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Foraging strategy of the black oystercatcher Hartwick, Earl Brian 1973

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c l FORAGING STRATEGY OF THE BLACK OYSTERCATCHER by EARL BRIAN HARTWICK B.Sc, University of Toronto, 1966 M.Sc, University of Toronto, 1968 A Thesis submitted i n P a r t i a l Fulfilment of the Requirements for the Degree of Doctor of Philosophy In the Department of Zoology We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA JULY, 1973 In presenting t h i s thesis i n p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t freely available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of The University of B r i t i s h Columbia Vancouver 8, Canada Date 4-4 X M FORAGING STRATEGY OF THE BLACK OYSTERCATCHER ABSTRACT The study was concerned with the foraging strategies of animals and, i n p a r t i c u l a r , with a recent model of optimal foraging proposed by Royama i n 19 70. The black oyster-catcher Haematopus bachmani Audubon was chosen as a study animal because i t foraged i n an open habitat where i t was possible to observe both the hunting of the adult and the feeding of the young. Low s u r v i v a l rates r e s u l t i n g from predation and drowning indicated that e f f i c i e n c y i n feeding would have sur v i v a l value. A natural history study revealed that the birds moved t h e i r chicks to the feeding area. Thus the model was examined under three d i f f e r e n t feeding contexts, (1) adult feeding, (2) feeding of chicks at the nest, (3) feeding of chicks at the feeding area. Interac-tions with g u l l s prevented the movement of some chicks to the feeding area and the e f f e c t s of t h i s on the foraging of the parents were recorded. The feeding area was studied i n some d e t a i l and a natural zonation t y p i c a l of rocky shore habitats was found to e x i s t . Shore-level size gradients were found i n many of the species. The birds exhibited a number of d i f f e r e n t hunting modes on the feeding area and a tendency to select larger than average sized prey items. Values were obtained for the parameters of Royama's model, including handling time, average weight of prey, rate of successful search and prey density. P r o f i t a b i l i t i e s were calculated for some prey i n the three feeding s i t u a t i o n s . The most p r o f i t a b l e food items were mussels when they were vulnerable. A study of the diet indicated that mussels formed the greatest part of the die t i n terms of weight. This more pr o f i t a b l e food item occurred even more frequently i n the diet of chicks at the nest. The adults c a r r i e d large items to the chicks while feeding on smaller items themselves. When chicks were moved to the feeding area, small items prev-iously lacking i n t h e i r d i e t suddenly appeared quite frequen-t l y . Risks not considered i n the model are thought to account for t h i s . Crabs which appeared i n the d i e t only when chicks were present may play a special role i n the d i e t , although an explanation i n terms of p r o f i t a b i l i t i e s may also be reason-able. In the s i t u a t i o n where chicks remained i s o l a t e d from the feeding area, mussels formed an increasing percentage of the d i e t of the chicks. A number of d i f f e r e n t responses to pote n t i a l prey were recorded during the study. The e f f e c t s of prey size and encounter rate were examined experimentally. The birds showed a rapid conditioning to large prey with s p e c i f i c searches being ca r r i e d out a f t e r only one contact. The s p e c i f i c search continued i n spite of a high encounter rate with smaller prey. The need for a j o i n t i n t e r e s t i n strategies and t a c t i c s was discussed. The study i l l u s t r a t e d the advantage of i v c a r e f u l l y choosing a study animal i n order to test and u l t i -mately improve models advanced by theoreticians. TABLE OF CONTENTS TITLE PAGE ABSTRACT TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES ACKNOWLEDGEMENTS INTRODUCTION STUDY ANIMAL STUDY SITE POPULATION AND NATURAL HISTORY STUDY METHODS RESULTS (a) Breeding Behaviour (b) Population Size (c) Egg-laying and Clutch Size (d) Survival of eggs and young (e) Interactions (f) T e r r i t o r i e s i n r e l a t i o n to breeding success DISCUSSION THE PROFITABILITY HYPOTHESIS -FORAGING OF BLACK OYSTERCATCHERS - GENERAL DESCRIPTION (a) The foraging area (b) The prey of the black oystercatcher (c) Hunting and attack behaviour ABUNDANCE (N^) AND SIZE OF PREY IN THE HABITAT Page i i i .. v v i i i x v i i 1 5 7 8 9 11 11 12 16 18 20 22 26 30 32 32 33 34 36 v i MATERIALS AND METHODS 36 38 RESULTS (a) Survey of the Fauna (b) Abundance and Size of Prey DISCUSSION TIME SPENT HANDLING PREY (T R) METHODS RESULTS (a) Adult Feeding (b) Feeding of Chicks at the Nest (c) Feeding of Chicks at the i n t e r t i d a l THE AVERAGE WEIGHT OF PREY (WA) METHODS RESULTS (a) Measurement of Prey Size (b) The Average Weight of Prey 72 RATE OF SUCCESSFUL SEARCH (a) 75 METHODS 76 RESULTS 76 THE PROFITABILITY OF PREY 78 THE DIET OF THE BLACK OYSTERCATCHER 82 METHODS 83 Adult Diet 8 Chick Diet 8 RESULTS 8 Adult Diet - composition 8 Chick Diet - composition 8 38 40 59 61 61 61 61 63 66 68 68 68 68 v i i COMPARISON OF ADULT AND CHICK DIETS 88 Composition 88 Prey Size 95 THE EFFECTS OF LOCATION ON THE DIET OF CHICKS 97 DIET IN RELATION TO PROFITABILITY 105 THE RESPONSE TO PREY 109 (a) General Remarks and Observations 109 (b) Experimental Presentation of Prey 111 Observations and Results 113 Discussion 116 FORAGING IN TIME AND SPACE 118 METHODS 118 RESULTS 120 1. Foraging i n time 120 (a) T i d a l E f f e c t s 120 (b) E f f e c t s of Young 125 (c) E f f e c t s of Interactions 125 2. Foraging i n Space 126 GENERAL DISCUSSION 131 BIBLIOGRAPHY 136 v i i i LIST OF TABLES TABLE Page I Census of Oystercatchers - 1972 13 II Clutch size of black oystercatchers 16 III Hatching and fledging success 19 IV Comparison of oystercatcher behaviour on Kayak and Cleland Islands 27 V Zones present on Cleland Island - based on Ricketts and Calvin (1968) 32 VI Numbers and sizes of organisms found i n square foot quadrats i n zones 1 and 2. 42 VII Densities of organisms within reach of oystercatchers i n zone 3 (25 quadrats) 45 VIII Density (number per quadrat) of vulnerable mussels over tide cycle 53 IX Organisms found i n square foot quadrats i n the interface area 56 X Organisms i n zone 4 (based on 8 quadrats) 58 XI The feeding area - Summary 60 XII E f f e c t s of Size on handling time of mussles 62 XIII Adult Prey size 69 XIV Calculation of the Rate of successful Search 77 XV P r o f i t a b i l i t i e s of Prey items 81 XVI Composition of adult d i e t determined from observed feedings 85 XVII Composition of chick d i e t from 517 observed feedings 86 XVIII Shells c o l l e c t e d by nests 87 XIX Limpets carried whole to chicks 87 XX Frequencies of prey items i n adult and chick diets based on observed feedings 89 XXI The adult d i e t before and during the presence of chicks 92 ix XXII Observed feedings of adult birds XXIII Results of multiple regression analysis of proportion of d i e t consisting of small items XXIV Changes i n prey with chick location XXV E f f e c t s of location on chick feeding rate XXVI P r o f i t a b i l i t y and the use of mussels and limpets i n the three feeding contexts XXVII Selection of limpets from groups of large and small ones during f i r s t feeding bout XXVlllThe e f f e c t s of s i z e and encounter rate on prey s e l e c t i o n XXIX Size s e l e c t i o n of limpets XXX E f f e c t s of young on frequency and length < foraging t r i p s XXXI Sequence of areas v i s i t e d while foraging XXXII The timing of t r i p s to distant reefs X LIST OF FIGURES Figure 1. The v a r i a t i o n i n the size of the roosting flock with time i n the ti d e cycle as i l l u s t r a t e d by counts made during ebb and flood tides on two separate occasions. 2. Size of flock at high tid e 3. Clutch commencement of black oystercatchers on Cleland Island 1970-72 4. Survival i n eggs and chicks i n 1971 5. D i s t r i b u t i o n of nests i n r e l a t i o n to predation and exposure to storms - 1971 6. D i s t r i b u t i o n of nests i n r e l a t i o n to predation and exposure to storms - 1972 7. Size d i s t r i b u t i o n of mussels N = 2287 8. Number of mussels per quadrat at d i f f e r e n t t i d e l e v e l s 9. E f f e c t s of shore l e v e l on average size of mussels 10. Size of limpets on the mussel bed 11. Density of limpets at d i f f e r e n t levels i n the mussel bed 12. Comparison of limpets i n adult d i e t with those i n the mussel bed 13. Size of mussels i n the d i e t of adults 14. Comparison of mussels c a r r i e d to chicks with those present i n the habitat 15. Comparison of adult and chick diets from observed feedings 16. Size of limpets i n adult and chick diets 17. Number of mussel and limpet s h e l l s found by one nest i n 1971 - one chick 18. Number of mussel and limpet s h e l l s found by one nest i n 1971 - two chicks 19. Size of mussels and limpets c a r r i e d whole to a single chick i n 1971 x i 20. Size of mussles and limpets ca r r i e d whole to two chicks i n 1971 104 21. Ef f e c t s of tide range on time of i n i t i a t i o n of foraging 121 22. Foraging A c t i v i t y over tide cycles 122 23. Foraging time and tide l e v e l 124 x i i ACKNOWLEDGMENT I am esp e c i a l l y g r a t e f u l to Dr. C.S. Holling, my research supervisor, for his support throughout the project. I wish to thank Mr. F. Maurer, who was a valuable source of advice on technical matters. Mr. J . Duncan and the 'Bare Island Crew' were most he l p f u l i n setting up f i e l d f a c i l i t i e s . I am gratef u l to Dr. R. Drent for his advice on a study s i t e . Special thanks go to the McLory family i n Tofino for t h e i r h o s p i t a l i t y . I wish also to thank my wife Heather for typing numerous copies of the thesis under almost impossible conditions. Dr. C. Walters, Dr. D. McPhail, Dr. R. Liley,. and Dr. W. Rees read the thesis and provided valuable suggestions for i t s improvement. Fin a n c i a l assistance was provided by the Dept. of Zoology, U.B.C., the National Research Council of Canada and Carling Breweries. 1 INTRODUCTION The possible application of optimality p r i n c i p l e s to biology was discussed by Rashevsky i n 1938 and more recently by Rosen ( 1 9 6 7 ) . Both authors emphasize that there are a number of competing solutions to any p a r t i c u l a r b i o l o g i c a l problem. The adaptive 'design' of the organism i s then considered to be an optimal design i n the sense that i t minimizes a cost function for the set of competing solutions. Recently such arguments have been applied to the feed-ing behaviour of organisms. This has led to the development of a number of d i f f e r e n t models for optimal foraging. These models are generally of two types. On the one hand there are models which involve both time and energy expenditure i n the cost of predation. The strategy i s one of maximizing net energy intake i n a given feeding time. Schoener (1971) has reviewed the l i t e r a t u r e on foraging strategies and proposed a model of t h i s type. The other class of models involve only time and some measure of return l i k e biomass y i e l d . In p a r t i c u l a r , Royama (19 70) has developed a model which predicts the hunt-ing behaviour of a predator i n time and space. Following a study which included detailed observations on the food items carried to nestlings by adult titmice, he sought to explain why, at times, very abundant prey were not taken by the birds while larger but much less abundant prey were. His data supported the conclusion of Tinbergen (1960) that there was no simple c o r r e l a t i o n between the composition of the 2 nestlings d i e t and the r e l a t i v e abundance of the prey i n the habitat. As an alternative to the search image theory proposed by Tinbergen, Royama developed his 'hunting by p r o f i t a b i l i t y 1 model. The problem, i n Royama's words, i s "to discover the most productive way for i t (the t i t ) to alloca t e i t s hunting time between d i f f e r e n t prey species". He reasoned that from the predator's point of view i t was not the density of a prey species that was necessarily important but rather how much of that prey the predator could c o l l e c t i n a given time spent hunting. He c a l l e d t h i s amount, the p r o f i t a b i l i t y of the prey species and suggested that i t was predictable from the equation developed for the functional response of predators (Holling, 1965). According to Royama a b i r d w i l l move about the environment sampling d i f f e r e n t prey but spending most of i t s time hunt-ing species which are most p r o f i t a b l e . Since the pr o f i t a b -i l i t y of a prey may change over time due to changes i n the quantity or qu a l i t y of the prey or i n the r e s p o n s i b i l i t i e s of the adult forager, the birds must continually sample d i f f e r e n t areas and d i f f e r e n t prey to monitor these changes. Royama's theory of hunting by p r o f i t a b i l i t y was a t t r a c t i v e for a number of reasons. I t was r e l a t i v e l y simple and appeared to be testable i n the f i e l d . More-over, Holling (1968) has argued that optimal strategies developed i n theory may not be capable of complete expression i n nature because of t a c t i c a l l i m i t a t i o n s . By using the Holling 'disc' equation, Royama combines both t a c t i c s and 3 strategies, providing at least an i n i t i a l step towards bridging the gap between these two rather d i f f e r e n t approaches in ecology. The present study was i n i t i a t e d i n order to examine Royama*s theory i n a f i e l d s i t u a t i o n . The need for an increased integration of theory and empirical work has been stressed i n the l i t e r a t u r e (see, for example, Watt, 1962) . Indeed, the answer to many of the current c r i t i c i s m s of t h e o r e t i c a l models seems to l i e i n the lack of the ' r i g h t 1 kind of data. Much f i e l d work has not been directed and interpreted by theories (Watt, 1962). Both Holling (1965) and Watt have argued that such fragmented studies cannot greatly advance a science. In view of t h i s , c a r e f u l con-sideration was given to the s e l e c t i o n of a species which would lend i t s e l f to the study of foraging theory. An examination of the l i t e r a t u r e on optimal foraging revealed a lack of d i r e c t observations on the foraging of adults. Indeed, Royama's model was based only on the feed-ing of nestlings by adults. This led to a search for a species whose foraging could be observed and where both adult and chick d i e t could be determined. The Black Oystercatcher, Haematopus bachmani Audubon, was selected for the present study. Research on the forag-ing habits of t h i s shorebird was i n i t i a t e d i n 1971 and con-tinued into 1973. The major portion of the work took place during the 1971 and 1972 breeding seasons (MayfAugust) with short f i e l d t r i p s being made during the winters and i n the 1973 breeding season. The immediate objectives of the present study were as 4 follows: 1. to study the natural history of the oystercatcher e s p e c i a l l y as i t related to the requirements of foraging. 2. to look for evidence that maximization of feeding rate would be advantageous to the oystercatcher. 3. to determine the prey of the oystercatcher and to calculate p r o f i t a b i l i t i e s for each of them. 4. to determine the composition of the d i e t of the oystercatcher and to record the circumstances under which the foraging took place. 5. to examine the d i e t , and the foraging i n time and space, i n r e l a t i o n to the r e l a t i v e p r o f i t a b i l i t i e s of d i f f e r -ent prey species as predicted by Royama. 5 STUDY ANIMAL The Black Oystercatcher, Haematopus bachmani i s a shore-b i r d belonging to the family Haematopodidae of the order Charadriiformes. Small numbers of these oystercatchers inhabit most of the shoreson the rocky open coast of western North America. The adult b i r d i s black and brown with an unmistakable red, l a t e r a l l y compressed b i l l . Information on the biology of t h i s species was coll e c t e d by Webster who published a series of papers dealing with plumage, growth, breeding and feeding habits (Webster, 1941). However, his studies on breeding and feeding were limited and many of his statements concerning the behaviour of the birds are not supported by the present study. No work has been car r i e d out on the foraging behaviour of the black oyster-catcher i n r e l a t i o n to the a v a i l a b i l i t y of food i n the habitat. Websters' study of feeding habits was limi t e d to i d e n t i f i c a t i o n , of the food of the oystercatcher as indicated by the sh e l l s that he found near young on several small islands near Alaska, and to stomach analysis of ten adults. Only four of these adults were co l l e c t e d i n the breeding season and l i t t l e i n -formation was available i n these. The black oystercatcher inhabits the surf-swept rocky coast. In the breeding season i t defends a t e r r i t o r y which includes a section of i n t e r t i d a l where foraging takes place. The p o s s i b i l i t y of observing adult foraging and chick feed-ing made t h i s b i r d an excellent candidate for studies on foraging i n r e l a t i o n to present theory. 