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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 on three Welsh Islands: Skomer, Skokholm and Middleholm. The census was made by counting 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 incubation period and applying a correction for the response rate (the proportion of males which respond). The response rate was determined by playing the tape-recordings down a set of burrows in which eggs had been laid and where the incubating birds could be examined. Knowing the total number of burrows, the number of burrows sampled, the number of responses and the response 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 probably the largest colony of Manx Shearwaters in the world; the estimates for Skokholm and Middleholm were 46000 and 3000 respectively. These figures are compared with earlier censuses undertaken using traditional methods.