TROPHIC RELATIONS OF SEABIRDS 



101 



schooling fish, squid, and crustaceans that 

 occur near the surface. For these very abun- 

 dant shearwaters, that, unfortunately, is 

 close to the extent of our knowledge both for 

 the North Pacific, where they winter, and the 

 South Pacific, where they breed. Most petrels 

 remain in oceanic habitats, but shearwaters, 

 particularly the sooty shearwater (Puffinus 

 griseus), and sometimes fulmars feed close to, 

 if not within, the inshore neritic habitat. A 

 much better understanding of the diets of this 

 group is sorely needed. 



Knowledge on the food of gulls, shorebirds, 

 and related species is surprisingly scanty in 

 view of all that is known about their breeding 

 biology and social behavior. Little is known 

 about the marine food of phalaropes, but by 

 inference from their association with storm- 

 petrels, plankton-feeding whales, and conver- 

 gence lines (Martin and Myers 1969), their 

 tiny size, and their method of feeding (picking 

 at minuscule items on the water surface), one 

 can guess that they feed on zooplankton and 

 detritus. Skuas (Catharacta skua) and jaegers 

 (Stercorarius spp.) apparently eat what they 

 can find at the surface, as well as whatever 

 they can steal from gulls and terns. Almost all 

 the literature on their feeding (Bent 1946) 

 dwells on accounts of their stealing from 

 other birds. That spectacular behavior would 

 seem to be so energetically costly, though, 

 that it is probably less important than we 

 have been led to believe. Rather surprisingly, 

 the question of what foods the gulls and terns 

 eat in the eastern North Pacific is difficult to 

 answer from the literature (Tables 9 and 10). 

 Some information exists for five of the larger 

 larids at isolated places, but little is known 

 about food elsewhere in their respective 

 ranges, and the diets of the seven smaller 

 gulls and the terns are practically unknown. 

 Studies on gull diets in the Atlantic region 

 (e.g., Spaans 1971; Harris 1965) provide infor- 

 mation on what to expect from the same 

 species in the Pacific, but that information 

 must be considered only in general terms be- 

 cause, the birds being somewhat opportu- 

 nistic, their diets differ greatly from one 

 locality to another (Ingolfsson 1967). A few 

 observations are available for arctic terns 

 (Sterna paradisaea) in Alaska, but little infor- 

 mation exists for other terns (Table 10). Bent 

 (1921) noted that Aleutian terns (S. aleutica) 



sometimes associate with arctic terns during 

 feeding. 



Finally, we must include raptors, particu- 

 larly the peregrine (Falco peregrinus) and bald 

 eagle (Haliaeetus leucocephalus), because 

 they are important predators on the smaller 

 seabirds (White et al. 1971, 1973). Peregrines 

 have, in fact, been observed feeding on storm- 

 petrels far at sea (Craddock and Carlson 

 1970). 



Trophic Relations Within 

 Seabird Communities 



We have compared and summarized in 

 general terms the food partitioning by species 

 in five rather broad oceanographic regions 

 and their subdivisions in the northeastern 

 North Pacific and Bering Sea, based on the 

 specific details on diets presented in Tables 1 

 through 10. The five broad regions, defined 

 oceanographically by Dodimead et al. (1963) 

 and Favorite et al. (1976) and modified by 

 Sanger (1972), are shown in Fig. 1. The five 

 oceanographic regions (domains) were divided 

 further into inshore neritic, offshore neritic, 

 and oceanic habitats (Sanger and King, this 

 volume). We did not include estuarine habi- 

 tats or sheltered bays in the analysis. 



The oceanic habitat includes waters of the 

 photic zone overlying the deep ocean and con- 

 tinental slopes beyond the continental or 

 insular shelves. The Bering Sea and central 

 subarctic domains are largely made up of 

 oceanic habitat. The other three domains in- 

 clude both inshore and offshore neritic as well 



Fig. 1. Schematic oceanographic domains of the 

 subarctic Pacific regions (defined by Dodimead et 

 al. (1963) and Favorite et al. (1976) and modified 

 by Sanger (1972). 