6 A r e l a t i v e l y large number of oystercatchers was known to inhabit the area around Clayoquot Sound on the west coast on Vancouver Island. In 1971 a study was i n i t i a t e d to investigate the feeding ecology of the Black Oystercatcher on Cleland Island i n Clayoquot Sound. 7 THE STUDY SITE - CLELAND ISLAND The study s i t e i s a small i s l a n d , about 0.6 kilometers long by 0.4 kilometers wide, located i n Clayoquot Sound on the west coast of Vancouver Island (latitude 49° N. longitude 126° W). The isl a n d consists largely of bare rock with an extensive grass-brush b e l t covering the higher areas. Driftwood i n many places suggests that waves sweep much of the i s l a n d during winter storms. Exposure varies around the i s l a n d with some protection given to the north-west shores by the presence of a large reef. Winds are common with a d a i l y pattern of low winds in early morning, increasing to moderate or fresh by a f t e r -noon. South-east winds usually bring r a i n but have less e f f e c t on surf than the strong westerlies or the very common north-west winds. Tides are mixed semi-diurnal with a mean tide of 2.6 meters and a large tide of 3.9 meters. Low tides reveal an extensive i n t e r t i d a l t y p i c a l of the rocky exposed coast. The i s l a n d i s a breeding s i t e for many birds including glaucous-winged g u l l s , black oystercatchers and various a l c i d s . In addition, the i s l a n d i s used as a roost by cormorants and as a stop-over for many migrating shorebirds. 8 POPULATION AND NATURAL HISTORY STUDY Data on population and natural history were co l l e c t e d throughout the study. Such data were expected to provide support for the choice of the oystercatcher as a to o l for studying foraging strategies. Moreover, foraging i s necessarily influenced either d i r e c t l y or i n d i r e c t l y by the requirements of the birds to perform other a c t i v i t i e s associated with breeding. A study of these a c t i v i t i e s and the factors a f f e c t i n g them seemed to be necessary background information for the interpretation of observed behaviour patterns. 9 METHODS Blinds constructed of wood or canvas were set up at si t e s where, i f possible, both a nest and the feeding area associated with i t could be observed. Pairs of birds were then observed by telescope from these blinds and t h e i r general behaviour and t h e i r interactions with other birds were recorded. Data on the population were co l l e c t e d by making regular t r i p s around the i s l a n d . Counts of single birds, pairs or flocks were recorded along with the area with which they were associated. The presence of a flock of non-t e r r i t o r i a l birds was noted and i t s size was monitored throughout the season as well as over p a r t i c u l a r t i d a l cycles. A l l counts were made at high tide when birds were roosting and easier to locate. With time i t was possible to associate certain areas with p a r t i c u l a r pairs of birds . These ' t e r r i t o r i e s * were c a r e f u l l y searched for nests which, when located, were assigned a nest number and checked regularly thereafter i n order to record the number of eggs or young present. Attempts were made to determine the causes of mortality to both eggs and young and the circumstances under which the loss occurred. Situations where s u r v i v a l of eggs or young appeared to be threatened were also recorded. Data on the population i n 1970 were also available from J . Ward who recorded the nests and t h e i r contents while carrying out a study of the Glaucous-winged g u l l . Some data on nesting success i n other areas were also 10 available from a general b i r d census carried out by D. Hatler for the P a c i f i c Rim Park region. Banding of adult birds was ca r r i e d out i n 1971. One adult from each of nine pairs of breeding birds was caught and banded with both a colour band and a C.W.S. band. No adults were banded i n 1972 but a l l chicks close to fledg-ing were banded i n both years. 11 RESULTS (a) Breeding Behaviour After the f i r s t few t r i p s around the i s l a n d i t became clear that p a r t i c u l a r pairs of birds were associated with d e f i n i t e areas and that the pairs a c t i v e l y defended these areas, d r i v i n g other oystercatchers o f f or "bowing" them o f f i n a piping ceremony. This ceremony consisted of much bobb-ing of the body accompanied by a series of loud, high-pitched sharp notes given while the b i r d i s i n a peculiar 'hunched-forward' posture. Such ceremonies occurred frequently at the boundary between two t e r r i t o r i e s and a l l four birds were involved. The t e r r i t o r y included both a nest s i t e and a section on the i n t e r t i d a l . In most cases these formed a single area but i n a few cases the nest s i t e was i s o l a t e d from the feeding area by water or by other t e r r i t o r i e s . The oystercatchers nested over most of the open rocky areas of the i s l a n d but were not found i n the extensive grass-brush b e l t i n the central part. Two types of nests were found. In many cases the nest consisted of s h e l l fragments l i n i n g a depression i n the rock surface while other nests were simple depressions i n the s h e l l beaches. Observations of nest-building behaviour indicated that both members of the p a i r became deeply involved i n the process with co-operative building l a s t i n g as long as an hour at a time. Mating was an extremely simple behaviour with no apparent displays, sometimes occurring on the i n t e r t i d a l during 12 foraging or during nest building. After a period of several weeks egg-laying began with clutches of up to three eggs being l a i d . Incubation was approximately'twenty-six days with both adults taking turns incubating during a t i d a l cycle. T e r r i t o r i a l c o n f l i c t s with neighbouring birds some-times involved both members of a pair and at these times the eggs were t o t a l l y unprotected. After hatching, the chicks remained i n the nest for a short time before moving to a position near the nest. In one case a parent was observed to remove the egg s h e l l from a nest with newly hatched chicks and deposit i t around forty feet away. When the chicks were young one of the parents brooded them while the other parent foraged and carr i e d food items one at a time to the chicks. Such feeding t r i p s i n i t i a l l y involved f l i g h t but within a few days the adults began to walk to and from the feeding area. Shortly thereafter the chicks were moved i n the d i r e c t i o n of the feeding area. This move was carried out i n stages. The f i r s t move was generally short and occurred as early as two or three days after hatching. The age at which the chicks reached the feeding area varied from as young as two days to as old as twenty-five days. In a few cases which are described l a t e r under 1 Interactions' the chicks remained at the nest u n t i l fledging at an age of approximately forty days. (b) Population Size When censusing began i n early May pairs of birds were already associated with d e f i n i t e t e r r i t o r i e s and c o n f l i c t s with neighbouring pairs or intruding birds were common. 13 In 1972 regular counts of the number of breeding pairs and the numbers of birds i n the flock which roosted on the island indicated that the t o t a l number of adult birds on the island increased over the season from less than eighty birds i n May to over one hundred birds i n August (Table 1). TABLE I Census of Oystercatchers - 1972 Census Period Counts Number of Birds (mean) Range May 22 - June 5 4 75 68-80 June 8 - Aug. 15 4 98 95-103 The flock which roosted i n one part of the i s l a n d apparently consisted of n o n - t e r r i t o r i a l b i r d s . The size of thi s flock varied over the t i d a l cycle (Fig. 1) with sharp increases occurring midway through flood tide and decreases through ebb t i d e . Such changes i n the flock size were accompanied by f l i g h t s of smaller flocks which engaged i n 'piping' interac-tions with t e r r i t o r i a l pairs around the i s l a n d . Birds belonging to the flock were often seen f l y i n g i n from reefs some distance away and presumably these were returning from feeding grounds. Counts of the number of birds i n the flock at high tide indicated a seasonal change i n the size of the flock. The flock increased i n size throughout the season (Fig. 2) apparently reaching a higher number i n 1971 than 1972. In one case a banded adult was observed i n the flock a f t e r the adult had l o s t i t s clutch of eggs. Later i t was observed back on i t s t e r r i t o r y . 14-.? FIGURE I. The v a r i a t i o n i n the size of the roosting flock with time i n the tide cycle as i l l u s t r a t e d by counts made during ebb and flood tides on two separate occasions. H O U R S F R O M HIGH T I D E I5f FIGURE 2. Size of flock at high t i d e 16 (c) Egg-laying and Clutch size Egg-laying began early i n May and continued into July with a peak i n clutch commencement i n the f i r s t week of June in 1970 and 1971 and an e a r l i e r peak i n 1972 (Fig. 3). Clutch size varied between one egg and three eggs (Table II) with an average clutch size of 2.1 eggs. TABLE II Clutch Size of Black Oystercatcher Clutch Size Number of Nests Total 1970 1971 1972 3 eggs 9 14 13 36 2 eggs 38 26 34 98 1 egg 14 8 11 33 167 I7P FIGURE 3. Clutch Commencement of Black Oystercatchers on Cleland Island 1970-72 17 o a> N . 0) CD ZD —3 CO CM CD HI z ZD aaivuiNi S3HOimo d o aaaiAjriN CM .CO o CM — i — o o CM O O O CM I o CO 18 (d) Survival of eggs and young The nest records for Cleland Island give hatching data for a t o t a l of 167 clutches (1970-1972). Hatching success varied from a low value of 25% of 120 eggs l a i d i n 1971 to a high value of 46% of 102 eggs l a i d i n 1972 (Table I I I ) . This was accompanied by low fledging success i n a l l three years with only 28% of 47 young fledging i n 1972. The number of chicks fledged per breed-ing pair varied from 0.19 i n 1971 to 0.31 i n 1972. The two main sources of mortality to both eggs and chicks were storms which washed out the nests or drowned the chicks, and predation or destruction by other birds l i k e g u l l s and crows. In a few cases chicks were found apparently drowned i n small pools close to the nest. Usually i t was possible to determine the cause of death of eggs or chicks although i n some cases they d i s -appeared without a trace, presumably ca r r i e d o f f by predators. In 1971 forty clutches suffered losses of eggs or chicks to storms and predation. Storms accounted for nineteen (48%) of these cases and predation accounted for the r e s t . TABLE III Hatching and Fledging Success Year No. of Clutches Eggs Percent Young Percent Chicks fledged Breeding pairs (total) (total) hatching (total) Fledging per breeding pa 1970 56 60 117 34 40 38 0.27 1971 57 59 120 25 30 37 0.19 1972 42 48 102 46 47 28 0.31 20 The pattern of mortality appeared to be si m i l a r i n 1970 and 1971 with l i t t l e or no loss i n eggs occurring u n t i l the second week i n June when a high mortality occurred at a time when the egg-laying of g u l l s was peaking. In 1971 a severe storm occurred on June 24 r e s u l t i n g i n a heavy loss of eggs (Fig. 4). In 1972 clutch commencement was e a r l i e r and a greater percentage of eggs hatched. Although winds were common no major storms occurred and as a r e s u l t only eight nests suffered losses to storms. However, the loss to predation was high with much of i t occurring at the chick stage so that the percent fledging was lower than the previous two years. (e) Interactions (1) Between Oystercatchers C o n f l i c t s between adults often occurred at the border of adjacent t e r r i t o r i e s and were accompanied by 'piping ceremonies'. In some cases these interactions became 'vicious' battles including attacks on birds which were s i t t i n g on the nest. Adults were also observed to chase single intruders o f f the t e r r i t o r y and to engage i n 'piping f l i g h t s ' with the flocks which often flew around the i s l a n d . Adults were observed to attack young oystercatchers i f a parent attempted to move them through the t e r r i t o r y of another p a i r , and one case was observed where a fledged b i r d was attacked by adult birds while f l y i n g out over the water. 2lf FIGURE 4. Survival i n eggs and chicks i n 1971 22 (2) With other species C o n f l i c t s with g u l l s were common and occurred i n a l l phases of breeding. Adult oystercatchers did not hesitate to attack g u l l s that approached t h e i r young but g u l l s were also observed chasing oystercatchers away. Given the chance g u l l s attack and k i l l young oystercatchers which enter t h e i r t e r r i t o r i e s , and i n some instances prevent adult oyster-catchers from moving t h e i r chicks to the i n t e r t i d a l . This resulted i n a s i t u a t i o n where some chicks remained near the nest s i t e u n t i l fledging. Such a s i t u a t i o n occurred i n two nests i n 1971 and 1972. One of the pairs was unsuccess-f u l i n 1972 and the r e s u l t s were uncertain i n 1971. The other pair managed to fledge one chick i n both years. In addition to t h e i r interactions with g u l l s , the oystercatchers were observed to chase crows away from the nest area and occasionally pigeon guillemots i f they were near the chicks. Several instances were observed where adults chased other shorebirds o f f the i n t e r t i d a l . (f) T e r r i t o r i e s i n r e l a t i o n to breeding success The nests of the oystercatcher occurred on the open rock surfaces and s h e l l beds around the edge of the i s l a n d . Glaucous-winged g u l l s nested on the open rock as well but t h e i r density was highest higher up near the edge of the vegetation (John Ward, personal commmnication), The distance from an oystercatcher's nest to i t s feeding area varied considerably from approximately twelve meters to over sixty meters. The location of the nest seemed to have a large e f f e c t on the chances of success. While the loss of 23 eggs or young to gu l l s or crows occurred throughout the is l a n d , much of the loss from surf took place on the r e l a t i v e l y low exposed southern t i p of the is l a n d (Fig. 5 and 6). Survival seemed highest i n that part of the west side of the island protected by reefs. This area was also protected from the d r i v i n g r a i n c a r r i e d by south-east storms. A study of figures 5 and 6 suggests that successful breeding occurred i n the same areas i n both 1971 and 1972. Indeed, several pairs were successful i n both years, but i n other cases pairs were successful i n only one of the years. Successful breeders generally nested away from the higher density of g u l l s occurring near the vegetation but i n an area that was not greatly exposed to surf. In 1971 an in t e r e s t i n g concentration of nests occurred i n a trough on the south-east side of south point near the roost s i t e of the flock. The nests were unusual as there did not seem to be a feeding area associated with them. The area where they occurred was extremely vulnerable to south-east storms and a l l of these nests were washed away by waves. Such a concentration of nests did not occur i n 1972. Of the nine adults banded i n 1971, three were observed on the same t e r r i t o r i e s i n 1972 while the other s i x were not found. No birds banded as chicks were sighted. A b r i e f check i n 1973 indicated that one b i r d was associated with the same t e r r i t o r y for three years i n a row. None of the banded birds returning i n 1972 were successful i n breeding i n 1971. FIGURE 5. D i s t r i b u t i o n of nests i n r e l a t i o n to predat and exposure to storms - 1971 25 f FIGURE 6. D i s t r i b u t i o n of nests i n r e l a t i o n to predat and exposure to storms - 1972 25 DISCUSSION Research on the breeding of the black oystercatcher in the Sitka region was c a r r i e d out by Webster (1941). His data and interpretations seem to be based on r e l a t i v e l y few observations and the differences between what he reports for Kayak Island and the s i t u a t i o n on Cleland Island are s u f f i c i e n t to warrant a comparison (Table IV). The differences are rather large and i t seems unreasonable that such differences would be r e a l . Webster's suggestion that the birds take turns feeding the chicks over a whole tide cycle seems u n r e a l i s t i c . On Cleland i t was often easy to dis t i n g u i s h between members of a pair because they were banded, or t h e i r t e r r i t o r y was c l e a r l y v i s i b l e so that they could be kept track of, or morphological differences were noted (for example, there were often differences i n the eye, and l o r a l regions). Although the behavioural differences reported above seem u n r e a l i s t i c , differences i n breeding success may exist between areas. The res u l t s of the population study indicated a consistently inhospitable environment for black oystercatchers on the outside coast. The low success rate found on Cleland Island appears to be representative of the outer coast as indicated by nest checks c a r r i e d out by D. Hatler i n the Ucluelet and Barclay sound areas. However, production figures for ten pair-seasons on