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Skomer Island, Pembrokeshire, Wales is believed to have one of the largest colonies of Manx Shearwaters Puffinus puffinus in the World. In 1998 a census was made of the whole island, and the adjacent islands of Skokholm and Middleholm, in order to try to establish the size of the breeding population; the Skomer population was estimated to be just over 101,000 breeding pairs. A second census was carried out in 2011. First, a set of study burrows was opened and a tape of the male call (normally only males respond to these) was played down each burrow several times during the course of incubation in order to establish the male response rate. Then the same tape was played down all the burrows in each of 288 randomly selected plots across the island and the number of responses recorded. Extrapolating responses from census plots to the whole island yielded an estimate of 125,112 (CI 16,445) responses. Adjusting this figure to take account of the response rate yielded an estimate of 316,070 (SE 41,767) breeding pairs. This figure is greatly in excess of the estimate made just 13 years earlier. Possible reasons for this are discussed.