of a large brood to leave the t e r r i t o r y sooner and return from a foraging t r i p more quick ly than a parent of a small brood. The fac t that hunger w i l l inf luence the time that a parent remains on the t e r r i t o r y was demonstrated by an experiment out l ined in subsection D.3.b. Up to t h i s po in t , the d i s -cussion has centered on the mechanisms which i n i t i a t e feeding and adjust the foraging frequency. The ca l l amplitude discussed i n sub-section D.4 may enhance the charac ter is t i cs of the ca l l behaviour and be instrumental in terminat ing a feeding when the chicks are nearly sa t ia ted . 68 In summary, there are three mechanisms f o r c o n t r o l l i n g parental feeding. F i r s t l y , the pecking behaviour i n i t i a t e s a feeding, secondly, the c a l l i n g behaviour is probably instrumental in both the i n i t i a t i o n of feeding and the adjustment of the foraging frequency and t h i r d l y , the ca l l amplitude provides f o r the enhancement of the c a l l i n g behaviour and a means f o r the terminat ion of a feeding. The problems associated wi th c lutch size and food a v a i l a b i l i t y could be magnified by the parental feeding control system. In sub-section D.3.c i t was shown that the parents of the 3 broods were most f requent ly together on the t e r r i t o r y . The chicks would undoubtedly su f fe r less predation i f two parents, rather than one, were on the t e r r i t o r y , so th i s fac to r would probably contr ibute to the evolut ion of a c lutch size of three. Parents wi th less than 3 chicks would not develop a strong parental bond wi th the chicks which would ca l l i n f r e -quently because they would be sa t ia ted . There would be l i t t l e a t t r a c t i o n to the t e r r i t o r y so tha t f requent ly only one parent would remain on the t e r r i t o r y . Parents wi th more than 3 chicks would be st imulated to forage to such an extent tha t they would spend l i t t l e time on the t e r r i -t o r y . In Chapter 1 i t was indicated that parents of supernormal broods spend less time on a foraging t r i p than parents of normal broods. In Chapter 2, i t was suggested that the cumulative c a l l i n g behaviour of the brood determined the length of the foraging absence. I f food is less avai lable f o r the Mandarte parent, then the parent may return w i th in the l i m i t s of the foraging absence, wi thout a maximally successful catch. 69 Generally the Mandarte parents do present fewer feedings to the chicks per foraging t r i p ; th is may mean that the parents wi thout a large catch are return ing to the Mandarte colony. In terms of feeding, t h i s behaviour would be maladaptive because time would be wasted commuting to and from the feeding area. I f , however, predation has contr ibuted to the evolu-t i on of th i s behaviour then i t i s probable that t h i s behaviour is a compromise between increasing the foraging opportunity and increasing the time that both parents spend on the t e r r i t o r y . Ear l i e r in th i s discussion, I have mentioned that the attendance of two parents on the t e r r i t o r y is greatest fo r broods of three. Perhaps t h i s compromise alluded t o , is one of the factors responsible fo r the evolut ion of the g u l l ' s c lutch s ize. In summary, a number o f signals are used by the chicks to communicate t h e i r needs and the parents w i l l respond in a predictable manner. The evolut ion of the parental feeding behaviour may have con-s is ted of a number of compromises optimal f o r a c lutch size of three. CHAPTER 3 THE MODEL A. Introduction Af ter the f i r s t f i e l d season, a qua l i t a t i v e model (Table 14, F ig . 20) of feeding regulation was developed and a number of hypotheses were formulated. In th i s chapter, the model i s presented so that the basic character i s t i c s of the control system are summarized. The flow chart demonstrates that the decisions based on behavioural assessments determine the outcome of a parent-chick i n te rac t i on . The fort ran pro-gram provides a more deta i led quant i tat ive descr ipt ion of the sequence of events preceding a feeding. A verbal descr ipt ion of the 43 s ta te -ments of the fort ran program i s given in the l a s t sect ion. B. Description of the Model I have constructed a flow sheet descr ibing the commands and decisions necessary for the control of feeding behaviour. The purpose of th i s model i s to co-ordinate the aspects of feeding which I have discussed throughout th i s paper. A b r i e f account of the 43 steps should demonstrate the s imp l i c i t y of th i s descr ipt ive model of parental feeding behaviour. 