Mandarte Island on the east coast of Vancouver Island indicated much greater success where with 0.7 young raised 27 TABLE IV Comparison of oystercatcher behaviour on Kayak and Cleland Islands Behaviour nesting nest building clutch size incubation look-out post of b i r d not incubating piping f l i g h t s feeding of chicks feeding of chicks chick behaviour Kayak Island -Webster 1941 fi v e types of nests single i n d i v i d u a l c a r r i e s rock flakes from great distance a clutch of four was found pair take turns, changing every twelve hours Cleland Island -Hartwick 1973 two types of nests both of pa i r involved rock flakes are tossed behind the b i r d rather than c a r r i e d d i r e c t l y to nest largest clutch was three eggs pairs take turns, changing frequently sometimes several times i n one hour never at greater i n some cases i t was elevation than nest at greater elevation even though such places than nest available not made afte r eggs hatched continued a f t e r eggs hatched one parent feeds chicks birds change places over t i d e cycle while several times per hour other guards, then and both feed chicks changes places once they are moved to feeding area only v/hole mussels carr i e d to chicks chick does not follow foraging adult u n t i l t h i r d to f i f t h week flock behaviour no piping a f t e r June f l i g h t s to distant feeding grounds discontinued a f t e r eggs hatched i n many cases only the meat i s ca r r i e d to the chicks chick may follow adult within one week piping continued throughout season continued through-out season 28 on average by each pair (Drent et a l . , 1964), while Webster (1941) reported only 0.4 young per pair for the Sitka region. Boyd (1962) has summarized data for the European Oystercatcher (Haematopus ostralegus) i n three d i f f e r e n t areas. Young fledged per pa i r varied from 0.1-to 2.1 so that i t does seem that success may vary considerably between di fferent areas. On Cleland, 155 pairs raised only 39 chicks i n the three years that were monitored, i n d i c a t i n g approximately an eight year period of production required to sustain the population. Figures on adult mortality are not available but no loss of adults was observed during the breeding season and checks made during the winters indicate a seasonal move-ment into the protected i n l e t s where, unless food i s i n short supply, s u r v i v a l should be f a i r l y high as well. Successful breeding on Cleland Island depended on protection from both predators and storms. The location of the nest was important. If a b i r d nested i n the higher areas near the edge of the vegetation i t s chances of success-f u l breeding were low because of the higher density of g u l l s which may either destroy the eggs or k i l l the chicks. Although there may be an advantage i n nesting near g u l l nests to lessen the chance of crow predation, the oyster-catcher requires a safe route to lead chicks from the nest to the i n t e r t i d a l and g u l l s were observed to i n t e r f e r e with these moves. Birds that nest close to the i n t e r t i d a l on the other hand, escape the threat of g u l l s but are highly vulnerable to wash-out by waves. 29 Thus the b i r d i s faced with the problem of incubating i t s eggs, and feeding and protecting i t s young i n an area some distance from i t s food source and i n the v i c i n i t y of h o s t i l e g u l l s with a high r i s k of wasted e f f o r t i f a storm s t r i k e s . While chicks are near the nest only one adult can forage at a time since the other adult remains to provide protection for the young. Since they carry only one item up to the chicks on any one t r i p , time i s spent t r a v e l l i n g back and forth between the chicks and the feeding area. Sometimes t e r r i t o r i a l c o n f l i c t s involve both of a pair and thus the chicks are l e f t unguarded i n the v i c i n i t y of h o s t i l e g u l l s . The dangers increase as the young oyster-catchers become more mobile and the g u l l s themselves become more aggressive as t h e i r own eggs hatch and young move about. Under such circumstances i t seems reasonable to expect the adults to forage e f f i c i e n t l y . E f f i c i e n c y i n foraging would be an advantage i f rapid growth increased the chances of surv i v a l of chicks or i f adults were able to spend more time caring for t h e i r young. Some of the possible advantages i n moving the chicks to the feeding area are obvious at t h i s point. Once the chicks are moved, they are free from harassment by g u l l s and both adults can forage for and protect the young at the same time. However, the chicks are s t i l l vulnerable to storms and remain so presumably u n t i l fledging. 30 THE PROFITABILITY HYPOTHESIS P r o f i t a b i l i t y i s the amount of a prey species that a predator can c o l l e c t i n a given time spent hunting that prey. A measure of t h i s p o t e n t i a l harvest rate i s given by the Holling 'disc' equation. P r o f i t a b i l i t y = P = N, W_ = a (T-TTT N J N W = a M W, 1 A A H A' o A Q A T T 1 + a T N H o where NA = number of prey attacked a = rate of successful search T = time prey are exposed T = H handling time W = A average weight of prey N = o prey density Royama suggests that the composition of the d i e t and the selection of s p e c i f i c items from a c o l l e c t i o n of varying sizes and abundances can be explained on the basis of t h i s model. The composition r e s u l t s from the tendency to hunt prey according to how p r o f i t a b l e i t i s and to spend more time hunting where p r o f i t a b i l i t y i s highest. Since p r o f i t a b i l i t y can change over time and space the animal must continually sample to monitor these trends. In order to t e s t and explore Royama1s hypothesis i t i s necessary to determine values for the parameters i n the model. These include the density of prey (N o), the average weight of the prey (W ), the time spent handling the prey (T ), 31 and the rate of successful search of the predater (a). P r o f i t a b i l i t i e s for the various prey items can then be calculated on the basis of equation (1). The hypothesis can be tested i n part by determining the composition of the diet of the birds and comparing th i s with the calculated p r o f i t a b i l i t y indices for each prey. According to Royama's theory, oystercatchers should respond rather rapidly to changes i n v u l n e r a b i l i t y of prey i n the i n t e r t i d a l , and for periods of time when d i f f e r e n t prey are exposed to predation the die t of the birds should r e f l e c t the r e l a t i v e p r o f i t a b i l i t i e s of the prey. The black oystercatcher i s superbly suited for t h i s p a r t i c u l a r study because values for each of the parameters in the model can be calculated and the hypothesis can be examined under the following d i f f e r e n t feeding contexts. 1. Feeding of adults 2. Feeding of chicks at the nest 3. Feeding of chicks at the feeding area 32 FORAGING OF BLACK OYSTERCATCHERS - General Description The c a l c u l a t i o n of parameter values for the pr o f i t a b -i l i t y equation requires an intimate knowledge of the hunting behaviour of the predators (Royama, 1970). A b r i e f des-c r i p t i o n of t h i s a c t i v i t y i s presented here as necessary background information for determining p r o f i t a b i l i t i e s . (a) The Foraging Area The c l a s s i f i c a t i o n scheme of Ricketts and Calvin (1968) can be used to describe the i n t e r t i d a l . The shore habitat i s open coast, rocky type, being mainly unprotected and surf-swept. The shore i s d i v i s i b l e into zones according to t i d a l exposure although the extent and l e v e l of the zones tends to be d i f f e r e n t from that given by Ricketts and Calvin. The zones on Cleland are both higher and wider because of the great exposure of the island but the general description of the zones i s sim i l a r (Table V). Tides i n the area are mixed semi-diurnal so that the i n t e r t i d a l i s exposed to varying extents twice d a i l y . TABLE V Zones Present on Cleland Island - Based on Ricketts and Calvin (1968) Zone General Description 1. Uppermost horizon Mainly a region of bare rock 2. High i n t e r t i d a l Rockweed, bare rock toward upper l i m i t s 3. Middle i n t e r t i d a l Mussel bed 4. Low i n t e r t i d a l Laminarians and c o r a l l i n e s 33 (b) The Prey of the Black Oystercatcher Based on observations and the c o l l e c t i o n and i d e n t i f -i c a t i o n of sh e l l s of animals attacked by the bir d s , the Black Oystercatcher feeds to a varying extent on the following organisms. Molluscs Arthropods Annelids Sipunculids -Echinoderms -Nemertines mussels gastropods gastropods chitons crabs isopods Mytilus californianus  Acmaea d i g i t a l i s Nereis worms Peanut worms sea cucumbers ribbon worms Acmaea mitra Acmaea pelta Acmaea scutum Acmaea persona Crepidula nummaria  Katharina tunicata  Mopalia muscosa  Mopalia l i n e a t a  Petrolisthes cinctipes  Oedignathous inermis  Idothea sp. L i g i a sp. Nereis sp. unidentified Cucumaria sp. uniden t i f i e d 34 (c) Hunting and Attack Behaviour The prey of the oystercatcher e x i s t i n a variety of microhabitats and under conditions which often require a d i f f e r e n t type of hunting behaviour for d i f f e r e n t prey. Moreover, the substrate had some e f f e c t on the a b i l i t y of the birds to move around i n search of prey. For example, the birds often slipped on the wet laminaria i n the lower zone. The presence of weeds required probing and moving the weeds aside i n search of prey. While the birds were hunting i n p a r t i c u l a r zones a number of d i f f e r e n t hunting modes were apparent. For example, birds foraging on the mussel bed were observed at times to feed on small limpets and cucumbers which were f a i r l y numerous on and i n between the mussels or gooseneck barnacles. While foraging for these small items the birds generally moved slowly, often turning, and they usually probed weeds as they moved about. At other times the birds moved quickly past these smaller prey items and went down to the water's edge where they foraged on gaping mussels. At such times the birds are often unresponsive to small prey, do not probe weeds as they pass and appear to cover greater distances while forag-ing. In some cases the use of p a r t i c u l a r areas of the i n -t e r t i d a l was obvious. For example, under ce r t a i n circum-stances (see chick feeding) the adults spent considerable time foraging along the bottom edge of the mussel bed where the crab Oedignathous inermis was av a i l a b l e . The major prey of the black oystercatcher were mussels 35 and limpets. A b r i e f description of the attack behaviour on these items i s worthwhile. European Oystercatchers (H. ostralegus) apparently have two methods of attacking mussels (Norton-Griffiths, 1967). Some mussels are 'hammered' u n t i l the s h e l l gives way while others are 'stabbed' when they are gaping. Only the 'stabbing' method of opening mussels was observed on Cleland Island. In t h i s case the b i r d searches for gaping mussels, orients i t s e l f to the valves and d e l i v e r s a sharp downward stab with the b i l l . With rapid levering and b i t i n g the b i r d draws out the meat. In some cases the whole mussel including the s h e l l comes loose and the b i r d usually c a r r i e s i t up to a higher location where i t then withdraws the meat. A necessary part of the attack seems to be the cutting of the adductor muscles i n the f i r s t stab since birds were occasionally seen struggling to withdraw t h e i r b i l l s a f t e r a mussel closed on them. Limpets appear to pose no problem for the oystercatchers. The b i r d generally lowers i t s head and delivers one or more sharp jabs on the edge of the limpet s h e l l . The dislodged limpet i s then placed, s h e l l down, and the b i r d r a p i d l y bites around the edge of the s h e l l , f i n a l l y picking the body up and shaking the s h e l l o f f . The meat i s then swallowed i n one gulp. Limpets removed from shallow pools or steeper areas were sometimes carr i e d up and positioned, s h e l l down, on a higher area or on s h e l l beach. 36 ABUNDANCE (NQ) AND SIZE OF PREY IN THE HABITAT The i n t e r t i d a l on Cleland Island was studied for two reasons. On the one hand intensive studies were ca r r i e d out i n order to determine the densities of various prey vulnerable to attack by oystercatchers. In addition, general c o l l e c t i o n s and i d e n t i f i c a t i o n s were made i n order to provide a l i s t of species found there. MATERIALS AND METHOD Different parts of the i n t e r t i d a l were studied both intensively and extensively throughout the season. As well as making general c o l l e c t i o n s for i d e n t i f i c a t i o n and record, quadrats were used to obtain measures of the densities and size d i s t r i b u t i o n s of various organisms. Sampling consisted of tossing a foot square quadrat behind the sampler and- counting and measuring a l l organisms within the quadrat. In some cases these quadrats were part of transects running down the i n t e r t i d a l at randomly chosen points. In the spec i a l case of small pools very small samples measuring two inches square were removed e n t i r e l y to give some in d i c a -tion of the organisms present and t h e i r s i z e . Although some samples of the mussel bed were also removed completely and examined to see the variety of organisms present, only those organisms within b i l l depth are considered as vulnerable to the birds and thus, i n most cases, only items i n t h i s category were recorded (a probe of the r i g h t length was used - average b i l l length from museum specimens 7.5 cms.). 37 The lev e l s of the i n t e r t i d a l zones and the sampling areas were measured with the use of tide tables and a ver-t i c a l pole. Error was minimized by proceeding at short steps from the water l e v e l and by using a l e v e l . Several square foot plots were marked o f f at each l e v e l of the mussel bed at a number of s i t e s on the i s l a n d . The t o t a l number of mussels i n each plot was counted and counts of mussels gaping i n the plots were made throughout the t i d a l cycle. The c r i t e r i o n of gaping depended i n i t i a l l y on whether or not a probe could be inserted between the valves but l a t e r gaping was determined simply by sight. Tide l e v e l and surf were also recorded before each count of gaping mussels. Gaping mussels were co l l e c t e d and measured. 38 RESULTS (a) Survey of the fauna Over 58 species of organisms were found inhabiting the i n t e r t i d a l with Arthropods, Molluscs and Echinoderms being most highly represented. The following i s a l i s t of the organisms found during the present study. Those organisms known to be eaten by the oystercatcher are starred. Not a l l organisms have been i d e n t i f i e d at thi s time. A L i s t of Organisms found i n the i n t e r t i d a l Phylum Species Coelenterata Anthopleura xanthogrammica Platyhelminthes unidentified Nemertea Annelida Sipunculida Arthropoda •unidentified Serpula vermicularis *Nereis~sp. •unidentified Balanus cariosus Balanus glandula Balanus n u b i l i s P o l l i c i p e s polymerus *Petrolisthes cinctipes Pugettia producta Hemigrapsus nudus Hemigrapsus oreganensis Pagurus sp. *Oedignathous inermis Fabia subquadrata Common Name green anemone flatworms ribbon worms tube worms nereis worms peanut worms barnacle barnacle barnacle goose barnacle porcelain crab kelp crab purple shore crab shore crab hermit crab crab pea crab Phylum Arthropoda Mollusca Species  Cirolana sp. *Idothea sp. unidentified * L i g i a sp. *Katharina tunicata *Mopalia muscosa  Cryptochiton s t e l l e r i  T o n i c e l l a l i n e a t a *Mopalia c i l i a t a  Hinnites multirugosus  Entodesma saxicola *Mytilus californianus  Searlesia d i r a  Amphissa columbiana  Thais emarginata  Diodora aspera *Acmaea d i g i t a l i s *Acmaea mitra *Acmea pelta *Acmaea t e s t u l i n a l i s *Acmaea persona  H a l i o t i s kamtschatkana  Tegula funebralis  L i t t o r i n a scutulata  Ceratostoma foleatum  Crepidula adunca *Crepidula numararia Common Name isopod isopod amphipods isopod (rock louse) leather chiton mossy mopalia gumboot chiton lined chiton chiton rock scallop rock-dwelling clam C a l i f o r n i a mussel dire whelk wrinkled amphissa s n a i l rough keyhole Limpet fingered limpet whitecap limpet shi e l d limpet plate limpet mask limpet northern abalone black top s h e l l s n a i l Checkered l i t t -orine s n a i l leafy hornmouth s n a i l hooked s l i p p e r -s h e l l white s l i p p e r -s h e l l 4 U Phylum Species Common Name Mollusca D i a u l u l i a sandiegensis nudibranch Anisodoris n o b i l i s nudibranch Echinoderma Dermasterias imbricata leather sea star Pisaster ochraceus purple sea star Henricia l i v i u s c u l a red sea star Strongylocentrotus drobachiensis green sea urchin Strongylocentrotus purpuratus purple sea urchin Eupentacta quinquisimita white sea cucumber *Cucumaria sp. sea cucumber Pycnopodia helianthoides sunflower sea star In addition, an octopus inhabited one of the tid e pools. One blenny (Pisces) was taken i n a sample of the mussel bed. (b) Abundance and Size of Prey The res u l t s of sampling are presented for each of the i n t e r t