=Table 10=. _Diets of terns in different localities (x = major prey species)._
------------------------------------------------------------------------ CRUSTACEAN FISH ------------------------------------------------------------------------ |"Crustacean"|_Mallotus_ | |Euphausiid| |Cottid Diet | | |Amphipod| | |_Ammodytes_ Location | | | | | | larvae ------------------------------------------------------------------------ =Arctic tern= _(Sterna paradisaea)_ Pribilof Islands (Preble and McAtee 1923) x x Alaska (Bent 1921) x x x No. Atlantic (Hartley and Fisher 1936) x =Common tern= _(S. hirundo)_ E. No. America (Bent 1921) x x ------------------------------------------------------------------------
The oceanic habitat includes waters of the photic zone overlying the deep ocean and continental 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 include both inshore and offshore neritic as well as some oceanic habitat. The boundary between 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 _(Aptenodytes 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 common and usual members of the seabird community 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 phytoplankton (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 exception at times might be the least auklet _(Aethia pusilla)_ when it feeds on small copepods (see Bédard 1969_b_).