70 71 Table 15 This Simple Flow Chart Summarizes the Information Discussed in th is Thesis. The 43 Steps Describe the Sequence of Events, Commands, and Decisions which must be made During the Parental Feeding Act 1. I n i t i a l Condit ions: AW = 6 0 AFT = 0 TD = 0 DT = 10 KPN = 2 LA = 1 KLPSV = 0 2. Threshold Values: CCF = .84 CICIQ = 75 CPF = 20 CLCITF = 85 JCLPSV= 1 3. Read: JBS,KLPSV Computations 4. LCFQ = 2.6(12 DT)+4.2 GO TO 7 24. IF (LA-20) 26,26,25 5. LCFT = 4.3 (12 DT)+4.4 GO TO 7 25. IF (LA-21) 26,21,21 6. LCFF = 7.5 (12 DT)+31.2 GO TO 7 26. IF (LCIQ-CLCIQ) 30,28,28 7. LDIN = 5.7 JBS + 3.4 27. IF (LCITF-CLCITF) 30,281,281 8. FT = f (DIN) LCIQ = 7 4 . 8 - 9 . 6 5 e " ( * 0 4 8 D T^G0 TO 11 271. IF (JB-3) 28,28,272 9. 272. DT = 3 GO TO 282 10. LCITF= 8 4 . 6 - 1 0 . 8 e " ^ ' 0 4 1 D T^G0 TO 11 28. GMF= 0.16 AW + 5.3 GO TO 29 11 . LPF = 0.75 (60 DT) + 26.1 282. CMF= (o . l 6 AW + 53)/JBS 12. IF (LA-14) 15,15,13 29. AW = AW + GMF (.26) 13. IF (LA-24) 16,16,14 30. DT = DT + 0.5 14. IF (LA-35) 17,17,17 31 . AFT= AFT - 0.5 15. IF (LCFF-CFF 30,18,18 32. TD = TD + 0.5 16. IF (LCFT-CFT) 30,18,18 33. IF = (TD-18) 34,37,37 17. IF (LCFQ-CFQ) 30,18,18 34. AW = AW - AW (.10) 18. IF (KLPSV-JCLPSV) 19,23,23 35. LA = LA + 1 19. IF (AFT-FT) 20,21,21 36. IF (LA - 45) 37,40,40 20. KPN = 1 GO TO 30 37. IF (LA - 14) 4,4,38 2 1 . KPN = 1 38. IF (LA - 24) 5,5,39 22. AFT = 0 GO TO 30 39. IF (LA - 35) 6,6,6 23. IF (LPF-CPF) 30,24,24 40. Output 72 INITIAL CONDITIONS e.g. time of day, brood size, chick hunger etc. THRESHOLD VALUES e.g. pecking,colling BEHAVIORAL ASSESSMENTS Callfreq. is a func.of hunger Pecking » » » " Coll intensity • » « DIN is a func.of BROOD size MAX. FORAGING absence is a func.of DIN Determine age of brood because threshold values differ between age groups DECISIONS IF CALLfreq> threshold I IFCALLin'.en.> IF PECK VALUE > " IF PARENT has some min. amount cf food IF REO.are satisfied a feeding ensues Could receive feeding (sated) prop, to age ANY NO .chicks wait for other parent or for hunger to increase HUNGER INCREASES SEQUENCE REPEATED F igure 20 The f low diagram summarizes the sequence o f events and de c i s i o n s which c o n s t i t u t e the con t r o l system. 73 1) These equal i t ies describe the basic condit ions necessary f o r the f i r s t few decis ions. AW = weight of the ch ick ; TD = time of day divided in to ha l f hour i n t e r v a l s ; KPN = the number of parents on t e r r i t o r y ; KLPSV, KLCSV = the amount of food (grams) in the stomach of the parent and ch ick , respect ive ly . AFT = the length of time a parent is absent from the t e r r i t o r y and on a foraging t r i p ; DT = the time since a sa t i a t i on feeding, i . e . , depr ivat ion t ime; LA = the age (days) of the chicks. 2) Certain values are used to make decis ions. CCF = ca l l frequency f o r a successful feeding; CPF = the number o f pecks which e l i c i t a feeding; CLCIO, CLCCITF = the in ten-s i t y of ca l l necessary before a feeding occurs; JCLPSV = there must be a minimum amount of food in the stomach of an adul t before a feeding occurs. 3) Two factors which w i l l change the decisions made in the model are: JBS = brood s i z e ; KLPSV = the amount of food which the parent obtains on a foraging t r i p . 4) Each statement describes how ca l l frequency changes with depr ivat ion t ime. 5, 6) I was unable to describe accurately how ca l l frequency changes wi th the age of the chicks so that chicks are blocked in to one week, three week, and f i ve week age classes (4 , 5, 6 respec t i ve ly ) . 7) The amount of time that ca l l s are heard from a t e r r i t o r y i s a funct ion of the brood s ize . 