^8 SEA-BIRDS 



Southern, who has carefully studied the problem of the differential 

 distribution of the bridled guillemot, thinks that its 'bridle' is probably 

 controlled by a single Mendelian factor, which appears to control also 

 a slight difference in the skull structure and the shape of the tail- 

 feathers. He organised counts of the percentage of bridled guillemots 

 throughout Britain in the years round 1939 and again in those round 

 1949; and he has also collected as much evidence as he could from the 

 rest of the guillemot's range. Two main conclusions are apparent: 

 first, the percentage of bridled birds increases from SSE to NNW 

 (with a reversal in Iceland) ; and secondly the percentage is not always 

 constant at any one place — there are signs of trends towards increase 

 or decrease, and of shifts, or drifts, of the balance. Possibly the 

 possession of a bridle gives a guillemot an advantage over other guille- 

 mots in some environments, and a disadvantage in others, though we 

 do not know why: the alternative is that possession of the bridle is 

 the result of an advantageous mutation that is spreading through the 

 population; which is unlikely to be the case on the evidence, though 

 Southern has been careful to show that the possibility still exists. 

 There is no indication that bridled guillemots prefer to mate with 

 each other rather than with unbridled guillemots; mating in a 

 mixed colony appears to be completely, or almost completely, at 

 random. 



Southern shows that the percentage of bridled birds marches 

 fairly closely with humidity and cloudiness; but, as he points out, 

 many other factors may be involved. The changes between r.1939 

 and ^.1949 may be linked with the climatic amelioration, but "might 

 very well be due to random fluctuation." The actual percentages as 

 recorded in the paper of Southern and Reeve (1941) and Southern 

 (1951), and in a few notes published by other observers, are shown on 

 the maps (Figs. 9a, 9b). The results of Southern's enquiry of 1949 

 have shown that out of the very many colonies studied in Britain 

 at only five has a significant'^ change been recorded in ten years, four 

 of which show decreases of the percentage of bridled birds and one 

 an increase. One of the decreases is at St. Kilda, where the expedition 

 of 1939 found 16.5 per cent, of the guillemots bridled and that of 

 1948 only 10.3 per cent, (one of us took part in both counts). Other 

 decreases in Britain have been significant, as at the Isle of May, 5.3 to 



*This means, here, not only mathematically significant by the ordinary X^ test 

 (not likely to occur by chance more than once in twenty times), but also based on 

 counts by observers whose reliability has been checked against other observers. 