i d a l zones. Zones 1 and 2 - Upper and Uppermost Horizons The uppermost horizon i s the splash zone or L i t t o r i n a zone and i s mainly a region of bare rock with some green algae. The upper horizon or high i n t e r t i d a l i s the zone above the mussel bed and i s t y p i c a l l y rockweed but bare toward i t s upper l i m i t s . On Cleland these zones were usually ten feet and higher depending on the degree of exposure. The areas of bare rock were largely devoid of macroscopic animal l i f e except for s n a i l s (Littorina) although crevices 41 harboured the rock louse L i g i a and occasionally small numbers of dark amphipods. On the steeper rock faces the limpet Acmaea d i g i t a l i s was found i n clumps often i n areas not accessible to oystercatchers because of steepness. The rockweed area made up zone 2 and consisted of bare rock with varying amounts of weeds l i k e Ulva, Fucus and Endocladia. The d i v e r s i t y and abundance of organisms was low with only very small limpets, a few amphipods and a single mussel being found i n f i v e quadrats taken i n the north and west part of the i s l a n d . Many of the organisms were found only by moving the weeds aside, similar to the methods used by the birds (Table VI). In some areas shallow pools, some weeded, others cl e a r , remained as the tid e dropped and both the abundance and d i v e r s i t y of organisms appeared higher i n these pools as compared with other parts of the upper zones. Table VI Numbers and Sizes of organisms found i n square foot quadrats i n Zones 1 and 2 Number Organisms Density Size (cms) Quadrats Present mean range mean range 6 limpets - - 1.1 0.8-1.6 limpets 19 J -2 - 84 0.8 0.6 - 1.4 mussels 9 0 - 53 3.0 1.7-4.2 snails 6 0 - 2 5 isopods less than 1 - -amphipods less than 1 - -43 Zone 3 - Middle I n t e r t i d a l This zone included the mussel bed and was the most discrete and the most complex. It was also the zone most heavily used by the oystercatchers and as such i t was studied in greater d e t a i l . The extent of the mussel bed varied considerably from at least 86 feet wide on gentle slopes on the exposed north-west to 9 feet wide on steep slopes i n the r e l a t i v e l y protected south-east. In some areas the mussel bed began at a tide l e v e l of 5 feet and extended to 10 feet or more but i n very exposed areas i t was often from 6 feet to 12 feet or more. The zone consisted of t i g h t l y packed mussels with goose-neck barnacles d i s t r i b u t e d i n clumps throughout. Limpets were found both on the mussels and on the goosenecks. In between the mussels there was a moist microhabitat containing sea cucumbers and small limpets and s n a i l s . Occasionally nereis and other organisms found deeper i n the mussel bed were seen from the surface. Although some of the organisms i n t h i s zone were usually found deep i n the mussel bed and thus were only occasionally v i s i b l e from the surface i t was of in t e r e s t to remove several complete samples from the mussel bed to determine what organisms were there and i n what numbers. These samples give a f a i r representation of what i s i n the mussel bed. Since t h e i r absolute numbers are of l i t t l e i n t e r e s t the organisms are simply l i s t e d here i n order of numerical abundance: sea cucumbers, mussels, porcelain crabs, limpets, amphipods, isopods, barnacles, worms (including sipunculids and nereids), 44 nudibranchs, t u r b e l l a r i a n s , small chitons, s n a i l s , s t a r -f i s h and blennies. Other samples taken included only those organisms with-in b i l l length of the surface. Twenty-five foot-square quadrats were taken at a variety of l e v e l s and locations i n the mussel bed. This sampling indicated that very l i t t l e other than mussels, limpets and sea cucumbers was obtainable from the mussel bed surface although the birds were seen to take other items i n small numbers, eg. nereis (Table V l l ) . The numbers of sea cucumbers varied from place to place and t h e i r v u l n e r a b i l i t y to oystercatchers probably depended on the tightness of the packing of surface mussels and the depth of the mussel bed. I t i s assumed that v u l n e r a b i l i t y of these organisms i s not affected by the time exposed -ie. drying doesn't drive organisms deeper into the mussel bed reducing t h e i r v u l n e r a b i l i t y . This seems reasonable since the regions i n between mussels tended to remain moist even when the surface of the mussel bed became dry. Limpets appeared to be r e l a t i v e l y e a s i l y taken under various con-di t i o n s of drying. TABLE VII Densities of organisms within reach of oystercatchers i n zone 3 (25 quadrats) Organism Mean number per square foot Range Mussels 230 81 -- 505 Limpets 43 19 -- 116 Sea Cucumbers 27 0 -- 242 Snails 3 0 -- 12 Nereis 0.5 0 - 4 46 The mean size of mussels for the entire mussel bed was 4.1 cms. with a range of 0.3 - 9.9 (figure 7). A general examination of the mussel bed suggested a gradient i n size and abundance of mussels and t h i s impression was confirmed by sampling. Surface mussels become larger and fewer i n number at lower leve l s with a peak i n mean size at the 6 - 7 foot l e v e l (figures 8 and 9). The limpets found on the surface of the mussel bed were almost e n t i r e l y Acmaea d i g i t a l i s and A. persona with almost equal representation of the two species. In the open mussel bed there are t y p i c a l l y large numbers of very small limpets (0.5 cms.) and a low density (3^14) of limpets between 0.5 cms. and 1.5 cms. Larger limpets (both A. persona and A. pelta) were found at the base of sea palm (Postelsia) i n the lower mussel bed and at the4to5 foot l e v e l near the lower weeds (Figure 10). Large numbers of large limpets (A. pelta , A. scutum, A. mitra) were found on the sides of deep pools i n t h i s zone. Crevices and rock faces also provided habitat for the larger limpets. Sampling of limpets on the open mussel bed suggested a trend of decreasing density with increasing height (Figure 11) . Combining samples for the mussel bed excluding pools gives a mean size of 0.8 cms. for limpets with a S. E. of less than 0.1. FIGURE 7. Size d i s t r i b u t i o n of mussels, N = 2287 47 48f FIGURE 8. Number of mussels per quadrat at d i f f e r e n t t i d e levels (range i s indicated) 500n 450H 400-350H LO _ J UJ to 300' CO ZD UJ o < u_ cn ZD CO 250H 200' 150H 100H 50 H 0' • <5' SAMPLE SIZE (7) 5'-6' (5) 6'-7' (2) 03 7^8' ' 8^9' ' $A0' ' 10-12' ' 12^ (8) (10) (3) (3) (2) TIDE LEVEL FIGURE 9. Effects of shore l e v e l on average size of mussels (s.e. i s indicated) 7-0 60-50-oo < 3-0-2 2-0-.4-5 5-6 6-7 7-8 8-9 9-10 10-11 11-12 TIDE DEPTH. FEET. FIGURE 10. Size of limpets on the mussel bed (s.e. i s indicated) 51 f FIGURE 11. Density of limpets at d i f f e r e n t l e v e l s i n the mussel bed (range i s indicated) 51 12CH 5^ 6 e-7 7-8 £h9 9M0 10 S H O R E L E V E L (FT) 52 The transect counts of gaping mussels gave the following r e s u l t s . Mussels began to gape when splashed by a r i s i n g tide and remained gaping for a short time a f t e r becoming exposed again. However, not a l l mussels began to gape at the same time and t h i s resulted i n a pattern of an increasing density of gaping mussels at any one l e v e l between the time when water began to splash that l e v e l and the time when the l e v e l became heavily washed and unavailable to birds . During ebb tide the density of gaping mussels at any one l e v e l rapidly decreased as the l e v e l became exposed. The pattern of events i s i l l u s t r a t e d i n Table V l l l for a whole t i d e cycle. At anytime while the tide i s washing the mussel bed there i s a band approximately two feet i n width i n which mussels are vulnerable. Increases i n the number of gaping mussels occurred rapidly at any given l e v e l but the maximum counts made just before the area becomes completely washed by water and thus unavailable were s t i l l far below the t o t a l number of mussels present. The general impression was that within a l e v e l , larger mussels began gaping e a r l i e r than smaller ones but the actual c o l l e c t i o n of gaping mussels proved d i f f i c u l t . However, a sample of forty gaping mussels was obtained from various lev e l s during one tide and these ranged i n size from 2.6 cms. to 11.5 cms. with a mean length of 7.2 cms. TABLE V l l l Density (Number per quadrat) of vulnerable mussels over a t i d e c y c l e . Wash - l e v e l which i s washed by waves Regular - common waves Maximum - l e v e l reached by high-est waves Numbers are given for each Quadrat Tide 1.1 f t . at 0815 hrs. Lines indicate no longer 10.3 f t . at 1440 hrs. available 4.2 f t . at 2020 hrs. Time Tide Wash (ft.) Number of mussels gaping at l e v e l hrs. f t . regular max. 4-5 5-6 6-7 7-8 8- 9 9-10 10-11 11-12 1000 3.5 4. 4.5 8 , 4 1 0 1 0 0 0 0 1030 4.0 5 . 5.5 -,24 -,28 4 0 2 0 0 0 0 ui 1100 4.5 5.5 6.5 - 21+ 31 35 16 19 5 , 5 , 2 3 0 0 0 1145 6.0 7.5 9.0 - - - 22+ 1 0 , 14 0 0 0 1200 6.0 8.5 10.5 - - - - 1 3 , — t 8 11+ 4 , 1 3 , 2 , 0 0 1230 7.0 10.0 11.0 - - - - - 3 , 2 2 1245 7.5 11.0 12.0+ - - - - - - , 5 1 4 , 4 1300 8 . 12.+ _ — 1400 Time hrs. Tide f t . Wash regular (ft.) max. Number of mussels 4-5 5-6 6-7 7-gaping at 8 8-9 l e v e l 9-10 10-11 n - i : 1500 1600 9.0 11.0 13. _ - - - - -1700 7.5 9.5 12 — — — — * — 6, 4 4 ,5, 14 9, 7 very small 1730 6.0 9.0 10.5 _ - 2, 3 4 , 3, 7 2, 1 1800 5.5 8.0 9.5 — — — —* 11, 7 1, 1,1, 1, o 4, 1 deep ones 1830 5+ 6.0 8.0 - - 14 5, 1 0, 1,0, 0, 0 0, 0 1845 5.0 6.0 7.0 - - 6 1, 0 0, 1 0 0 1900 5.0 7.0 7+ 13 2 0 0 0 0 55 MUSSEL BED - laminaria Interface General c o l l e c t i n g and s p e c i f i c searching indicated that the interface was the source of the crab Oedignathous  inermis, the abdomens of which were sought a f t e r by the adults for chick food. T y p i c a l l y the interface was steep and consisted of very large mussels and occasional bare or weed covered rock. The crabs were d i f f i c u l t to locate and withdraw from t h e i r locations i n between the large mussels and i n p i t s and deep crevices. Samples were taken on both the lower edge of the mussel bed and from the weed area just between the mussel bed and the laminaria. Mussels were the most abundant organism and t h e i r size depended on whether the sample was just below the mussel bed or r i g h t on the lower edge. Limpets were few i n number but there were some r e l a t i v e l y large ones present. No Oedignathous. crabs turned up i n the sampling i n t h i s zone although they were found i n small numbers down i n the laminaria. However, they were known to be i n f a i r numbers i n the interface on the west and north sides of the i s l a n d . Table IX summarizes the res u l t s of sampling i n the interface area. TABLE IX Organisms found i n sq. f t . quadrats i n the interface area Organism Mean number Range per sq. f t .  mussels 52 50-55 limpets 7 5-11 sea cucumbers 5 0-14 sna i l s 0.6 0-2 crabs 0.3 0-1 worms 1.0 0-2 amphipods 0.3 0-1 sea stars 0.6 0-1 Size usually either very large (over 10 cms) or very small (less than 2 cms) 1.3 cms. some large, most 1 cm. Zone 4 - Lower Horizon This zone was comprised of an i r r e g u l a r surface covered with laminarians, feather boa (Egregia) and c o r a l -l i n e s making i t extremely slippery. In places where these weeds were washed aside large chitons and sea stars were usually v i s i b l e . Sampling i n t h i s area revealed small numbers of small mussels and limpets with occasional large limpets. Both limpets and chitons were found i n every quadrat but many of the other organisms showed up i n only a few samples (Table X). TABLE X Organisms i n Zone 4 (based on 8 quadrats) Organism Mean number per sq. f t . Range i n number Size mussels 9.0 0 - 29 small (less than 2 cms) limpets sea cucumbers 6.0 0.3 1 -0 -11 2 small, occasionally very large crabs 1.0 0 - 5 -isopods 4.0 0 - 25 -amphipods 3.0 0 - 9 -worms 3.0 0 - 11 -chitons 2.0 1 - 5 -flatworms 0.6 0 - 2 -s n a i l s 0.3 0 - 2 -nereis 0.6 0 - 5 -sea stars 1.3 0 - 4 -nudibranchs 0.1 0 - 1 — 59 DISCUSSION For any p a r t i c u l a r tide cycle there w i l l be a pattern of changing v u l n e r a b i l i t y of prey species over space and time. Tides i n the area of Cleland Island are mixed semi-diurnal and t h i s means that the i n t e r t i d a l i s exposed to a varying extent twice a day. As the tide ebbs the area open to search by the oystercatcher increases reaching a maximum at low t i d e . The b i r d c l e a r l y has a number of options as to how i t forages both i n time and space and also as to the prey i t sele c t s . Organisms higher up i n the i n t e r t i d a l are a v a i l -able for much longer periods of time but of course are, on the average smaller. Changes i n v u l n e r a b i l i t y occur rather rapidly but within a small, well-defined area, so that i t seems r e a l i s t i c to expect the birds to be able to monitor the changes and respond rapidly to them. The res u l t s of the sampling program are summarized i n Table XI. TABLE XI The Feeding Area - Summary Zone Characte r i s t i c s Prey Density (number per quadrat) Mean Size (cms) upper and uppermost Horizons open rock rockweed Acmaea d i g i t a l i s 19 1.1 (zone 1) 0.8 (zone 2) middle i n t e r t i d a l Mytilus- Mytilus californianus 230 P o l l i c i p e s Acmaea d i g i t a l i s 43 Acmaea persona Cucumaria 27 4.1 0.8 Pools with A. pe l t a . A. scutum. A. mitra Medium sized Acmaea persona associated with P o s t e l s i a Interface lower edge of mussel bed rock, weed Oedignathous inermis  Acmaea pelta 1.3 Laminaria weed chitons limpets crabs 2 6 1 TIME SPENT HANDLING PREY (T ) H In order to calculate p r o f i t a b i l i t i e s for various prey items i t i s necessary to determine the amount of time that a predator takes i n handling the prey. This time was recorded for various prey items under the three d i f f e r e n t feeding circumstances. METHODS Handling times were recorded while the birds were being observed from the observation b l i n d s . The foraging adults were c a r e f u l l y watched by telescope and a stopwatch was v i s i b l e at a l l times so that both the beginning of an attack and the time when the prey was eaten were recorded. Handling time includes a l l of the time between the commencement of an attack and the beginning of renewed searching. In the case of adults feeding chicks T^ includes t r a v e l time to feeding area. RESULTS (a) Adult Feeding Mussels The time from the beginning of an attack to the point at which an adult l e f t a mussel was recorded for 10 6 attacks involving at lea s t seven d i f f e r e n t adults mean T (mussels) = 45 sec. H Range 7 - 1 2 5 The handling time of mussels by adults includes a l l the time spent by the adult on any p a r t i c u l a r mussel. 62 Some of the v a r i a b i l i t y i s e a s i l y explained. Cases of very high T were situations i n which the adult was chased away from the mussel by waves. As indicated i n another section the adult would continue to return to the same mussel u n t i l i t was completely eaten. In other cases the whole mussel was torn loose and car r i e d up higher where i t was placed and eaten. D i f f i c u l t i e s i n placing mussels resulted i n increased handling times. Attempts were made to see i f the length of the mussel was influencing T . Where possible, the size of H the mussel attacked was estimated r e l a t i v e to the length of the birds b i l l and the time spent handling the mussel was recorded (Table XII). TABLE XII Ef f e c t s of Size on Handling Time of Mussels Size Class Up to 1/3 b i l l h b i l l 3/4 f u l l b i l l mean T (sees) 35 54 51 H sample size 7 19 10 range 20-55 28-125 33-75 With such sample sizes and measurement techniques i t i s impossible to say whether there i s any e f f e c t . Limpets Handling time for limpets included a l l the time from when the limpet was knocked o f f the substrate to the point at which i t was swallowed. This was recorded for 63 attacks, mean T = 10 sees, range 3 - 2 5 H 63 V a r i a b i l i t y i n for limpets was largely due to d i f f i c u l t i e s i n placing the limpet where i t wouldn't s l i d e about too much. The whole process of s h e l l removal was very rapid and the meat was taken i n one gulp so that no e f f e c t of size was considered. Small unshelled items These included sea cucumbers and were simply picked up and swallowed so that handling time was minimal T = 3 H sees. (max.) (b) Feeding of chicks at the nest Early i n t h e i r development the chicks are close to the nest and the adult must transport the prey over long distances. Handling time i s very high at t h i s time since usually the adult spends time preparing the food by chewing i t , moistening i t i n a pool or s l i c i n g o f f small pieces. Moreover, the chick response i s slow and the parent often end's up holding the food i n i t s b i l l for some time or simply pointing at the food on the ground. If there i s no response the adult swallows the food item and returns to the i n t e r t i d a l . Any sudden disturbance l i k e piping w i l l often cause the b i r d to swallow the prey as wel l . To a cer t a i n extent the hand-l i n g time of prey eaten by the adult i s also affected by t h i s stage i n breeding. When chicks are present the foraging adult sometimes holds a prey item i n i t s b i l l for a short period before eating i t . There i s a