A census of the Manx Shearwater Puffinus puffinus on Skomer, Skokholm and Middleholm, west Wales

In 1997/98 a census was made of the Manx Shearwater populations oil three W elsh Islands: Skomer, Skokholm and Middleholm. The census was made by count ing all the burrows in the winter, playing a tape-recording of a male call (to which only males respond) down a sample of the burrows during the incub ation period and applying a correction for the response rate (the proportio n of males which respond). The response rate was determined by playing the tape-recordings dozen a set of burrows in which eggs had been laid and wher e the incubating birds could be examined. Knowing the total number of barro ws, the number of bro-rows sampled, the number of responses and the respons e rate enabled us to make an estimate of the population size on each island . Skomer, with an estimated population of some 102000 breeding pairs is pro bably the largest colony of Manx Shearwaters ill the world; the estimates f or, Skokholm and Middleholm were 46000 arid 3000 respectively. These figure s are compared with earlier censuses undertaken using traditional methods.