74 8) The time taken on a foraging t r i p is re lated to the size of the brood. I was unable to obtain enough data to show the dif ferences between each brood size but was able to demonstrate that the foraging t r i p in te rva l decreases as the brood size increases. 9, 10) The in tens i t y of the peer ca l l changes wi th the depr ivat ion time and the age of the chick. 11) The frequency of pecks increases wi th increases in the depr ivat ion t ime. 12, 13, 14) These three statements d i rec t the flow of commands depending on the age of the chicks. 15, 16, 17) The previous statements determine which of these three decisions is made. Each statement asks: " i s the ca l l frequency high enough to e l i c i t a feeding?" 18) I f the parent has more food than some minimum amount then the chicks are fed . 19) This statement says: " i f the actual feeding t r i p time is equal or greater than the foraging time expected fo r that brood s i ze , one parent w i l l leave (21) the t e r r i t o r y and the other parent might feed the chicks. I f the AFT s FT then the parent w i l l remain on the t e r r i t o r y u n t i l the AFT * FT (20 ) . " 20) Parent is alone on the t e r r i t o r y and cannot feed the chicks. 21) Parent is alone on t e r r i t o r y and may feed the chicks. 22) The actual feeding t r i p time is zero because the parents have re l ieved each other. 75 23) I f the pecking frequency is greater than some threshold value then the feeding is more l i k e l y to occur. 24, 25) These two statements determine the age of the chicks so that the next appropriate decision can be made. 26, 27) Depending on the age of the ch ick , the minimum ca l l i n t e n -s i t y necessary f o r a feeding is compared wi th the ca l l i n tens i t y as a funct ion of depr ivat ion t ime. This is the f i n a l statement which must be s a t i s f i e d before a feeding occurs. 28) The chick is fed a number of grams of food charac te r i s t i c f o r that age or weight c lass. 29) The proport ion of the feeding which is transformed in to chick f lesh is determined by the E.G.E. 30, 3 1 , 32) To the time of day, depr ivat ion t ime, and foraging t r i p t ime, a ha l f hour is added each time the program cycles. 33, 34) When the day is f i n i s h e d , the chick looses a proport ion of the body weight through the n igh t . 35, 36) One statement increases the age of the chick by one day and the other decides whether or not to stop the program. 37, 38, 39) Three statements which, depending on the ch ick 's age, d i r e c t the f low of command to d i f f e r e n t statements. 40) Output: The growth of the chick each day f o r 45 days is described. Statements 271, 272 and 282 w i l l modify the amount of food given to the superbroods and increase t h e i r depr ivat ion t ime. Each ch ick , as 76 i t were, would be checked through the program so that the hunger o f the hungriest chick would regulate the foraging a c t i v i t y of the parents. Many refinements of t h i s model could be made so that i t could be pro-grammed f o r a computer but simulat ion was not the in ten t of t h i s model. The only purpose of th i s model is to co-ordinate the ideas discussed throughout t h i s paper. I I . GENERAL DISCUSSION Diverse factors inf luence the p roduc t i v i t y o f a gu l l colony. F i r s t l y , c l imat ic factors could inf luence d i f f e r e n t phases of the breeding season so that p roduc t i v i t y could be modified at d i f f e r e n t stages of the breeding per iod. For example, hyperthermia i s a cause of death f o r young Mandarte chicks exposed to the sun whereas the wind and ra in may be a mor ta l i t y fac to r on Cleland Is land. Secondly, the degree of predation may d i f f e r between colonies. On both Cleland and Mandarte Is lands, resident crows do take both eggs and small ch icks, but the Cleland colony experiences more Bald eagle (Haliaeetus leuco- cephalus) attacks than does the Mandarte colony. T h i r d l y , the importance of i n t r a s p e c i f i c competition d i f f e r s between Mandarte and Cleland Is lands. The mean number of nests located w i th in a 15-foot c i r c l e centered on 10 randomly selected nests on Mandarte and Cleland is 3.9 and 2 .5 , respect ive ly . The small t e