very clear impression of a decision process at work here determining whether the adult w i l l carry an item up to the chick or not. Even while the chicks are s t i l l near the nest they become more adept at handling the prey and the adults spends less time preparing the prey and thus there i s a reduction i n handling time. Three instances of another way of reducing T were observed H for one nest i n which the adult ca r r i e d food up to the chick but simply handed i t to the other adult who prepared and pre-sented the prey to the chicks while the forager immediately returned to the feeding area. After a few days the adult begins to walk to and from the feeding area. This tends to increase handling time but may be necessary i n f a m i l i a r i z i n g the adult with a safe route to lead the chicks to the i n t e r t i d a l . In one nest under observation the parents foraged for themselves on a point of land but picked up prey items for the chicks i n a bay much closer to the nest. This u t i l i z a t i o n of l o c a l resources would also reduce t r a v e l time. In t h i s p a r t i c u l a r example the adults eventually moved the chicks out to the point. HANDLING OF MUSSELS In most cases the adult simply tears a piece of the meat out of the s h e l l and quickly walks to the chick with i t . The adult may then return to the same mussel and carry another piece of meat to the chick or feed on the mussel i t s e l f . When the adult c a r r i e s only a piece of the meat to the chick, the handling time i s r e l a t i v e l y short, usually less than 50 sees, and at the very most would average 52 sees; or twice the handling time for limpets. T = 52 sees. Observed maximum = 54 sees. H In some cases the whole mussel, s h e l l included, breaks loose and the adult c a r r i e s i t to a position near the chicks where i t pecks out the meat which i t gives a piece at a time to the chicks. Handling time i s high i n t h i s case. Only six cases were observed where the adult fed the entir e mussel to the chick. In other cases where sh e l l s were ca r r i e d up the adult i t s e l f fed on part of the meat. T = 185 sees. (range 107-271 sees.) H HANDLING OF LIMPETS In the case of limpets, the adults either c a r r i e d the whole limpet to the chick and removed the meat from the s h e l l there or ca r r i e d ojnly the meat up af t e r removing i t from the s h e l l either on the i n t e r t i d a l or af t e r carrying i t up to some higher locat i o n . The chicks took a l l limpet meat i n one gulp. When the chicks were near the nest the main time consuming aspects i n t h i s case were t r a v e l time and the time spent holding the meat u n t i l the chick took i t . mean T H = 26 sees. HANDLING OF OTHER PREY Handling times were also recorded for some of the other types of prey given to the chicks. The times are based on r e l a t i v e l y few observations but they do give an indicatio n of the possible differences. 66 Prey Mean T (sees.) P a r t i c u l a r s nereis 56 preparation included b i t i n g along i t s length oedignathous crab 95 preparation involved removal of abdomen chitons over 97 (c) Feeding of Chicks at the i n t e r t i d a l Once the chicks are moved down to the feeding area t r a v e l time i s almost t o t a l l y eliminated. Moreover, the chicks soon begin following c l o s e l y behind the foraging parent and feeding becomes quite rapid at le a s t i n the case of small limpets and unshelled items on the mussel bed. Larger items l i k e mussel meat are car r i e d up to the chick by the adults. The adult usually runs up with the meat so that time again i s r e l a t i v e l y short. In many cases the chick i s quick to respond to the food - carrying adult. There was some in d i c a t i o n that feeding was becoming a teaching experience with more and more of the work being l e f t to the chicks. MUSSELS Handling time was recorded for pieces of mussel meat car r i e d to the chicks while they were down at the feeding area. mean T H = 29 sees, range (21 - 40) LIMPETS Handling time for limpets varied depending on whether the birds were giving small limpets on the mussel bed to the chicks or carrying up larger limpets from farther down. b V Overall mean T = 1 4 sees. Range 4-42 H * In the case of the small limpets, when the chicks were very close to the foraging adult handling time was only s l i g h t l y greater than that for the adults. 68 THE AVERAGE WEIGHT OF PREY (W ) A Royama argues that the density of prey i s not necess-a r i l y the important factor i n determining the predator's re-sponse. The size of the prey must also be taken into account. Attempts were made to determine the size of various prey items u t i l i z e d by the oystercatchers. METHODS The f i r s t step i n determining the average weight of various prey types was to obtain a measure of the s h e l l length of the mussels and limpets eaten by oystercatchers. Adult birds were observed feeding and then c o l l e c t i o n s were made of the mussel and limpet s h e l l s . These were then i d e n t i f i e d , and measured with a vernier c a l l i p e r . In some cases the size of prey was estimated r e l a t i v e to the b i l l length of the b i r d s . I t was also possible to obtain very good measures of the size of prey car r i e d to the chicks while they were at the nest s i t e . In any feeding bout a cert a i n percentage of the food items were carried whole to the chicks and the s h e l l s were then removed at the nest. This resulted i n an accumul-ation of s h e l l s of mussels, limpets, chitons and crabs which could be i d e n t i f i e d and measured. These s h e l l s were inspected on a regular basis for a number of nests around the i s l a n d . RESULTS (a) Measurements of Prey Size Although i t was more d i f f i c u l t to estimate prey size for adults than i t was for chicks, a small sample was obtained and measurements were made. Moreover, observations of adult feedings gave indications of prey size r e l a t i v e to the b i l l length and i t seems certain that the samples are representative at l e a s t i n the case of limpets. The mean size of mussels and limpets eaten by adults was 4.9 cms. and 1.3 cms. respec-t i v e l y (Table XIII). TABLE XIII - Adult Prey Size Sample Species Mean size (SE) range Size (cms.) 34 Acmaea d i g i t a l i s 1.3 (0.0) 0.9 - 1.8 Acmaea persona 53 Mytilus californianus 4.9 (0.3) 1.7 - 9.2 Comparison of the size of prey i n the d i e t with that i n the habitat suggests that the adults tend to choose medium size limpets between 1.0 and 1.5 cms. (Fig. 12). The picture for mussels i s not as clear because of the small sample and apparent b i modality (Fig. 13) so that no comparison i s made. For at l e a s t some of the time the mussels taken by adults appeared si m i l a r i n size to those given to the chicks. Since the sample of the mussels given to the chicks i s much larger, comparisons with size i n the habitat are made l a t e r using those data. Extensive c o l l e c t i o n s of s h e l l s c a r r i e d to the chicks i n a variety of nests resulted i n good measures of the size of food fed to chicks. Mussels tended to be medium to large with a mean size of 5.2 cms. based on 2647 mussels. Limpets, chitons and crabs tended to be quite large. Mean size of 70f FIGURE 12 Comparison of Limpets i n adult d i e t with those i n the mussel bed 100 UJ CQ 80i H 60^ O LL o UJ o DC UJ Q_ 40H 20 0 H A B I T A T [ ~ I 0 A D U L T DIET L_ 0.5 1.0 1.5 2.0 L I M P E T S I Z E ( C M S ) 2.5 *7lf FIGURE 13 Size of mussels i n the d i e t of adults NUMBER OF MUSSELS o -T* C H ' <-ro GO m <2 on N m GO 0) 00 CD CO r\D O) _ l I I I L_ ro o TZ. 72 limpets based on a sample of 698 was 1.7 cms. A comparison of the siz e d i s t r i b u t i o n of mussels fed to chicks with that of mussels i n the habitat indicates that the birds are selec-t i n g larger than average mussels (Fig. 14). (b) The Average Weight of Prey Once the lengths of the prey were determined, dry weights could be calculated from the double logarithm re-gressions given by Menge (1971). Mytilus californianus i s assumed to be similar to Mytilus edulis i n the length -weight r e l a t i o n s h i p . Species Length L - dry weight W Relationship  Acmaea d i g i t a l i s log W = 2.826 Log L - 1.782 Mytilus edulis log W = 3.355 Log L - 2.45 On the basis of these equations the limpets eaten by adults on the mussel bed would average 0.03 g. No exact measure was made of the size of pieces of mussel meat ca r r i e d to chicks but i t was always many times larger than the size of the small limpets. In the case where a l l of the meat i s given, 0.89 g. would be the average biomass intake. When a b i r d feeds on a mussel i t usually takes between one and four pieces before i t i s fi n i s h e d . In 68% of the times when feeding chicks i t was the f i r s t piece that was carri e d to the chick while the res t of the times the b i r d c a r r i e d up what was l e f t a f t e r i t had taken one or more pieces. A reasonable assumption might be that the b i r d draws out, on the average, 50% of the meat present each time. 73 f FIGURE 14 Comparison of mussels car r i e d to chicks with those present i n the habitat 480 INTERTIDAL N = 2287 l 1 r" i l i i i -1 l I i i CHICK FOOD N = 2915 I I I 1 i I 0 4 5 6 SIZE CLASS (CMS) 8 10 74 This would mean that between one quarter and one half of the meat would be car r i e d to the chick depending on whether the second piece or the f i r s t piece of meat was taken. The weight of meat given to the chick would then be 0.45 g . i f i t was the f i r s t piece and 0.22 g. i f i t was the second piece. 75 RATE OF SUCCESSFUL SEARCH (a) The component (a) i n the functional response equation can be broken down into a number of subcomponents (Holling, 1965). I t i s largely determined by the searching a b i l i t i e s of the predator including i t s v e l o c i t y , the distance at which i t w i l l react by attacking a prey, and by the capture success of the predator. The l a t t e r subcomponent i s equal to the proportion of prey coming close enough to be attacked, that are successfully attacked. It includes a term which i s r e a l l y a measure of the p a l a t a b i l i t y of the prey, i e . the proportion of contacted prey to which the predator reacts. In order to calculate p r o f i t a b i l i t i e s for the prey of the oystercatcher i t was necessary to obtain a measure of (a). Observations on the hunting behaviour of the birds indicated that they could operate at times i n a number of d i f f e r e n t modes. For example, while the birds foraged on the mussel bed there were times when they were observed to hunt s p e c i f i c a l l y for cert a i n types of items. At times the birds were searching for gaping mussels and ignoring smaller items, while at other times the birds fed on small limpets on the mussel bed and were at l e a s t temporarily unresponsive to mussels which were vulnerable at the same time. An estimate of the rate at which the birds could f i n d prey can be obtained from observations of the birds when they were operating i n these p a r t i c u l a r modes. This has been done for birds foraging on the mussel bed and u t i l i z i n g either mussels or small limpets. 76 METHODS A measure of the parameter a', the rate of successful search, i s obtained from the equation a = N A (T - T N ) N~ H A O This i s v a l i d when independent measures are made of each of the parameters. The number of prey attacked i n a given time spent hunting either mussels or small items i n the mussel bed was recorded for a number of d i f f e r e n t b i r d s . A value of N for o small limpets was taken as the mean density over the mussel bed while the value of N for mussels was taken as the mean o number of mussels gaping, taken at a l l lev e l s of the mussel bed over a whole t i d a l cycle. RESULTS The values of 'a' for an adult b i r d searching for mussels only or limpets only were 0.0067 and 0.0012 respec-t i v e l y . These values were based on 137 prey captures of mussels and 279 prey captures of small limpets on the mussel bed (Table XIV). TABLE XIV Calculation of the Rate of Successful Search Prey Type Density Hunting Time Prey eaten Total handling _ N (per sq.ft.) T (sees.) N time T N . a = N f l - A H A (sees.) A (T-T N )N H A o limpets - 43 279 0.0012 small unshelled 8560 111 3123 items mussels 8.3 8627 137 6165 0.0067 78 THE PROFITABILITY OF PREY The basic assumption i n Royama1s theory of hunting behaviour i s that an animal t r i e s to maximize i t s hunting e f f i c i e n c y . Each type of prey can be assigned a value or p r o f i t a b i l i t y which i s simply the amount of the prey which the predator could c o l l e c t i n a given time spent hunting that prey. The determination of t h i s value i s a t a c t i c a l problem which Royama suggests can be calculated from the Holling disc equation. P = N W = a N W, A A o A T 1 + a T N H o P r o f i t a b i l i t i e s then depend on the size and density of the prey, handling time, and the searching a b i l i t i e s of the predator. Since the handling time of prey given to nestlings i s much higher than that of prey eaten by the adult, Royama predicts a difference i n the diets of adults as opposed to chicks. This i s i l l u s t r a t e d i n the figures below (from Royama, 1970) i n which the hunting e f f i c i e n c y i s plotted against prey density for two d i f f e r e n t sized prey. In the f i r s t figure handling times are given for the case where the adult i s feeding i t s e l f . The second figure i l l u s t r a t e s the change i n the order of p r o f i t a b i l i t y which might occur when the prey i s used as food for nestlings and the handling time i s greatly increased. This assumes that any r e l a t i o n between prey size and handling time d i f f e r s according to whether the food i s for the chick or for the adult. PREY B 80 Royama used the term 'niche' to describe the place where the prey species mainly occurs and the concept of pro-f i t a b i l i t y was applied to the prey species i n t h i s niche. Although the o r i g i n a l use of the term may r e f e r to prey which are separated i n space i t seems reasonable to extend the concept to prey which occupy d i f f e r e n t microhabitats i n the same location or which require a d i f f e r e n t method of hunting. At t h i s time p r o f i t a b i l i t i e s have been calculated for mussels, and for the limpets i n the mussel bed (Table XV"). These are prey items which at c e r t a i n times i n the t i d e cycle are simultaneously vulnerable to predation. In a l l three feeding situations mussels are more pr o f i t a b l e than the smaller limpets i n the mussel bed even though the l a t t e r are more abundant. Interestingly, i t appears that the birds can transfer more biomass i n a given time by giving pieces of mussel meat to the chicks rather than carrying up whole mussels. According to the p r o f i t a b -i l i t y model then the birds should respond to t h i s p r o f i t a b i l i t y difference by concentrating more on mussels than limpets when they are both vulnerable. TABLE XV P r o f i t a b i l i t i e s of Prey Items Feeding Context Adult feeding Chicks at nest Chicks at feed-ing area Prey N T mussels 8.3 limpets 43 whole mussels limpets 8.3 piece of mussel meat 8.3 43 piece of mussel meat 8.3 limpets 43 H W (sees.) (g) 45 10 185 0.89 0.03 0.89 0.0067 0.0012 0.0067 45 0.22-0.45 0.0067 26 .03 .0012 29 0.22-0.45 .0067 14 .03 .0012 P r o f i t a b i l i t y (g/sec.) 0.014 0.001 0.0044 .0035-.0071 0.00066 0.0047-0096 0.00089 82 THE DIET OF THE BLACK OYSTERCATCHER In studies of animal foraging i t i s common practice to compare the die t of the animal with the apparent availab-i l i t y of foods i n the habitat. In the case of b i r d studies t h i s comparison has usually involved only the di e t of n e s t l -ings. The general conclusion reached i s that there i s no clear and simple r e l a t i o n s h i p between the composition of the d i e t and the composition of food species i n the habitat. In the section on the i n t e r t i d a l of Cleland Island i t was pointed out that over 58 species were found during sampling and general study of the shore habitat. Moreover, the organisms were associated i n a natural zonation and there were large differences i n both the size and the r e l a t i v e abundance of the various species. The oystercatchers were observed to feed to varying extents on roughly 31% of the species recorded i n the i n t e r t i d a l . Feeding occurred under three rather d i f f e r e n t circumstances (1) adult feeding (2) feeding of chicks at the nest (3) feeding of chicks at the i n t e r t i d a l . Differences e x i s t i n the values of the parameters i n the p r o f i t a b i l i t y equation for each of these feeding s i t u a t i o n s . The diets of the oystercatchers and of t h e i r young were recorded i n order to see i f there were differences i n the use of various prey items as adult food or as chick food and whether such differences r e f l e c t e d the p r o f i t a b i l i t i e s of the prey. 83 METHODS - Adult Diet Observation blinds were set up close to feeding areas. Two observers would approach the b l i n d with only one remain-ing and the other leaving the area. The birds were then observed by telescope for varying periods of time but usually for the duration of a t i d a l cycle. After a study of the i n -t e r t i d a l organisms