TROPHIC RELATIONS OF SEABIRDS 



115 



Table 16. Number of seabirds of different oceanographic regions having different numbers of 



categories of food in their diets. 



Number of categories in the diets 8 



a These are the "food categories" of Tables 11-15. Items included in diets are not included here. 



ing an inshore neritic feeding element are the 

 only ones that include herbivores, and even 

 then, few of these species exist in significant 

 numbers in the marine environment (discount- 

 ing estuaries and sheltered bays). 



It is readily apparent from the foregoing 

 comparisons that much overlap exists in the 

 prey eaten by seabirds within each com- 

 munity. The question whether real competi- 

 tion ever exists is academic. Competition per- 

 haps exists only rarely because seabirds parti- 

 tion resources through use of different feeding 

 methods, selection of different-sized prey, and 

 habitat zonation. Table 18 lists feeding meth- 

 ods (after Ashmole 1971 and Ainley 1977) and 

 the body size and bill length of each species 

 considered in this review. Bill length is 

 usually related directly to body size (Ashmole 

 1968; Bedard 19696), but note, for instance, 



that the longer species of the two kittiwakes 

 has the shorter bill. Body weight would be a 

 better measure of relative size than body size, 

 but few reliable weight data are available for 

 seabirds. 



The use of different feeding methods by 

 species in each community grossly assigns 

 birds to feeding at different depths. Thus, 

 whereas shearwaters, puffins, and small gulls 

 (Xema sp., Rissa spp.) overlap almost entirely 

 in prey categories and even prey species, the 

 gulls can capture these organisms only at the 

 surface; the shearwaters capture them at shal- 

 low depths; and the puffins capture them at 

 much deeper depths. Direct field observations 

 of this phenomenon are few but Gould (1971) 

 and Sealy (1973a) compared the diets of birds 

 feeding in mixed-species flocks. An example of 

 how even finer divergence in feeding methods 



Table 17. Number of species feeding at different trophic levels within seabird communities and 

 habitats of the northeastern North Pacific Ocean and Bering Sea. A single species can be rep- 

 resented in more than one level. (Trophic level I = vegetarian, II = secondary carnivore, III 

 = tertiary carnivore, IV = upper level carnivore, Sc = scavenger [II-IV].) 



a Proportion based on the arbitrary assumption that half (5) of the 11 species in question catch and eat 

 birds at sea. 