r r i t o r y size on Mandarte is probably corre lated wi th the intense i n t r a s p e c i f i c s t r i f e witnessed on the i s land . Large numbers of chicks which intrude in to neighbouring t e r r i t o r i e s are pecked to death on Mandarte but rare ly on Cleland. Four th ly , the feed-ing biology of the two areas is s t r i k i n g l y d i f f e r e n t . Three aspects of the feeding biology which could produce di f ferences in the produc-t i v i t y of Mandarte and Cleland are (1) the a v a i l a b i l i t y of food, (2) the morphology of the prey, and (3) the ca lo r i c content of the food. A r e s t r i c t i o n on the a v a i l a b i l i t y of food through e i the r spat ia l or temporal patterns could inf luence the p roduc t i v i t y of a colony. This 77 78 measure of a v a i l a b i l i t y should also be re lated to the density of the predator, f o r Mandarte has a much larger gu l l population than Cleland, and several other large gu l l colonies ex i s t and presumably compete f o r food in the immediate area of Mandarte. As I have already mentioned, the occurrence of garbage in the d ie t of the Mandarte chicks i s co in-cident wi th the peak in hatching. This blossoming Mandarte population could have a de ter io ra t ing inf luence on the food resources. Although garbage can be gathered in the v i c i n i t y of the colony, many breeding gu l ls have been seen on the Vancouver refuse p i l e ( J . Ward pers. comm.) during the chick stage. This journey would en ta i l an eighty mile round t r i p from Mandarte to Vancouver and back. To compound the problem of food a v a i l a b i l i t y , the pr inc ipa l food, he r r ing , i s only avai lab le during r e s t r i c t e d times when the f i s h are feeding on zooplankton. Even during stormy weather, the foraging success of Cleland gu l ls was not impaired and there are no apparent r e s t r i c t i o n s on the times avai lable fo r fo rag ing. Frequently I saw the Cleland gu l ls feeding w i th in 400 metres of the is land whereas only rare ly did I see Mandarte gu l l s feeding near the i s land . Not only is there a d i f ference in the temporal and spat ia l pat tern of prey abundance near the two colonies, but also a d i f ference in the type of food ava i lab le . Sandlance (Ammodytes hexapterus) of Sand Eel is a small th in s i l v e r y f i s h normally found in schools near the water surface. Herring (Clupea p a l l a s i i ) is much larger and is normally found in schools ascend-ing and descending at dusk and dawn, respect ive ly . The physical charac-t e r i s t i c s of the prey may also inf luence the growth of the brood. A 79 100 gm feeding of 20 sandlance can eas i ly be d is t r i bu ted to a brood; a' 100 gm feeding of herr ing may consist of one herr ing which can eas i ly be monopolized by one chick. I f one chick, perhaps the more aggressive i n d i v i d u a l , succeeds in monopolizing several feedings, obvious var ia t ions in weight would evolve in the brood. I would pre-d i c t that the variance f o r the mean of the growth rate and the asymptotic weight would increase with the brood s ize . In add i t i on , the. variance f o r the mean of growth rate and asymptotic weight would be greater on Mandarte. For the brood s izes , one to th ree, the variance f o r the mean growth rate increases wi th the brood size and is greater f o r Mandarte (Table 15). The data f o r the supernormal broods do not support my Table 15 These Data, taken from J . Ward's Thesis, Show not only that the Variance of the Growth Rate Increases wi th Brood Size but also that the Variance is Greater on Mandarte. The Trend Does not Persist i n t o the Supernormal Broods which are Experimentally Enlarged Broods Variance of Growth Rate in Relation to the Brood Size Brood Size Mandarte Cleland 1 .2037 x 10 7 gms. .1899 x 10 7 gms. 2 .2331 " .1904 3 .3740 " .2446 4 .3370 " .3640 5 .2772 " .2770 6 .1297 .3789 80 predic t ion because only the successful superbroods contr ibuted to the analysis. A l i g h t weight chick may receive j u s t enough food in a normal brood but the competition f o r food in a supernormal brood may be so intense that l i g h t weight birds quickly d ie . These