i t was possible to i d e n t i f y prey by sight. Since the attack behaviour of the b i r d d i f f e r e d for various prey i t was also possible to determine prey types from the behaviour of the b i r d . Observations were made when parents were brooding eggs or caring for chicks as well as for special cases where a clutch or brood was l o s t to predation. Some of the prey were c o l l e c t e d from the i n t e r t i d a l a f t e r a bir d had foraged i n an area. These were i d e n t i f i e d and measured. The sizes of the adult prey were also estimated r e l a t i v e to the b i l l length of the adult. Chick Diet Two sources of information on the chick d i e t existed. When the chicks were some distance from the i n t e r t i d a l the adult would carry some of the prey whole to the chicks l o c -ation and removal of meat from the s h e l l was then car r i e d out. Regular c o l l e c t i o n s of these s h e l l s were made for various nests around the isl a n d and a l l were i d e n t i f i e d and measured. In addition the feeding of chicks was observed from the b l i n d . If the actual presentation of food was not seen, the adult was usually v i s i b l e for various lengths of time while carrying up the food item. 84 RESULTS - Adult Diet - Composition Small to medium sized limpets (Acmaea d i g i t a l i s and A. persona) occurred most frequently i n the feedings observed and together with small unshelled items accounted for 66% of the food items taken. Mussels formed 29% of the die t i n terms of frequency while crabs, worms, chitons and sna i l s made up the res t (Table XVI). However, mussels c l e a r l y form the greatest part of the die t i n terms of weight. Chick Diet - Composition Based on observed feeding both at the nest and at the feeding area, mussels and limpets made up 40% and 37% of the diet respectively with small unshelled items l i k e sea cucumbers at only 4% (Table XVII). Other items including chitons and nereis worms made up 19% of the t o t a l feedings. Another source of information was the shelled items car r i e d whole to the chick. C o l l e c t i o n of these sh e l l s i n -dicated a high dependence on mussels (Table XVIII). Out of 5537 shel l s c o l l e c t e d , 64% were mussels, 29% were limpets and the rest were chitons, crabs and s n a i l s . The crabs were a l l Oedignathous inermis and only the abdomens had been removed. TABLE XVI Composition of Adult Diet Determined from observed feedings Prey Type mussels limpets small unshelled items nereis s n a i l s crabs chitons worms Number of feedings observed 368 554 281 11 10 9 2 15 Percent of t o t a l feedings 29.4 44.3 22.4 0.9 0.8 0.7 0.1 1.2 t o t a l s 1,250 99.8 TABLE XVII Composition of chick d i e t from 517 observed feedings Prey Type Number of observed feeding Percent of Diet mussels 207 40. limpets 193 37. small unshelled items 19 4. nereis 36 7. sn a i l s 1 0.5 crabs 31 6. chitons 16 3. worms 12 2. isopods 2 0.5 517 100. 87 TABLE XVIII Shells Collected at Nest Year Number of nests mussels limpets chitons crabs s n a i l s 1971,72 25 3562 1621 191 155 8 (64%) (29%) Co l l e c t i o n of limpet s h e l l s permitted i d e n t i f i c a t i o n of the chick food. Many of the limpets were Acmaea  persona, with almost equal numbers of A. d i g i t a l i s and A. pelta and a smaller number A. scutum (Table XIX). TABLE XIX Limpets Carried Whole to Chicks Number of Total A. d i g i t a l i s A. persona A. pelta A. scutum Nests s h e l l s 7 631 150 296 158 36 88 COMPARISON OF ADULT AND CHICK DIETS Composition A comparison of the frequencies with which various prey items were observed being eaten indicates a s i g n i f i c -ant difference ( V=222.6, p<.001), between the die t of the adult and that of the chick (Table XX). Mussels formed a greater percentage of the chick di e t than the adult diet while limpets were s l i g h t l y less represented i n the chick d i e t ( f i g . 15). In the case of both adults and chicks, mussels and limpets combined to form approximately the same percentage of the d i e t , 73% and 77% respectively. However, the species of limpets d i f f e r e d with a greater use of Acmaea persona, A. pelta and A. scutum i n the chick d i e t . One obvious difference i n the diets i s the apparent heavy use of small unshelled items by the adult and the r e l a t i v e lack of these i n the chicks d i e t . Items l i k e Nereis and chitons which were rare i n the adult d i e t showed up more frequently i n the chick d i e t . A point of i n t e r e s t i s the r e l a t i v e l y high number of crabs i n the chick d i e t . These were Oedignathous crabs and they appeared only i n the diets of the birds a f t e r chicks were present even though they were present e a r l i e r i n the habitat. These form a source of r e l a t i v e l y large prey when the t i d e i s very low and yet none were observed i n the diets u n t i l adults began carrying the abdomens of these crabs to the chicks. I t seems unlikel y that the numbers of crabs changed d r a s t i c a l l y during the period and c e r t a i n l y no TABLE XX Frequencies of prey items i n Adult and Chick diets based on observed feedings Prey Type Number of feedings observed Adult diet Chick d i e t mussels limpets small unshelled items nereis crabs chitons worms 368 554 281 11 9 2 15 207 193 19 36 31 16 12 V = 222.61 , p < .001 d f = 6 90f FIGURE 15 Comparison of adult and chick diets from observed feedings 501 VO o MUSSELS LIMPETS CUCUMBERS NEREIS SNAILS CRABS CHITONS WORMS ISOPODS 91 such change was apparent i n sampling. Nor was there any change i n a v a i l a b i l i t y of the crabs since they appeared as d i f f i c u l t to f i n d and extract throughout the season. I t may be that these crabs form a special part of the d i e t of the chicks. They did begin appearing i n the adult d i e t as well. This suggested that the d i e t of the adult may be affected by the presence or absence of chicks (Table XXI). However, there appeared to be considerable v a r i a b i l i t y i n the observed frequencies of prey items i n the adult d i e t on d i f f e r e n t days and on d i f f e r e n t tide regimes (Table XXII). In some cases the adults appeared to be feeding largely on mussels while on other occasions small limpets and unshelled items formed most of the d i e t . A multiple regression was performed to see what proportion of the v a r i a b i l i t y i n the proportion of the d i e t consisting of limpets and small un-shelled items was att r i b u t a b l e to the stage of breeding ( i e . incubation, chicks or other), the day i n the season, the range and d i r e c t i o n of the tid e and the nest l o c a t i o n . The analysis included weighting by sample size and an angular transformation to percentages. Only the range of the tide and the location of the nest were found to have s i g n i f i c a n t e f f e cts (Table XXIII). The f r a c t i o n of the variance of the dependent variable that was att r i b u t a b l e to the multiple regression was 0.22. Clearly other variables not included i n the regression are a f f e c t i n g the value of the dependent variable. TABLE XXI The adult d i e t before and during the presence of chicks Stage Prey items observed small nereis & mussels limpets unshelled items other worms chitons Before or during incubation 160 260 121 17 0 if. Chicks 163 201 125 9 11 X= 18.9 p < .005 df = 4 TABLE XXII Observed feedings of Adult birds Observation Mussels Limpets 1 9 18 2 1 34 3 0 4 4 29 38 5 32 63 6 6 1 7 1 77 8 6 1 9 3 62 10 6 12 11 14 18 12 31 14 13 38 29 14 27 6 15 8 15 16 2 28 17 11 3 18 44 16 19 15 2 20 11 2 21 5 8 22 2 29 23 0 0 24 0 0 25 0 0 26 1 19 27 0 6 28 29 1 29 10 1 30 14 9 31 13 7 32 0 8 33 0 2 34 0 8 35 0 4 36 0 9 Small unshelled items Other Prey 6 1 10 0 0 0 15 1 34 1 9 0 34 1 7 0 14 2 22 0 11 0 12 3 18 3 3 0 5 0 3 2 2 1 1 0 0 1 3 0 8 2 13 6 5 0 5 0 2 0 7 1 1 0 2 1 0 0 10 6 8 5 4 7 0 0 0 0 3 1 4 2 94 TABLE XXIII Results of multiple regression analysis on the proportion of the d i e t consisting of small items 2 Step R P r o b a b i l i t y Variable F-Ratio P r o b a b i l i t y 0.125 .05 nest location 4.87 .05 P a r t i a l c o r r e l a t i o n of tid e range = 0.33, p c .05 0.22 .05 nest location 4.64 .05 tide range 4.11 .05 P a r t i a l c o r r e l a t i o n c o e f f i c i e n t s of breeding stage, tide d i r e c t i o n and day i n season not s i g n i f i c a n t . PREY SIZE Collections of s h e l l s c a r r i e d to the chicks indicated that the birds selected r e l a t i v e l y large items as chick food at least when the chicks were at the nest. The adults, on the other hand were observed to feed often on small, unshelled items l i k e sea cucumbers and these were never c a r r i e d to the chicks at the nest. Data on the size of mussels taken by adults are i n s u f f i c i e n t to make a comparison with those given to chicks. However, the limpets c a r r i e d to the chicks indicated a clear difference with adults tending to feed on small to medium sized limpets and carrying up large limpets to the chicks (Fig. 16). After the chicks were moved to the i n t e r t i d a l the birds were observed feeding both themselves and the chicks on small limpets and sea cucumbers, i e . on items which had previously been ignored as chick food. 9£f FIGURE 16 Size of limpets i n adult and chick diets 100-1 80H O I- 60 U J 40-o 20-\ '"I A D U L T 0 C H I C K I 1 0 T" 4 L I M P E T S I Z E ( C M S ) 97 THE EFFECTS OF LOCATION ON THE DIET OF CHICKS The data presented i n the previous section were com-prised of feedings observed lar g e l y while the chicks were by the nest but included were observations af t e r chicks were moved to the i n t e r t i d a l . If these two rather d i f f e r e n t situations are separated then the ef f e c t s of the location of the chicks on the food selection become apparent. Information on the composition of the d i e t of chicks i s obtainable d i r e c t l y from the feeding observed from the blinds (Table XXIV). While the chicks are by the nest they are fed most frequently on mussel meat (59%) , with large limpets forming much of the rest of the d i e t (25%) . However, after the chicks are moved to the i n t e r t i d a l they are fed more frequently on limpets (50%) with mussel meat forming 21% of the d i e t . At the i n t e r t i d a l small limpets and small un-shelled items are fed frequently to the chicks and the d i e t then resembles that of the adults. The change i n die t with the change i n location was accompanied by an increase i n the feeding rate as indicated by frequency of feeding. This i s i l l u s t r a t e d by the obser-vations on one nest given i n Table XXV. This high rate of feeding was not reached by adults whose chicks remained at the nest. Two hours of observation on two 2 3 day old chicks remaining near the nest s i t e indicated a feeding rate of 9 feedings per hour. Of course a higher frequency of feeding does not mean a greater food intake since prey tended to be d i f f e r e n t i n the two si t u a t i o n s . TABLE XXIV Changes i n Prey with Chick Location Chick Location Prey By the nest Near the feeding area Mussels 154 53 limpets 64 129 small unshelled items 3 16 nereis 18 18 * sna i l s 0 1 crabs 15 16 * chitons 5 11 * worms 3 11 * isopods 0 2 259 257 V = 86.82 df = 6 p < .001 TABLE XXV Effects of Location on Chick Feeding Rate Location Whole Hours Average Number Range observed of feedings/hour at nest 9 4.2 1 - 7 at i n t e r t i d a l 4 (8 days) 18.3 15 - 25 100 The question arises as to whether chicks would eventually be fed the smaller items even i f they were not moved to the feed-ing area. The cases where g u l l s prevented the movement of young birds to the feeding area provided information on t h i s point. Data from the c o l l e c t i o n of s h e l l s of items c a r r i e d whole to chicks indicate that mussels form an increasing pro-portion of the die t of chicks that remain i s o l a t e d from the feeding area (Figs. 17, 18). The number of large limpets did not appear to change over time. In spite of the great increase i n the number of mussels brought to chicks per day the mean size of mussels and limpets remained r e l a t i v e l y con-stant (Figs. 19, 20). toi f FIGURE 17 Number of mussels and limpet s h e l l s found by one nest i n 1971 - one chick (average was calculated when c o l l e c t i n g days were missed, t o t a l s h e l l s present days since l a s t c o l l e c t i o n 52 48 H 40H O — O M U S S E L S L\ A LIMPETS 6 A V E R A G E o a —o v O 9—9-I .©—9 ©—Q—a' o / -o b-6 O M / \ / \ / —© —a. V \ / A \ / 6 14 16 18 20 22 24 AGE OF CHICKS (DAYS) 26 28 30 T 32 34 36 38 FIGURE 18 Number of mussel and limpet s h e l l s found by one nest i n 1971 - 2 chicks (average was calculated when c o l l e c t i n g days were missed, t o t a l s h e l l s present . days since l a s t c o l l e c t i o n 60 56 H 52 48 H 0 O—o M U S S E L S a-—A L IMPETS A V E R A G E o A ll 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 AGE OF CHICKS (DAYS) 103 P FIGURE 19 Size of mussels and limpets c a r r i e d whole to a single chick i n 19 71 MUSSELS o o ° o o o o o o O o O ° 00 S 2 I h-o z LU 0- -| r - i r - J 1 1 1 1 1 1 I f ~ < LU 4H LIMPETS 21 o ° o o o o 0-1 1 1 T 1 1 1 1 1 1 — • l I " I I - 1 r 4 8 12 16 20 24 28 32 CHICK AGE (DAYS) 0 I (HP FIGURE 20 Size of Mussels and Limpets car r i e d whole to two chicks i n 1971 8-2H MUSSELS 6-1 o o 0 o o o o o 0 0 o o o o o o ° co • ^2-I i -O -z LU . _j OT 1 > " 1 1 1 1 1 1 1 1 • 1 1 1 1 1 1 1 r z 2j . | LIMPETS ° o o o 00o0o ° o o ° o o ° 0 o o o o o 0-1 1 1 1 — : 1 1 1 1 1 1 1 1 1 1 1 ' 1 ' ' I 0 4 8 12 16 20 24 28 32 36 40 CHICK AGE (DAYS) 105 DIET IN RELATION TO PROFITABILITY The calculations of p r o f i t a b i l i t y indices indicated that mussels, when vulnerable, were more p r o f i t a b l e than the numerous small limpets on the mussel bed. I t seems l i k e l y that at any time when mussels are vulnerable they would be more p r o f i t a b l e than any of the other prey. Of course, when the t i d e i s low and mussels are not vulnerable then there are a number of d i f f e r e n t options open to the birds. They can search i n the lower i n t e r t i d a l for large items or simply stay on the mussel bed feeding on small items. P r o f i t a b i l i t i e s were calculated only for mussels and for the limpets on the mussel bed. The r e l a t i v e use of these items i n the three feeding contexts can now be examined (Table XXVI). P r o f i t a b i l i t i e s were calculated only for the limpets occurring i n the mussel bed. In the case of adults feeding and the feeding of chicks at the i n t e r t i d a l most of the limpets were from t h i s zone. However, most of the limpets car r i e d to chicks at the nest were much larger and only a small part of the 25% consisted of limpets from the mussel bed. The diet figures are based on r e l a t i v e l y long periods of time and the exposure times of both mussels and these limpets w i l l d i f f e r considerably during these periods. Mussels are vulnerable to attack only when they are washed or are pa r t l y submerged whereas limpets are vulnerable whenever they are not submerged over b i l l length. A number of points are clear without c a l c u l a t i n g exposure times for TABLE XXVI P r o f i t a b i l i t y and the use of mussels and limpets i n the three feeding contexts Adult Feeding Chick at Nest Chick at I n t e r t i d a l H-. Food Item P r o f i t a b i l i t y % Diet P r o f i t a b i l i t y % Diet P r o f i t a b i l i t y % Diet ° mussels 0.014 29 0.0071 59 0.0096 21 limpets 0.001 C44 0.00066 <<25 0.00089 <50 107 the two d i f f e r e n t prey. The birds concentrate on mussels as food for the chicks while they are at the nest. Indeed, t h i s more p r o f i t a b l e food forms an increasing percentage of the d i e t of chicks that remains at the nest s i t e for long periods. Small limpets on the mussel bed have a low p r o f i t -a b i l i t y (0.00066) and presumably are ignored as chick food for t h i s reason. Even when mussels are not vulnerable, small items from the mussel bed do not form a s i g n i f i c a n t proportion of the food c a r r i e d to chicks while they are at the nest. Presumably larger items l i k e crabs, chitons and the large Acmaea pelta and A. scutum limpets are more p r o f i t a b l e at t h i s time. However, no p r o f i t a b i l i t i e s were calculated to t e s t t h i s . Mussels are always more p r o f i t a b l e than the small limpets so that the r e l a t i v e l y heavy use of the smaller limpets as food for the chicks when they are at the i n t e r -t i d a l requires explanation. Small limpets and small un-shelled items suddenly appear frequently i n the d i e t and often the young are being fed on these items when mussels are vulnerable. However, there was some ind i c a t i o n that the birds were s t i l l foraging e f f i c i e n t l y . Often one of the adults would remain with the chicks higher up i n the i n t e r t i d a l feeding them on the small items while the other parent hunted farther down and c a r r i e d mussel meat or other larger items to the chicks. In t h i s way the chicks are not greatly exposed to the surf as they would be i f they followed the parents around when they searched for mussels. Moreover, there i s always one parent with the chicks and t h i s parent can both guard and feed the chicks at the same l o a time. Clearly r i s k s are not taken into account i n the p r o f i t a b i l i t y model. 