Il8 SEA-BIRDS 



more near marshy places, though it will nest on sand. At most British 

 colonies where there are common and Sandwich terns together the 

 two species usually nest apart, though it is difficult to detect any 

 particular preference in the type of ground favoured. 



So far among closely related sea-birds there have been few studies 

 of difference in feeding-habits, though much is now known in the 

 North Atlantic about the difference in their nesting-site preferences. 

 Such pairs of closely related species as have been carefully analysed 

 show that the principle of Gause is in general correct; they do not 

 have the same ecology and there are marked differences in their food- 

 patterns. D. Lack (1945) points out that if one was to rely on the 

 information given in general works on British birds and by W. E. 

 Collinge (1924-27) one might come to the conclusion that the cormorant 

 and the shag eat mainly the same type of food. Fortunately the econ- 

 omic issues involved (because these species might be thought to 

 compete with fishermen) led to G. A. Steven (1933) carefully investigat- 

 ing the food of these two species round the shores of Cornwall. Steven's 

 investigations made it abundantly clear that the difference was very 

 great. For instance the shag ate more sand-eels than any other prey; 

 the cormorant none. The cormorant ate more fiat-fish than any other 

 prey; the shag a very small amount. The shag ate fairly large numbers 

 of sprats and sardines and other small fish, whereas the cormorant 

 appeared only to swallow them by chance. The cormorant, on the 

 other hand, ate great quantities of prawns and shrimps, which the 

 shag appears to swallow only by chance. In general the cormorant 

 fed primarily on animals which live on or close to the bottom of the 

 sea, while the shag fed mainly on free-swimming forms. 



Later Lumsden and Haddow (1946) examined the food of the shag 

 in the Clyde area and came to similar conclusions about its preferences 

 for sand-eels, small clupeoids and sprats, sardines etc., though the 

 Clyde birds preferred sand-eels to clupeoids. 



A rather wider investigation was made by C. H. Hartley and 

 Fisher in Spitsbergen in 1933. In a fjord in the centre of Spitsbergen 

 they studied the food of the many sea-birds which were feeding in a 

 special food-rich zone at the face of a glacier running into the sea. 

 In spite of the super-abundance of one particular crustacean, Thysa- 

 noessa inermis, which all but one of the ten sea-birds present ate in 

 large quantities, the food-pattern of each proved to be distinct. Thus, 

 besides Thysanoessa the kittiwake seemed particularly fond of a crusta- 

 acean Euthemisto libellula; the fulmar of offal; the arctic tern of a 