unsuccessful supernormal broods are not included in the analysis so that the var-iance i s calculated on supernormal broods which may contain chicks that are equally competit ive and successful in feeding. The variance f o r s ix broods on Mandarte is the smallest f o r a l l broods on both Mandarte and Cleland Is lands; t h i s may indicate that l i g h t weight chicks are el iminated quickly so that only those chicks w i th in a cer ta in weight range w i l l surv ive. Only those broods comprised of equally competit ive and healthy chicks survived f o r 45 days so that they were used in the calculat ions of the growth rate f o r a s ix brood. Another aspect of the food, i . e . , the q u a l i t y , may also be re lated to the success of supernormal broods. The ca lo r i c content of the food presented to the chick w i l l obviously a f fec t the growth and subsequent surv ival o f the chick. The ca lo r ic values of sandlance and Paci f ic herr ing are assumed to be the same so that d i f ference in qua l i t y is not re f lec ted i n these two foods. The Mandarte chicks receive a greater var ie ty of foods such as crabs, mussels, s t a r f i s h and garbage, which would not contain the equivalent number of calor ies f o r an equal weight or volume of herr ing or sandlance. Although a s i g n i f i c a n t d i f ference does not ex is t between the volumes of food fed to the chicks at var-ious ages f o r Mandarte and Cleland, an obvious trend indicates that older Cleland chicks receive more food than s i m i l a r l y aged Mandarte 81 chicks. These two aspects, lower ca lo r i c value per un i t volume of food and lower t o ta l quan t i t i es , would probably account f o r some of the di f ferences in growth between Mandarte and Cleland Island chicks. These three aspects of the feeding b io logy, i . e . , a v a i l a b i l i t y , morphology and qua l i t y of the food, could possibly inf luence the growth of the chicks. The a b i l i t y of the parent to cope wi th these d i f f e r e n t problems depends on the s k i l l o f the foraging parent and aspects of the parental feeding control system. Factors such as age, experience, and pheno-deviance w i l l be re f lec ted in the d i f f e r e n t i a l a b i l i t y of parents to successful ly forage and rear young (Coulson 1968). The remaining d i s -cussion w i l l be r e s t r i c t e d to a descr ip t ion of the parental feeding control system and i t s re la t ionsh ip to the p roduc t i v i t y of an i s land . I f my analysis and in te rp re ta t i on of the control signals which regulate parental feeding behaviour are co r rec t , one must admit that the gu l ls have a sophist icated language which communicates subt le d i s -t i nc t i ons in the motivat ion leve ls . The control system, as I see i t , consists of two main mechanisms: 1) that which determines the feeding frequency w i th in a t r i p 2) that which determines the foraging t r i p frequency. The two mechanisms are not , o f course, mutually exc lus ive, because the foraging frequency w i l l probably d i r e c t l y inf luence the feeding frequency. The feeding frequency is adjusted by (a) the pecking behaviour (b) c a l l frequency and (c) ca l l i n t e n s i t y . The foraging t r i p frequency is adjusted by (a) the c a l l frequency and (b) the "d in " level on the t e r r i t o r y . The feeding is i n i t i a t e d by the appropriate pecking and c a l l i n g frequency and 82 is terminated by xhanges in the ca l l i n tens i t y unless the food is con-sumed f i r s t . The de ta i l s of the control provided by these signals are described in the discussion fo l lowing each sect ion in Chapter 2 so that I w i l l address myself only to those de ta i l s per ta in ing to the quest ion, "Why is there a d i f ference in p roduc t i v i t y between Mandarte and Cleland Islands?" The problems associated with c lutch size and food a v a i l a b i l i t y could be magnified by the parental feeding control system. In Chapter 2 i t was shown that the parents of 3 broods were most f requent ly together on the t e r r i t o r y . The chicks would undoubtedly su f fe r less predation i f two parents rather than one were on the