109 THE RESPONSE TO PREY (a) General Remarks and Observations The theories of search images and p r o f i t a b i l i t y r e l y r t o a great extent on the observations made on the feed-ing of nestlings rather than the actual hunting behaviour of the adult. They are based on differences i n the occur-rence of items i n the d i e t compared with that expected on the basis of random encounter with prey i n the habitat and the tendency for the same prey to be brought on successive occasions. This tendency was also observed i n the case of the oystercatcher and i s i l l u s t r a t e d with the following data on items c a r r i e d to a chick during a flood t i d e with a low of - 0.1 feet. items ca r r i e d to chick - crab, crab, limpet, unidentified item, crab, crab, crab, crab, crab, crab, limpet,limpet, chiton, crab, mussel, crab, limpet, crab, nereis, mussel, limpet, mussel, mussel, mussel, limpet, limpet, limpet, limpet, limpet, mussel, mussel, mussel. The crabs brought were a l l Oedignathous crabs and only the abdomens of the crabs were given to the chick. Almost a l l of the crabs (70%) came from the same general area, a r e l a t i v e l y small, very exposed section of the feed-ing area. I t was often possible to make detailed observations on the foraging of adult birds and i n the course of doing t h i s the following responses to potential prey were noted. 1. Lack of Response If the oystercatcher attacked every prey that i t encountered chances are that i t would remain i n the upper 110 i n t e r t i d a l and c e r t a i n l y the pr o b a b i l i t y of encountering gaping mussels would decrease as the area exposed increased during an ebb t i d e . But thi s i s c l e a r l y not the case and the b i r d s , at times, show d e f i n i t e searching for p a r t i c u l a r prey, i e . a 'single mindedness'. On numerous occasions the birds were observed to move quickly over the exposed mussel bed past many poten t i a l prey items to go down to the water's edge to forage on mussels. Indeed, i t was sometimes possible to see by telescope pot e n t i a l prey items l i k e limpets close to the forager but apparently unnoticed or ignored i n the search for mussels. In the case of the search for crabs i n the example above the b i r d flew or walked past areas suitable for foraging to go to the exposed section of the feeding area. 2. Persistence i n Attack Under ce r t a i n circumstances a predator w i l l continue to pursue a prey on which i t has made an attack i n spite of being continually driven away by surf. This kind of response was observed only for attacks made on mussels. The bi r d i s often chased up higher by waves but moves quickly back down to locate the same mussel u n t i l he has successfully fed on " i t . The birds were also observed returning to a mussel a f t e r carrying a piece of meat up to a chick. 3. Abandoning of Prey In c e r t a i n cases attacks that were i n i t i a t e d were halted and the prey was either tossed aside or simply aban-doned. Prey items that were abandoned included nemertine worms, limpets, crabs, mussels, and chitons. The circum-stances under which t h i s occurred varied but appeared to 1±± involve u n s u i t a b i l i t y i n type or size of the item attacked, long handling time, escape of the prey, detection of other prey close by and d i s t r a c t i o n by other b i r d s . The observation that the birds ignored pote n t i a l prey and searched at times for s p e c i f i c items stimulated further investigation of t h e i r response to prey. Royama suggested that the 'conditioning* of a predator to a p a r t i c u l a r prey can occur rapidly, even from a single experience. With the time constraints imposed on prey v u l n e r a b i l i t y by the tides such a rapid conditioning would seem advantageous to the oystercatcher. According to Royama a predator may search for a prey even i f the density of the prey i s very low i f i t proves to be p r o f i t a b l e . The response of the oystercatcher to prey of d i f f e r e n t sizes and abundances was investigated by del i b e r a t e l y setting prey where they would be encountered by foraging birds and noting the responses, (b) Experimental Presentation of Prey METHODS A preliminary presentation of large and small limpets indicated that the birds selected the large sized limpets f i r s t and fed these to t h e i r chicks. Limpets appeared to be suitable for t h i s type of presentation experiment since they could be set out e a s i l y and t h e i r handling time was not affected by si z e . Thus limpets were c o l l e c t e d i n a moist container and placed i n an area known to be frequented by foraging bi r d s . In a l l cases of presenting prey to the oystercatchers the te s t area was i n the rockweed - open rock area just above the mussel bed and prey were set out during a flood t i d e . T y p i c a l l y the birds were foraging below the test area i n the i n t e r t i d a l , gradually moving up higher as the tide came i n and foraging largely on limpets, nereis and small items. D i f f i c u l t i e s a r i s i n g from surf covering the t e s t area resulted i n experiments of t h i s p a r t i c u l a r kind being ca r r i e d out on only one pair of birds and usually a number of d i f f e r e n t pre-sentations being made at the same time. On three separate days large and small limpets were placed i n groups on the feed-ing area. The groups consisted of equal numbers of large and small limpets i n an alternating array, more small limpets than large, a l l small limpets, or a l l large limpets. In each case the limpets within a group were spaced two inches apart and the groups themselves were one foot apart. After the limpets were placed i n p o s i t i o n , obser-vations were made from a b l i n d overlooking the feeding area. The behaviour of the birds was then recorded from the time that they encountered the t e s t area to the time when they l e f t i t and began other a c t i v i t i e s . In the f i r s t experiments the limpets within a group were only two inches apart and i t was d i f f i c u l t to t e l l how many limpets of the d i f f e r e n t sizes were actually perceived by the b i r d when encountering them i n groups. Since i t was r e a l l y the response to a low density that was of i n t e r e s t , the experiment was repeated i n the following way. Single large limpets were placed one foot apart i n an anay on the i n t e r -t i d a l . Small limpets (4 1.5 cms.), or medium sized limpets (1.5-2.0 cms.) varying i n number from zero to eleven were placed i n a t i g h t group around the large limpets forming 113 discrete groups each with a single large limpet. Two other groups consisted only of large numbers of small limpets. OBSERVATIONS AND RESULTS At the time that these experiments were ca r r i e d out the chicks of the tes t birds were about 18 days old and were i n the feeding area with the adults. When groups consisting of equal numbers of large and small limpets were encountered by a foraging b i r d , the b i r d began attacking the limpets i n the group, selecting the large limpets and giving these to the chick a short distance away. S i m i l a r i l y , a feeding bout i n a group with three times as many small limpets as large resulted i n the se l e c t i o n of large limpets to be given to the chick (Table XXVII). TABLE XXVII Selection of Limpets from Groups of Large and Small Ones during f i r s t feeding bout Size of Limpets Prey present i n group Prey attacked i n one bout large 14 11 small 14 2 large small 5 4 15 0 V = 12.0 p < .005 V = 6.23 p < .025 The birds usually l e f t a group before a l l of the prey were taken, spent some time at one or more other groups and then returned. Only after the large limpets had been given to the chick did the adult begin attacking the smaller limpets. The order i n which attacks occurred i s c l e a r l y not random as i l l u s t r a t e d by the following observed sequence large, large, large, large, large, large, large, large, large, large, large, small, small, small small, small, small, small, small, small, small small, small, small, small, (one sample runs test n, = 12, n 2 = 14. r = 2 p <.05) A l l large limpets were given to the chicks. When the large limpets were spaced out, an adult was observed to encounter a single large limpet, attack i t and begin moving about on the t e s t area attacking large limpets and carrying them to the chick. Analysis of attacks made before the b i r d began encountering groups already v i s i t e d , shows a d e f i n i t e selection of large limpets i n spite of en-countering many more small and medium sized limpets (Table XXVI11). TABLE XXVIII The e f f e c t s of size and encounter rate on prey s e l e c t i o n Size of Limpets large medium small 2 cms. 1.5-2 cms. 1.5 cms. Number encountered 9 11 20 Number attacked 8 1 2 = 15.88 p < .005 An examination of the limpets attacked during the whole experiment indicated a selection of large and medium sized limpets (Table XXIX). TABLE XXIX Size Selection of Limpets Size of Limpets large medium small prey present i n t e s t area 13 49 40 prey eaten 13 43 11 2 14.84 p < .005 Unfortunately t h i s type of experiment was c a r r i e d out with only one p a i r of birds although one other such presentation was c a r r i e d out with another p a i r under d i f f e r e n t circumstances. In t h i s case the prey were set out on a boundary between two t e r r i t o r i e s where adults were known to have frequent t e r r i t o r i a l c o n f l i c t s . Such a c o n f l i c t did occur and as usual the birds skulked around feeding on items i n the v i c i n i t y . On encounter-ing the t e s t area which contained three large limpets spaced one foot apart with four smaller limpets around each one, a b i r d made a sequence of attacks, moving to drink at a nearby pool between each feeding bout. The following sequence of attacks was observed (brackets are feeding bouts), (small), (small, small, l a r g e ) , (small, small), (large, small, small), (small, small), (small), (large, small, small). A one sample runs test for the above sequence indicates a random sequence of attacks l i b (n x = 3, = 12, r = 7,). The b i r d ate a l l of the limpets i t s e l f and afte r f i n i s h i n g them appeared to search the general area for more before beginning to roost. DISCUSSION In spite of the lack of r e p l i c a t i o n the res u l t s of the presentation experiments are i n t e r e s t i n g . The work on the d i e t of the birds had indicated small items as well as large were used for food for the chicks once they reached the i n t e r t i d a l . The sudden appearance of large limpets i n the test area brought an immediate response i n the foraging birds with clear selection of large items for the chicks and an apparent rapid conditioning to such items r e s u l t i n g i n s p e c i f i c searches being c a r r i e d out. Moreover, even among small and medium sized limpets there was an apparent tendency to select the largest ones for the chicks. Of i n t e r e s t i s a case where a b i r d entered the tes t area and f i r s t encountered a group of small and medium sized limpets. In contrast to the behaviour of the birds which encountered a large limpet, t h i s b i r d did not move about but simply fed i t s e l f and a nearby chick on the limpets i n that group. Medium sized limpets appeared to be taken before smaller ones. It i s d i f f i c u l t to say how general any of the res-ponses observed are. Ideally the experiments should have been car r i e d out on more pairs of birds under a variety of circumstances. The birds appeared to show a predictable response to the sudden appearance of a more p r o f i t a b l e food item. I t i s 117 not cer t a i n whether the birds would show a si m i l a r response i f they had no chicks to feed. The observation of the adult response to the limpets during the t e r r i t o r i a l i n t e r -action may represent a special case when the attention of the b i r d i s not s t r i c t l y on feeding. n o FORAGING IN TIME AND SPACE Optimal foraging theory applies not only to the selec-t i o n of prey but also to the temporal and s p a t i a l aspects of foraging (Schoener, 1971). C l e a r l y , the predator could d i s -t r i b u t e i t s hunting over both time and space according to the return. At any time an e f f i c i e n t predator should search for and locate areas which are more p r o f i t a b l e but there i s some advantage i n concentrating the periods of search at times when more prey are vulnerable. Moreover, according to the theory of Royama, the foraging patterns w i l l r e f l e c t both a tendency to contin-u a l l y sample the environment to determine r e l a t i v e p r o f i t -a b i l i t i e s and also a tendency to spend more time hunting prey which are most p r o f i t a b l e . Hunting for d i f f e r e n t prey may involve searching a d i f f e r e n t area or searching the same area i n a d i f f e r e n t way. Thus, i t was of in t e r e s t to investigate the temporal and s p a t i a l elements of foraging of the oystercatchers r e l a t i n g them to the pattern of changing prey v u l n e r a b i l i t y and the measures of p r o f i t a -b i l i t y . METHODS Observation blinds consisting of wood or canvas were set up early i n the season at s i t e s where pairs could be observed feeding. A telescope was used to observe the birds and a stop-watch was kept running i n front of the telescope so that the observer could record exact times for various events. A hand-held tape recorder was used to 119 record observations. Data on the temporal aspects of foraging were recorded during observations on ten pairs of birds but a few of the pairs were observed for only short periods of time. In each case the time when foraging began was noted along with the length of foraging t r i p s and and factors apparently influencing them. I t was possible in some cases to record the tide l e v e l s during the time when foraging was occurring and i n a l l cases such information was available by c a l c u l a t i o n from tide tables. In the case of the studies on foraging i n space detailed investigation of p a r t i c u l a r pairs and t h e i r t e r r i t o r y was required and t h i s was carr i e d out on only three pairs of bi r d s . Using natural zonation and crevices or other physical d i s c o n t i n -u i t i e s the t e r r i t o r i e s were divided into small areas which were assigned a number. The l e v e l of the areas i n the i n -t e r t i d a l was also determined. Coloured spikes were placed i n a l i n e i n d i c a t i n g the l e v e l of the ti d e as well as the l e v e l of the foraging b i r d . The time spent i n various areas and zones was recorded as well as the foraging success. 1ZU RESULTS 1. Foraging i n time The temporal aspects of foraging were affected by the t i d e regime, the presence of chicks and the occurrence of interactions with conspecifics and other species, (a) T i d a l e f fects Both the flock and the in d i v i d u a l breeding pairs showed a d e f i n i t e t i d a l rhythm with roosting at high tides and foraging during ebb or flood. The birds appeared to wait for the tide l e v e l to drop during an ebb before beginn-ing to forage. Observations over nineteen ebb tides i n d i c -ated that the birds began foraging on the average 1.7 hours (S.E. = 0.1) afte r high t i d e . While roosting at high-tide the breeding pairs were either both by the nest or one was by the nest and the other was roosting at some point closer to the feeding area. Very short periods of i n -a c t i v i t y were usually associated with low high tides and short ranges (Fig. 21). In some cases a b i r d began to forage r e l a t i v e l y early i n the tide cycle, but stopped a f t e r a short period of searching i n the exposed area. Foraging t y p i c a l l y began when the ti d e was i n the mid to low mussel bed and the upper mussel bed and rockweed were s t i l l wet. Once foraging began the time devoted to foraging showed a d e f i n i t e pattern over the t i d a l cycle, increasing over ebb tides and decreasing over flood tides (Fig. 22). Each set of observations for any p a r t i c u l a r pair of birds was consistent with the pattern of the means. There i s , FIGURE 21.