WHAT CONTROLS THE NUMBERS OF SEA-BIRDS? II3 



example, about a hundred years ago the puffin was by far the most 

 numerous bird on Ailsa Craig in the Clyde. Its numbers were described 

 by many observers up to about 1900 as phenomenal. There were the 

 usual expressions about their darkening the sky. Gibson states (though 

 on what grounds we do not know) that there were probably over a 

 quarter of a million puffins in the i86o's. Unfortunately in 1889, 

 rats got ashore from a wreck. Their effect was not immediate but the 

 decrease of the puffins became noticeable between 1900 and 1910, 

 and serious by 1927 — when indeed only a few were to be found. By 

 the 1930's the puffin was practically extinct, and on yearly visits to 

 the island between 1936 and 1942 one of us never saw more than a 

 hundred birds. By 1947 Gibson estimated that there were only thirty 

 birds on the whole island. He puts the decrease down to the combined 

 effects of rats, oil and gulls, though there seems to be no real evidence 

 that any but the first factor has operated importantly. Certainly 

 puffins in the Clyde area do not seem to get oiled in the same way as 

 guillemots and razorbills, and do not appear to have been involved 

 in the appalling oiling disasters of 1937 and 1948. Later Gibson 

 records a considerable recovery for in 1950 he found 246 birds 

 occupying twelve different parts of the island; only one part of the 

 island was occupied in 1947. Rats continue to inhabit the Craig. 

 Equally disastrous histories can be related for other islands, though 

 without so many definite facts. In about 1689 a swarm of rats, which 

 must have been black rats, infested North Rona, which was then 

 inhabited by a small human community. The rats are said to have 

 eaten up all the corn on the island and caused the death of the entire 

 human population (nothing is said about sea-birds). 



On Lundy (a Norse word meaning Puffin Island) puffins once 

 bred in "incredible numbers" — at least 100,000 pairs. Rats on this 

 island have reduced the puffins, it is beheved, to their present numbers 

 of 400 pairs in 1952. On one of the Shiant islands in the Minch, there 

 is a population of brown rats which, at the time of our visit, invaded 

 the house and kept the ornithologists awake. The big puffin colony 

 in the talus slopes of the Shiants still survives, and is very large, though 

 its size is certainly not up to some of the descriptions of it in the last 

 century. How important the decrease has been, and whether it is 

 due altogether to the rats it is difficult to measure. There are fortunately 

 no rats at some of the finest sea-bird stations in Britain, such as Handa, 

 St. Kilda, Sula Sgeir, North Rona and Skomer. There are rats on 

 Foula, though it is doubtful whether they can get anywhere near the 