t e r r i t o r y so that th i s fac to r would probably contr ibute to the evolut ion of a c lutch size of 3. Par-ents w i th less than 3 chicks would not develop a strong bond wi th the chicks which would ca l l in f requent ly because they would be sa t ia ted . There would be l i t t l e a t t r a c t i o n to the t e r r i t o r y so that f requent ly only one parent would remain on the t e r r i t o r y . Parents wi th more than three chicks would be st imulated to forage to such an extent that they would spend l i t t l e time on the t e r r i t o r y . An unguarded Mandarte chick is more l i k e l y to su f fe r from i n t r a s p e c i f i c competit ion than a Cleland chick because the Mandarte t e r r i t o r i e s are smaller and more densely packed. In Chapter 1 i t was indicated that parents of supernormal broods spend less time on a foraging t r i p than parents of normal broods. In Chapter 2 , i t was suggested that the cumulative c a l l i n g behaviour of the brood determined the length of the foraging absence. I f food is less avai lab le f o r the Mandarte parent, then the parent may return w i th in the 83 l i m i t s of the foraging absence, wi thout a maximally successful catch. Generally the Mandarte parents do in fac t present fewer feedings to the chicks per foraging t r i p ; th i s may mean that the parents wi thout an adequate catch are return ing to the colony. In terms of feeding, t h i s behaviour would be maladaptive because time would be wasted in commuting to and from the feeding grounds. I f , however, predation has contr ibuted to the evolut ion of th i s behaviour then i t i s probable that th i s be-haviour is a compromise between increasing the foraging opportuni ty and increasing the time both parents spend on the t e r r i t o r y . Ea r l i e r in th i s discussion I have mentioned that the attendance of two parents on the t e r r i t o r y is greatest f o r broods of three. Perhaps th i s compromise alluded t o , is one of the factors responsible f o r the evolut ion of the g u l l ' s c lu tch s ize . These two aspects of the control system demonstrate that the character of the feeding control system may magnify the detr imental e f fec ts of low food a v a i l a b i l i t y and large brood s ize . Two other aspects, f o r which I have no quan t i ta t i ve in format ion, may also contr ibute to the problems associated wi th low food a v a i l a b i l i t y , poor qua l i t y food, and large brood s izes. The long term adjustments made f o r spec i f i c nu t r i en t requirements f o r the chicks may be an important aspect of the control system. Some vertebrates show spec i f i c hungers (Hughes, et a l . , 1971; Roger 1967) but I am not aware of th i s a b i l i t y in g u l l s . The simplest way fo r a chick to receive a balanced d ie t is to refuse cer ta in foods. On Mandarte, the young chicks i n i t i a l l y refuse the french f r i e s , f a t , meat and bread which were presented by the parent , but a f t e r several successive refusals the chicks 84 accepted the garbage because, supposedly, they were in more need of calor ies than spec i f i c nu t r i en ts . Their hunger threshold had dropped below the threshold fo r unpalatable food (Hol l ing 1966). In addi t ion to the spec i f i c needs of the chicks, the ca lo r i c requirements of the parents should also be considered. One gets the impression that the parent feeds a l l i t s food to the chicks, but of course th i s is not the case. The outcome of oppos-ing motivations may have a s i g n i f i c a n t inf luence on the success of larger broods. From the evolut ionary perspect ive, the parent which is in good physical condi t ion a f t e r ra is ing three chicks may be more l i k e l y to survive than a weary parent who has reared s ix chicks. Assuming tha t food supply is in t imate ly associated wi th the evolut ion of brood s i z e , I would th ink that the requirements of the parents should also be con-s idered, especial ly in a r e l a t i v e l y longeval b i rd such as the g u l l . Then again, perhaps a greater genetic investment, i . e . , more progeny in the populat ion, is more important than the parent 's fu ture cont r ibu