WHAT CONTROLS THE NUMBERS OF SEA-BIRDS? I23 



plenty of room for more at most of the North Atlantic sea-bird 

 rookeries. 



However, before we finally admit the primary importance of food 

 we must give attention to the question of climatic amelioration in the 

 North Atlantic. In the present century there has been a northward 

 advance of very many species, which seems to be fundamentally due 

 to the effect of the improved weather on the plant life on both land 

 and sea; the birds which have moved north have presumably done so 

 because the most important elements in their food-supply have moved 

 north. These elements are, of course, either the plants themselves 

 (which include, of course, sea-diatoms) or other animals which depend 

 on plants and which stand between plants and birds. The little auk 

 (see p. 60) is becoming extinct in Iceland as a breeding species because 

 the edge of the polar pack-ice on which it seeks its main food is moving 

 annually farther and farther away, carrying with it the particular 

 plankton it is adapted to eat. This is an effect of the climate, but it is felt 

 not directly by the little auk but only through its food. Indeed, there 

 are only two ways in which climate could directly affect sea-birds, and 

 neither of them appear to us to be fundamental in the long run. 



Every now and then a cataclysm of nature may ^^pset the normal 

 migrations of birds and kill large numbers of them. Some sea-birds 

 are more vulnerable to storms and hurricanes, or sudden changes in 

 temperature, than others. One of the most vulnerable to storms is the 

 little auk, which, under certain irregularities of the weather, seems 

 doomed to "wrecks" or crashes on the coasts of north-west Europe 

 or eastern North America. Many hurricanes originate at irregular 

 intervals in the Caribbean area and occasionally farther up the sea- 

 board to New England. When these take place there is a great deal of 

 loss of sea-bird life. It has been adequately described by Murphy 

 (1936). While the immediate losses of life are sometimes spectacular 

 it seems that, in the long run, they have no fundamental effect on the 

 population of birds, though some species are more vulnerable than 

 others. 



It is also possible that birds may be directly prevented from breed- 

 ing by the weather. It is now clear from the observations of William 

 Rowan (1918), Frances Pitt (1929) and others, that abnormal weather 

 conditions may suddenly stop breeding activity even if birds have 

 arrived at the state of their glands compatible with reproduction. This 

 subject has recently been studied by A. J. Marshall (1949 and unpub- 

 lished) who examined the effect of the hard winter of 1946-47 on several 



