TROPHIC RELATIONS OF SEABIRDS 



109 



Table 10. Diets of terns in different localities (x = major prey species). 



Location 



CRUSTACEAN 



FISH 



Ammodytes 

 "Crustacean" Euphausiid Amphipod Mallotus Cottid larvae 



Arctic tern (Sterna 

 paradisaea) 

 Pribilof Islands (Preble 



and McAtee 1923) 

 Alaska (Bent 1921) 

 No. Atlantic (Hartley 



and Fisher 1936) 

 Common tern (S. hirundo) 

 E. No. America (Bent 1921) 



as some oceanic habitat. The boundary be- 

 tween the inshore and offshore neritic has yet 

 to be defined in terms of bird life, but it lies at 

 that line beyond which the bottom is too deep 

 for a diving bird to exploit. A depth contour 

 thus defines the boundary. In the antarctic 

 South Pacific, emperor penguins (Apteno- 

 dytes fosteri) dive to depths of 275 m, but so 

 far as is known, no comparable bird exists in 

 the North Pacific. Some marine ducks and 

 loons reportedly dive to 50-60 m (Kooyman 

 1974). The inshore-offshore neritic boundary 

 for seabirds may lie near the 70-m depth 

 contour. 



Food resource partitioning by seabirds in 

 the five oceanographic domains are shown in 

 Tables 11-15. Within each domain, the com- 

 mon and usual members of the seabird com- 

 munity are listed, and the major and minor 

 categories in each of their diets are shown (on 

 the basis of available literature, Tables 1-10). 

 The categories are grouped further, and 

 rather tenuously, according to the trophic 

 level at which a bird is presumably feeding: I 

 = herbivore, II = secondary carnivore, III = 

 tertiary carnivore, IV = final carnivore, and 

 Sc = scavengers (carnivorous) feeding at 

 many levels. Birds at level I feed on large 

 algae and seed plants and are not directly part 

 of the same food webs involving other species. 

 These food webs originate with phytoplank- 

 ton (Fig. 2). So far as is known, no bird feeds 

 on phytoplankton and few, if any, feed on 

 microzooplankton; hence birds do not 

 generally feed as primary carnivores. An ex- 

 ception at times might be the least auklet 

 (Aethia pusilla) when it feeds on small cope- 

 pods (see Bedard 19696). 



The above groupings are "tenuous" because 

 prey in each category may represent more 

 than one trophic level, and a single prey 

 species could occur at one level one day or 

 place and at another level the next day or 

 place, depending upon what it happened to be 

 eating. This is shown in Fig. 2, where the 

 parakeet auklet (Cyclorrhynchus psittaculus) 

 can occur in the food web at different levels, 

 depending both on the prey it is eating and on 

 what its prey is eating. Even without this 

 complication, many seabirds feed at more 

 than one level in the food web. For instance, 

 murres eating euphausiids would be feeding 

 at a different level than murres feeding on 

 larger fish. It might be "safer" to regard prey 

 organisms in level II as macrozooplankton, 

 prey organisms in level III as micronekton, 

 and prey organisms (seabirds themselves) in 

 level IV as macronekton (after Sverdrup et al. 

 1942). 



x x n m \ 



/' PARATHEMISTO ^\ V 

 - * LIBELLULA \ V 



*' / I - *\ lk\ V 



PHYTOPLANKTON ,' / ^9 



\ % 'i^ (Txli x)/Vt/5 L , 4 



^^^ ^^J<^CRISTATUS .* I I 



^^^ ^^^CALANUS X\/ '_ 



^C ALA NUS / >./ 

 FINMARCHtCUS 1 T - ) r 



I 

 ~~AMMODYTES H 



>^N!GAMMARID ! ~~ V MYSIDS 



RITUS^AMPHIPppS.-TTrT-.PpLY^HAETESr^r-r 



>^ ^VbAMMAKIU MT 



'.. .-.- .P^T. R !I u . s T A N'. p t! | . p PPS.T^-fP L . Y j : . H . A . E T. E . s . : 



Fig. 2. Schematic food web of the parakeet auklet in 

 the eastern Bering Sea (based on Bedard 1969a 

 and Dunbar 1946). Arrow sizes indicate relative 

 importance of prey and Roman numerals refer to 

 prey sizes (see text). 



