144 



E. H. DUNN 



oceans have a very high peak of productivity 

 in the summer, whereas the low arctic has a 

 lower, but longer-lasting, peak (Ashmole 

 1971). Fish stocks increase in summer as well 

 (Snow 1960; Pearson 1968; Sealy 1975a), and 

 decline or disperse in autumn (Potts 1968). 

 Catchability may also differ widely from year 

 to year (e.g., E. K. Dunn 1973). 



Marine foods are likely to have a patchy dis- 

 tribution, which may make food stocks diffi- 

 cult to locate, even in times of abundance 

 (Ashmole 1971; Sealy 1975a). Birds in locali- 

 ties with low food abundance frequently show 

 alterations in time and pattern of foraging, 

 sometimes even changing diets (Cramp 1972; 

 Henderson 1972; Hunt 1972; Lemmetyinen 

 1972). The time and energy expended in find- 

 ing and capturing food by different seabird 

 species must vary widely according to the 

 form of foraging used: plunge-diving, beach 

 scavenging, aerial robbing, underwater pur- 

 suit, and so on. Even when different species 

 have traveled the same distance to an identi- 

 cal food stock, therefore, the costs of procure- 

 ment differ. 



Time and energy spent foraging depends 

 not only on abundance and ease of capture, 

 but also on nutritional return, and on the age 

 and size of the bird. Fig. 3 shows that the 

 smaller species in a seabird community may 

 spend the most time foraging. Even though 

 this illustration is taken from the breeding 

 season when food demands of the young must 

 be taken into account, it suggests a difference 

 based on cost of living according to size. 



Age of the bird affects time and energy com- 

 mitment to foraging because younger birds 

 are often less skilled at capturing food. This 

 has been noted particularly in long-lived sea- 

 bird species (Orians 1969; Dunn 1972; LeCroy 

 1972; Buckley and Buckley 1974; Barash 

 et al. 1975). Older juveniles may be excluded 

 from feeding areas by more experienced, terri- 

 torial adults (Moyle 1966), whereas imma- 

 tures are not (Drury and Smith 1968; In- 

 golfsson 1969). 



Nutritional and energetic return obtained 

 from food is a very important factor in forag- 

 ing strategy that has not received the atten- 

 tion it deserves. Table 1 lists the caloric value 

 of various foodstuffs and illustrates how little 

 is known about foods eaten by seabirds. Al- 

 though caloric content and abundance of food 



1.8 



1.6 



UJ 



a. 



CO 



< 

 o 



o 

 o 



1.4 



1.2 



1.0 



CT 



M 



2.0 2.4 2.8 3.2 3.6 



LOG AVERAGE WEIGHT BIRD 



Fig. 3. Time spent foraging in the breeding season 

 as a function of body size. From Pearson (1968). 

 AT = arctic tern (Sterna paradisaea), CT = com- 

 mon tern (S. hirundo), ST = sandwich tern 

 (Thalasseus sandvicensis), K = black-legged 

 kittiwake, P = common puffin, M = common 

 murre, LBB = lesser black-backed gull, S = 

 shag. 



have often been accepted as the most impor- 

 tant determinants of foraging strategies 

 (Bookhout 1958; Emlen 1966; West 1967; 

 Bryant 1973), they may frequently be less im- 

 portant than nutritional value and digesti- 

 bility, also shown in Table 1 (Pulliam 1974). 



Since fish seem to be highly digestible, most 

 of the energy contained in them is available to 

 the consumer. There are, unfortunately, no 

 data on the digestibility of marine inverte- 

 brates, but those for insects suggest that di- 

 gestibility, at least of crustaceans with exo- 

 skeletons, is somewhat lower than that for 

 fish. A bird would therefore have to eat a 

 larger biomass of invertebrates than of fish to 

 satisfy the same energetic needs (although 

 cost of procurement might not be as high as 

 for fish). 



A bird must satisfy not only energetic 

 needs, but also nutritional requirements. Fish 

 are high in protein (Table 1), but what little is 

 known of marine invertebrates suggests a low 



