TROPHIC RELATIONS OF SEABIRDS 



117 



Table 18. Continued. 



a Information on body sizes (length) is from Robbins et al. (1966). 



b lnformation on bill lengths is from Palmer (1962), Dement'ev et al. (1968), and Friedmann (1950). 



c Feeding methods are from Ashmole (1971) as adapted by Ainley (unpubl. manuscr.). 



helps to partition food resources has been pro- 

 vided by Spring (1971) in his comparison of 

 the two murres. Both species feed by diving to 

 great depths, but the thick-billed murre is 

 able to hover over the bottom and thereby is 

 better able to capture benthic organisms. 



The scavengers (generalists) offer a good 

 example of how a range of bird and bill sizes is 

 usually represented among species having 

 similar diets and feeding methods. The pro- 

 gression of oceanic scavenger sizes is graded 

 rather evenly from the black-footed albatross 

 down to the northern fulmar, to the scaled 

 petrel, to the storm-petrel. All these species 

 capture prey that occur only at or near the 

 water surface. Recently Sanger (1973) re- 

 ported appreciable numbers of glaucous- 

 winged gulls (Larus glaucescens) and herring 

 gulls (L. argentatus), noted neritic scaven- 

 gers, out in the oceanic realm of the petrel. He 

 presented limited data that suggested an 

 overlap between the diet of these gulls and 

 that of black-footed albatrosses, as noted by 

 Miller (1940). It would not be surprising if 



these gulls were as much generalists in the 

 oceanic habitat as they are in the neritic. In- 

 terestingly, their bill and body sizes fall be- 

 tween those of the albatross and the fulmar, 

 thus in theory enabling them to invade the 

 oceanic habitat without great competition. It 

 is likely that their invasion occurred during 

 historical times and is related to their habit of 

 following fishing boats from shore out to sea 

 (Sanger 1973). If so, the gulls might be assum- 

 ing from other species part of a previously un- 

 contested resource. 



Another interesting group of species that 

 shows close similarities in diet consists of the 

 piscivorous loons, grebes, and mergansers. 

 All these birds, including seven or eight 

 species, apparently feed on fish occurring on 

 or near the bottom in the inshore neritic habi- 

 tat. Again, however, an even progression in 

 size exists: yellow-billed loon (Gavia adamsii), 

 common loon (G. immer), arctic loon (G. arc- 

 tied), red-throated loon (G. stellata), western 

 grebe (Aechmophorus occidentalis), red- 

 necked grebe (Podiceps grisegena), and com- 



