122 SEA-BIRDS 



of Other birds, notably the little auk, at all of whose breeding-places 

 in Greenland there is a dependent community of glaucous gulls. It 

 also takes fish and many crustaceans, thus overlapping with the Iceland 

 gull in the consumption of these foods which often, however, exist in 

 superabundance, notably at the faces of glaciers and near icebergs. 

 Unfortunately no stomach contents of the great blackback have been 

 investigated in Greenland, so it is not possible to make a comparison 

 between this species and the glaucous gull. It is certainly a predator 

 as well as a scavenger in Greenland, as it is in other countries. 



As far as we know nobody has ever investigated and compared 

 the food of Briinnich's and the common guillemots in the areas where 

 they overlap. No doubt there is a difference in their spectra even if it 

 is only a subtle one, as with their nest-site selection. For these two 

 very closely-related species breed on the same cliffs and the most 

 constant detectable difference in their habits is that on the whole 

 the Briinnich's guillemots tend to incubate on more sheltered ledges 

 and in cracks, thus taking up the niche occupied by the razorbill, 

 whose distributional range it touches but does not overlap. Its bill 

 is even like that of the razorbill (at least in winter form) and it may 

 occupy the same food niche: the razorbill eats rather less fish and more 

 crustaceans than the common guillemot. The rather scanty studies 

 of the food of the common guillemot indicate that more than half 

 of it is fish, about a third crustaceans and the rest marine molluscs. It 

 seems likely, from the studies in the arctic regions, that Briinnich's 

 guillemot eats a greater proportion of crustaceans, particularly plank- 

 tonic ones like Thysanoessa. 



To sum up, then, the evidence shows that Gause was right, and 

 that closely related species living in the same area do not show an 

 identical ecology. It seems logical, from this, to conclude that every 

 species of sea-bird (and, for that matter, every bird) possesses a charac- 

 teristic food-supply and that while this — its "food-spectrum" — may 

 change in the course of history (as it has done, for instance, with the 

 fulmar) in the long run the numbers of sea-birds depend on their 

 food-factors rather than on other factors in the environment, however 

 important. Probably the populations do not depend on the supply 

 of nest-sites unless this is directly threatened by man; we must record, 

 however, that in the North Atlantic, because of man, many safe 

 breeding-places were not so in the recent past and some species 

 (e.g. great auk and gannet) suffered as a result. With increasing 

 protection safe sites are once more available, and there is now 