- Ef f e c t s of tid e range on time of i n i t i a t i o n of foraging NITIATION O F F O R A G I N G - H O U R S A F T E R HIGH TIDE o o _ , N H D m JJ , > C O H z o m en CD' ^ co J 1 ' 1 1 i_ o 0 8 o o o TZT I2X-P FIGURE 22. F o r a g i n g a c t i v i t y over t i d e c y c l e s rzz 123 however, considerable v a r i a b i l i t y i n the rate at which the increase occurs over ebb tide and the reasons for t h i s are not c l e a r . The pattern of foraging over the t i d a l cycles suggested that a peak i n foraging a c t i v i t i e s existed and i t was of in t e r e s t to determine the l e v e l of the tide when most of the foraging occurred. An examination of the proportion of t o t a l foraging time spent during the hour i n which the t i d e was at a p a r t i c u l a r l e v e l indicated that a large percentage of foraging occurred when the tide was between three and eight feet with a peak at the six to seven foot l e v e l (Fig. 2 3). This does not indicate where the foraging occurred i n space but does suggest that most foraging occurred when the surf was washing the low to mid mussel bed, i e . when the greatest number of large mussels was vulnerable. FIGURE 23. Foraging time and tide l e v e l 30 20A 10H 0 0 ~~r 2 ' 4 6" 8 10 TIDE L E V E L - WHEN FORAGING O C C U R R E D (FT) 125 (b) E f f e c t s of Young Before the hatching of eggs the adults must forage only for themselves. However, with the a r r i v a l of young, t r i p s must be made back and f o r t h between the nest and the feeding area to transport food to the young. Moreover, the demands of the young w i l l increase with age. While the chicks are by the nest s i t e the adults apparently meet the new demands by foraging more frequently and for shorter times (Table XXX). TABLE XXX E f f e c t s of Young on frequency and length of foraging t r i p s Stage Trips per hour Average t r i p length (mins.) incubation 1.4 10.9 chicks 5.0 3.8 (c) E f f e c t s of Interactions Interactions with conspecifics usually had the e f f e c t of temporarily halting foraging, but sometimes a foraging t r i p began with a piping i n t e r a c t i o n . I t was observed many times that i f a piping f l i g h t took a b i r d out near or past i t s t e r r i t o r y the b i r d would drop down either to feed or to simply pick up a food item for the young. Clearly the time and energy spent i n such an a c t i v i t y i s made more worthwhile i f the b i r d turns i t into a feeding t r i p . I n t e r s p e c i f i c interactions had a s i m i l a r e f f e c t . For example, a forager would stop feeding and f l y up to chase away a crow. If an i n t e r a c t i o n was prolonged then i t might have a greater e f f e c t on foraging. 2. Foraging i n space The analysis of the s p a t i a l aspects of foraging i s incomplete. General observations and some of the space data analyzed so far do indicate that the birds may, at times d i s t r i b u t e t h e i r foraging over space according to pro-f i t a b i l i t i e s . The evidence to date i s summarized as follows:-1. Birds were observed to make t r i p s back and forth to p a r t i c u l a r areas. For example, a b i r d may carry mussel meat from a crevice to i t s chicks and then return quickly to the same crevice only to repeat the performance. 2. At times the birds were observed to spend a considerable period of time foraging i n one very small area. For example, one b i r d foraged i n a very small weedy pool (approximately two square feet i n area) for ten minutes, spending the whole time probing, struggling with items and turning frequently. 3. In general, the birds do move about the feed-ing area considerably so that the p o s s i b i l i t y for compar-ing p r o f i t a b i l i t i e s over space and also over time d e f i n i t e l y e x i s t s . Not only do the birds spend some time i n each zone while the zones are exposed but they also move about within a zone. This i s i l l u s t r a t e d by data i n which the location of a b i r d was recorded as i t moved about on i t s feeding t e r r i -tory. The t e r r i t o r y was sketched and was divided into small sections each of which was assigned a number (Table XXXI). 127 TABLE XXXI Sequence of areas v i s i t e d by a foraging bird during a flood t i d e T r i p Sequence of Area v i s i t e d 1 16, 13, 17, 10, 9, 4, 14 2 14 3 14 4 14, 16, 14, 16 5 6, 4, 9, 10, 11, 10, 9, 14, 4, 6, 5, 6, 5, 11, 6, 6 6, 7, 6, 1, 7, 6, 4, 10, 9, 4, 5, 6, 1, 6, 5, 4, 9 7 14 No attempt i s made to analyze or discuss the sequence of areas v i s i t e d at t h i s point and the data are simply pre-sented as evidence for the moving about of the bir d s . The discussion so far has dealt only with the forag-ing of adults on the feeding t e r r i t o r y . Mention must be made of foraging t r i p s away from the feeding area to reefs some distance away. The birds were, at times, observed to leave the t e r r i t o r y and f l y o f f . Early i n the season pairs were sometimes observed making these t r i p s but once eggs of chicks were present only one of a pair would ever leave. If chicks were present, an adult making one of these long distance t r i p s would always return carrying a large item for the chicks, usually a whole mussel. Birds with t e r r i t o r i e s i n the same general area were sometimes observed making these t r i p s around the same time but apparently not with the same destination. Some birds appeared to make such t r i p s more 128 often than others. There was some ind i c a t i o n that these t r i p s may be associated with low hunting success i n the feed-ing t e r r i t o r y . Often they were made at low tides when the i n t e r t i d a l was r e l a t i v e l y dry or during short range tides with high surf (Table XXXII). Clearly there i s no tendency to d i s -continue the t r i p s a f t e r the eggs hatch, as suggested by Webster (1941). TABLE XXXII Timing of Trips to Distant Reefs Date Tide Regime Time of Time of Time when T r i p Birds Breeding High Low High tide Low ti d e (hrs.) (hrs.) made (hrs.) involved stage May 26 9.8 0.0 June 2 7.9 4.2 JunelO 9.4 0.7 Jly.12 10.6 1.0 Jly.13 10.7 1.8 Aug.12 June29 3.6 1.1 1.1 1.1 Jly.5 4.4 Aug.7 9.6 4.6 1645 1725 0850 1420 0835 1015 1055 1235 1110 0915 0915 0915 1335 1805 0900 1420 0900 1200 1042 1414 1020 0930 0930 0930 1340 1500 both (of pair) both (of pair) one (of pair) one (of pair) one (of pair) one (of pair) one (of pair) one (of pair) one (of pair) one (of pair) ne pre-egg pre-egg egg chick chick chick chick chick chick egg egg chick TABLE XXXII Timing of Trips to Distant Reefs Date Tide Regime Time of Time of Time when T r i p Birds Breeding High Low High t i d e Low tid~e (hrsT) Thrs. made (hrs.) involved stage Aug. 7 4.6 Jly.20 7.5 6.0 Jly.31 11.1 '2.9 Aug.15 8.1 5.8 1235 0940 835 1805 1415 1035 1425 1735 1220 1300 1300 1045 1130 1130 one (of pair) one (of pair) one (of pair) one (of pair) one (of pair) one (of pair) one (of pair) chick chick chick chick chick chick chick 131 GENERAL DISCUSSION In his recent book, Emlen (1972) points out that 'the f i n a l a r b i t e r s of a theory's value are descriptive data'. The c o l l e c t i o n of such data has lagged behind the development of t h e o r e t i c a l models of optimal foraging. Indeed, st r a t e g i c models l i k e those proposed by Emlen (1966), and MacArthur and Pianka (1966) have been devel-oped on a r e l a t i v e l y poor data base. Moreover, i t i s often d i f f i c u l t to see how to obtain measures of the parameters of such models i n the f i e l d . Royama's model, on the other hand, was based on a precise t a c t i c a l model and the parameters of the model appeared to be open to f i e l d measurement. Moreover, recent studies appeared to support the idea of hunting by p r o f i t a b i l i t y . Goss-Custard (1970) found evidence that redshank tended to concentrate i n areas of high prey density and where feeding e f f i c i e n c y was highest. Croze (1970) working on carrion crows found evidence of area r e s t r i c t e d search and a tendency to associate cert a i n areas with c e r t a i n types of food. In the laboratory, Smith and Dawkins (1970) found that i n d i v i d u a l great t i t s responded to variations i n food density by spending a large proportion of t h e i r t o t a l searching time i n regions of highest food density. Actual p r o f i t a b i l i t i e s of prey items were not determined i n any of these studies. In the present study i t was possible to calculate the p r o f i t a b i l i t i e s of various prey items. This was largely due to the careful consideration given to the choice ± 32 of a study animal. Moreover, the study i s unique i n that i t provides data for foraging under three d i f f e r e n t circumstances; the feeding of young at the nest, the feeding of young at the i n t e r t i d a l and the feeding of adults. Each of these situations represents a change i n the values of the parameters i n the model. The study indicated that the d i e t of oystercatcher chicks d i f f e r e d considerably from that of the adults while the chicks were at the nest. P r o f i t a b i l i t y determinations indicated that the birds were concentrating on the most p r o f i t a b l e food item as food for the chicks. The study c l e a r l y indicated that, at times, pot e n t i a l prey were ignored and that t h i s behaviour was related to differences i n the p r o f i t a b i l i t y of food items. The strategy was one of maximizing biomass return i n a given hunting time. Other studies have also indicated differences between the diets of adults and those of chicks. Lind (1965) observed European Oystercatchers foraging on mudflats and apparently carrying larger Nereis worms to the chicks while often taking smaller worms for them-selves. More recently, Root (1967) showed that adult gnatcatchers 'broadened t h e i r foraging niche' when young were present. In the case of the Black Oystercatcher such changes i n d i e t and foraging appear to be related to the p r o f i t a b i l i t i e s of the various prey items. The increased food requirements involved i n feeding young are met i n part by a concentration on the most p r o f i t a b l e food item as well as an increase i n the use of larger items and possibly the use of prey not previously i n -cluded i n the d i e t . The r e l a t i v e l y heavy use of small limpets and other small items as food for chicks at the feeding area i s probably not predictable from p r o f i t a b i l i t y considerations. However, i t does not necessarily contradict the model when the p a r t i c u l a r circumstances are considered. Indeed, i t supports the idea that the birds maximize the feeding rate of the chicks while minimizing the r i s k s . Both the adult with the chicks and the mate searching at lower level s may be operating according to the p r o f i t a b i l i t y model. In the present study, p r o f i t a b i l i t y has been c a l -culated as the biomass that can be c o l l e c t e d i n a given time but, as Royama points out, i t could also be measured i n c a l o r i c values. While the 'calorie per gram' value of limpets i s s l i g h t l y higher than that of mussels (Menge, 1971) the predator can c o l l e c t more c a l o r i e s i n a given time by foraging on mussels when they are vulner-able. This, of course, i s e n t i r e l y d i f f e r e n t from sugg-esting that birds concentrate on 'energy r i c h ' foods (Emlen, 1972). One of the questions that a r i s e i n the present study i s why, i n f a c t , do the birds extend t h e i r d i e t beyond mussels to include other items but ignore c e r t a i n items which appear to be abundant and exposed for long periods of time eg. s n a i l s and barnacles. The tendency for the b i r d to include other items may be explained by Royama's model. In order to continually monitor pro-f i t a b i l i t i e s which do change over time, the b i r d must sample d i f f e r e n t prey items. Whether other items l i k e s n a i l s and barnacles are actually unprofitable for the birds was not determined i n the present study. One of the possible arguments against the use of the black oystercatcher as a study animal i s the complexity of the prey d i s t r i b u t i o n i n time and space. However, a num-ber of studies have indicated the possible importance of predation to the structure of the i n t e r t i d a l community. The prey of the black oystercatcher were di s t r i b u t e d ver-t i c a l l y i n the i n t e r t i d a l and t y p i c a l l y displayed v a r i a t i o n i n s i z e , abundance and length of time exposed to predation both within and between species. Shore-level s i z e gradients reported by Vermeij (1972) and Giesel (1970) were also found on Cleland Island. Thus species l i k e Acmaea d i g i t a l i s which were associated with high i n t e r t i d a l l e v e l s showed a tendency for increasing size with increasing l e v e l while the opposite was true for A. pelta and A. persona. The general interpretation of such gradients has been that they are a response to gradients in the nature and inten-s i t y of predation. P o s t l a r v a l prereproductives are then expected to inhabit a zone of minimal mortality within the v e r t i c a l range of the species (Vermeij, 1972). This zone would be at the base of the v e r t i c a l range of mobile gas-tropods associated with higher i n t e r t i d a l areas and near the top of the range of lower i n t e r t i d a l gastropods. Giesel (1970, believed that b i r d predation increased toward higher shore l e v e l s . The present study indicated that p r i o r to the hatching of eggs the oystercatchers preyed more on Acmaea d i g i t a l i s i n the lower part of t h e i r range but that limpets below a certai n size were probably unexploited. However, once chicks were present the adults searched forlarge items as food for the chicks and at that time the predation pressure may be di s t r i b u t e d i n such a way as to have greater impact on the higher l e v e l s . In clos i n g , i t i s surprising that more studies have not been carried out on the oystercatcher of the west coast. In Europe much research has centred around the European Oystercatcher, Haematopus ostralegus. However, the two species appear to d i f f e r considerably. The European species occurs i n very large numbers, at lea s t i n winter (Dare, 1966), and has been considered a major pest of s h e l l f i s h e r i e s (Davidson, 1968). Moreover, certa i n subspecies have recently shown strong tendencies to colonize inland areas (Heppleston, 1971). In contrast the black oyster-catcher appears i n r e l a t i v e l y low numbers along the coast at a l l seasons and although there i s a movement i n winter to more protected i n l e t s , breeding i s s t i l l l i m i t e d to coastal islands which are often inaccessible for research. 136 BIBLIOGRAPHY Boyd, H. 1962. Mortality and f e r t i l i t y of European Charadrii. Ibis 104: 368-387 Croze, H. 1970. Searching Image i n Carrion Crows. Z. Tierpsychol, Beiheft 5: 1-86. Dare, P.J. 19 66. The breeding and wintering populations of the Oystercatcher (Haematopus ostralegus L.) i n the B r i t i s h I s l e s . Fishery Invest. Lond. (Ser. II) 25 (5): 1-69. Davidson, P.E. 1968. The Oystercatcher - A pest of S h e l l -f i s h e r i e s . In 'The Problems of Birds as Pests', Academic Press. 141-155. Drent, R., G.F. van Tets, F. Tompa and K. Vermeer, 1964. The breeding birds of Mandarte Island, B r i t i s h Columbia. Can. Field-Nat. 78(4): 208-263. Emlen, J.M. 1966. The role of time and energy i n food preference. Amer. Nat. 100(916): 611-617. Emlen, J.M. 1972. 'Ecology: an evolutionary approach" Addison - Wesley Pub. Giesel, J.T. 1970. On the maintenance of a s h e l l pattern and behaviour polymorphism i n Acmaea d i g i t a l i s , a limpet . Evolution 24: 98-119. Goss-Custard, J.D. 1970. The responses of redshank (Tringa totanus L.) to s p a t i a l variations i n the density of t h e i r prey. J . Anim. Ecol. 39: ^ 91-113. Heppleston, P.B. 19 72. The comparative breeding ecology of oystercatchers (Haematopus ostralegus L.) i n inland and coastal habitats. J . Anim. Ecol. 41(1): 23-51. Holl i n g , C.S. 1959. Some c h a r a c t e r i s t i c s of simple types of predation and parasitism. Can. Ent. 91: 385-398. Holling. C.S. 1965. The functional response to predators to prey density and i t s role i n mimicry and population regulation. Mem. Ent. Soc. Can. 45: 1-60. Holling, C.S. 1968. The t a c t i c s of a predator. Symp Roy. Ent. Soc. Lond. 4: 47-58. 1 J V Lind, H. 1965. Parental feeding i n the oystercatcher (Haematopus o. ostralegus (L.). Dansk. orn. Foren. Tidsskr. 59: 1-31. MacArthur, R.H. and E.F. Pianka, 1966. On optimal use of a patchy environment. Amer. Nat. 100 (916): 603-609. Menge, B.A. 1971. Foraging Strategy of a S t a r f i s h i n r e l a t i o n to actual prey a v a i l a b i l i t y and environmental p r e d i c t a b i l i t y . Ecological Monographs 42 (I): 25-50. Norton-Griffiths, M. 1967. Some ecolo g i c a l aspects of the feeding behaviour of the oystercatcher Haematopus  ostralegus on the edible mussel Mytilus e d u l i s . Ibis 109: 412-424. Rashevsky, N. 1938. In 'Mathematical Biophysics, Physicomathematical foundations of Biology Vol. I I . p. 292. Ricketts, E.F., and J . Calvin, 1968. In "Between P a c i f i c Tides'. Stanford University Press. Root, R.B. 1967. The Niche Exploitation Pattern of the blue-gray gnatcatcher. Ecol. Monog. 37: 317-350. Rosen, C A. 1967. Optimality P r i n c i p l e s i n Biology. Butterworths, Lond. Royama, T. 1970. Factors governing the hunting behaviour and food selection of the great t i t (Parus major L.) J. Anim. Ecol. 39; 619-668. Schoener, T.W. 1971. Theory of Feeding Strategies. Ann. Rev. Ecol. Syst. 21: 369-404. Smith, J.N.M. and R. Dawkins, 1971. The hunting behaviour of i n d i v i d u a l great t i t s i n r e l a t i o n to s p a t i a l variations i n t h e i r food density. Anim. Behav. 19: 695-706. Tinbergen, L. 1960. The natural control of insects i n pinewoods. I. Factors influencing the in t e n s i t y of predation by song b i r d s . Arch. neer. Zool. 13: 265-343. Vermeij, G.J., 1972. I n t r a s p e c i f i c shore-level s i z e gradients i n i n t e r t i d a l molluscs. Ecology 53 (4): 693-700. Watt, K.E.F. 1962. Use of Mathematics i n Population Ecology. Ann. Rev. Ent. 7: 243-260. 138 Webster, J.D. 1941. Feeding habits of the black oyster-catcher. The Condor 43 (4): 175-180. Webster, J.D. 1941. The Breeding of the Black Oyster-catcher. The Wilson B u l l e t i n 53 (3): 141-156. 

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