t ion to the breeding colony. The t o t a l output of the parent may be more impor-tant during rapid population increase ( r - se lec t ion ) whereas the rate of increase may not be as important as the population reaches carry ing capacity (K-select ion) (MacArthur & Wilson 1963). Although these considerations are e x c i t i n g , the hypothesis is not readi ly tes tab le . Up to th i s point I have discussed the factors that may have been responsible fo r the evolut ion of the parental feeding control system. Only one other compre-hensive study of an avian parental feeding system e x i s t s , i . e . , Norton-G r i f f i t h s (1969). The g u l l ' s system is subs tan t ia l l y d i f f e r e n t from the 85 Oystercather's control system because of basic di f ferences in the feeding biology. Or ig ina l l y my hypothesis, s im i la r to Nor ton-Gr i f f i ths (1969), was that spec i f i c time in terva ls in the feeding sequence would change fo r various brood sizes so that the parent could have some measure of the hunger level of the brood. As i t turned out , t h i s hypothesis was rejected because no di f ferences were associated wi th the d i f f e r e n t brood sizes (Table 16). Although Nor ton -Gr i f f i t hs ' work is the only work Table 16 I n i t i a l l y i t was Thought that the Time Spent in Pre-feeding Behaviour would be some Function of the Hunger and Size of the Brood. The BEG-REG Interval is the Time Measured from the F i r s t Call or Peck to the Parent's Regurgi tat ion. PRES-REG is the Time Between the Presentation of the Parent's B i l l and the Subsequent Regurgi tat ion. The REG-ET In terva l i s a Measure of the Time that the Parent Offers Food to the Chicks. I t i s Obvious that No Con-s i s ten t Trends Exist in These Data Brood Size BEG-REG PRES-REG REG-ET n X n X n X One 27 59.4 36 47.4 28 71.8 Three 44 77.5 58 54.2 52 67.9 Six 87 54.1 91 40.8 72 71.1 comparable to th is study, the systems are rad i ca l l y d i f f e r e n t because of basic di f ferences in the feeding biology. The primary d i f ference in the control systems is that the gul l chick begs but the Oystercatcher chick 86 does not. The response of the Oystercatcher chick to the presentat ion of the food is the important control mechanism in N o r t o n - G r i f f i t h s ' work whereas the response of the chicks p r i o r to the presentation of food is the per t inent control mechanism in t h i s study. The control system has evolved in re la t i on to the a v a i l a b i l i t y of the food which the parent eats. The Oystercatcher forages e i ther on t e r r e s t r i a l invertebrates (Sa f r ie l 1967) or on i n t e r t i d a l invertebrates (Hartwick pers. comm.). I t can be said wi th some assurance that the a v a i l a b i l i t y of food to the adul t Oystercatcher does not f luc tua te great ly during a day so that the Oystercatcher need not ingest a large amount of food because the food is usual ly avai lable fo r long periods. The gul l feeds to capacity on a temporary abundance of food ( f i s h schools at the water surface) because the next opportuni ty to feed may be a f t e r some var iable period of t ime. The two birds have, there fore , evolved d i f f e r e n t stomachs; the gu l l has a large storage capacity whereas the Oystercatcher does not . The basic d i f ference in feeding behaviour may explain the essential d i f ference be-tween the control of parental feeding f o r the Oystercatchers and Glaucous-winged g u l l s . The gu l l chick must e l i c i t a regurg i ta t ion whereas the Oystercatcher chick simply takes the food from the b i l l of the parent. The previous statement is concerned wi th the maintenance of feeding because in both cases the parent has been st imulated to get food f o r the chicks. I have described the mechanism by which the chick st imulates the parent to forage but Nor ton-Gr i f f i ths simply says that only a hungry parent feeds i t s chick when the t ide is r i g h t . I would th ink that the chicks would have some s t imula t ing e f fec ts on the parent other than j u s t 87 being on the t e r r i t o r y . The re la t ionsh ip of the Oystercatcher ch ick 's ca l l behaviour to feeding was not