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).
[Illustration: Fig. 2. Schematic food web of the parakeet auklet in the eastern Bering Sea (based on Bédard 1969_a_ and Dunbar 1946). Arrow sizes indicate relative importance of prey and Roman numerals refer to prey sizes (see text).]
Table 11. _Use of food resources by seabirds in the Bering Sea coastal domain._ Information is from Tables 1-10. (Trophic level I = plants, II = secondary carnivore, III = tertiary carnivore, IV = upper level carnivore [on birds only in this table], Sc = scavenger on carrion, offal, or detritus [II-IV]; x = major food in diet, o = minor food, * = incidental food, ? = probable food.)
----------------------------------------------------------------------------- Habitat, bird trophic levels (I-IV. Sc), and food categories ----------------------------------------------------------------------------- Oceanic and offshore neritic Inshore neritic Inshore neritic ----------------------------------------------------------------------------- Key: II II III A. Crustacean J. Crustacean, midwater R. Fish, midwater B. Polychaete K. Crustacean, benthic S. Fish, benthic C. Coelenterate L. Coelenterate T. Cephalopod D. Fish/squid eggs & larvae M. Echinoderm IV. U. Birds III N. Mollusc Sc. V. Carrion/offal/ E. Fish Q. Fish/squid eggs & larvae detritus F. Cephalopod IV G. Birds Sc H. Carrion/offal/detritus I I. Plant