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.