invest igated so that he does not know i f the ca l l frequency would i n i t i a t e parental feeding. For the above reasons I contend that the model which I propose has a more'general value to control o f parental feeding in birds than the model advanced by Nor ton-Gr i f f i ths (1969). The ubiqui ty of the begging behaviour characterized by c a l l i n g and pecking (gaping probably serves a s im i la r funct ion as pecking), in addi t ion to the reports of intense begging associated wi th depr ivat ion (Hailman 1967; v. Haartman 1953; Kluyver 1961; Eisner 1963; Impekoven 1971), support my thesis that the mechanisms discussed may be charac te r i s t i c fo r other species of b i r d s , p a r t i c u l a r l y f o r nidicolous species f o r which there is circumstan-t i a l evidence. Although I have ascribed funct ions to the s igna ls , c a l l i n g and pecking, many experiments are needed before these funct ions can be des-cribed unequivocally. The general s t ructure and funct ion of the control mechanisms, however, has been s a t i s f a c t o r i l y explained so that some pre-d ic t ions could be made. The model is s u f f i c i e n t l y general in character so that i t is probably appl icable to many species of b i rds . SUMMARY ( i ) I t is suggested that the onset of breeding (egg lay ing) i s inf luenced by the a v a i l a b i l i t y of sandlance near Cleland Is land. ( i i ) The courtship feeding may provide energy f o r egg production in addi t ion to enhancing the pai r bond, ( i i i ) The d i f ference in the qua l i t y and quant i ty o f food given to the chicks f o r Mandarte and Cleland Islands may a f f e c t the growth rate and asymptotic weight of the chicks. ( i v ) Although both Mancarte and Cleland adults feed the chicks wi th the same frequency, Mandarte parents forage more f r e -quently than Cleland parents, (v) Cleland parental feeding occurs consis tent ly throughout the day whereas Mandarte feedings occur at three peaks during the day. These di f ferences are thought to be a r e f l e c t i o n of the prey a v a i l a b i l i t y during the day. ( v i ) The ecological growth e f f i c iency is compared wi th tha t of the Herring g u l l . ( v i i ) Hunger is defined as the number of grams of food necessary to sa t ia te a ch ick. The hunger level o f the f i e l d chicks is usual ly s l i g h t l y below the sa t i a t i on l e v e l , ( v i i i ) The foraging absence in re la t i on to brood size is s im i la r fo r both is lands. The adults decrease the foraging times as the brood size increases. 88 89 ( i x ) The ethogram f o r parental feeding behaviour is described; the Herring g u l l , which interbreeds in the region of range over lap, has a s im i la r behaviour pa t te rn , (x) A number of signals were quan t i t a t i ve l y and q u a l i t a t i v e l y described in re la t i on to parental feeding c o n t r o l . Pecking at the parent 's b i l l , c a l l i n g di rected to the parent , and i n tens i t y changes in the ca l l were measured a f t e r food depr i -va t ion . Field observations of b i l l pecking and ca l l frequency are recorded fo r successful and unsuccessful feeding attempts. ( x i ) A f i e l d experiment demonstrated that nest attendance by the adul t is inf luenced by the hunger of chicks. The chicks ex-press the i r hunger by pecking the parent and by increasing the ca l l frequency. Theore t i ca l l y , the foraging frequency i s i n -fluenced by the "d in " level on the t e r r i t o r y , ( x i i ) A descr ip t ive model is presented. The signals which determine the feeding and foraging t r i p frequency as well as the i n i t i a -t i o n and terminat ion of feeding are wr i t t en as a series of commands and decisions, ( x i i i ) The general aspects of the feeding biology of the two colonies are discussed wi th regard to the di f ferences in chick growth f o r the two is lands. 90 LITERATURE CITED Anand, B. K., G. S. Chhina, B. Singh. 1962. Ef fect of glucose on a c t i v i t y of the hypothalmic "feeding center . " Science 138:597. Andrew, R. J . 1964. Vocalizations in chicks, and the concept of 'st imulus con t ras t . 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