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.

The Manx Shearwater Puffinus puffinus is a burrow-nesting seabird which is very vulnerable to predation when on land. To minimise this, it comes ashore only at night and nests in burrows; all its major colonies are on islands which lack mammalian predators. Because of these habits, it is difficult to census, especially since in many of the colonies varying proportions of the burrows may be occupied, not by the shearwaters, but by European Rabbits Oryctolagus cuniculus and Atlantic Puffins Fratercula arctica. In recent years estimates of the number of breeding pairs have often been based on counting responses to a tape of the male call, played down the burrows. Normally only the male responds to the call, but not all males respond (Brooke 1978), so a correction factor based on the response rate must be used to arrive at an estimate for the whole population; in previous studies this response rate has been about 0.42, but has varied from study to study with consequent effects on the estimates of population size (Newton et al. 2004).

The two islands, Skomer and Skokholm, Pembrokeshire, Wales, are noted for their large populations of Manx Shearwaters. In 1998 a census of the two islands, plus the much smaller Middleholm, estimated a total population of 150,000 pairs with about 101,000 pairs on Skomer (Smith et al. 2001). With the caveat that numbers are poorly known and using the maximum figures quoted by Newton et al. (2004), some 90% of the of the World population (370,000 of 410,000 pairs) nest in the UK and Ireland, a higher proportion of the World population than is the case for any other bird species breeding in Britain and Ireland. Again using Newton et al. (2004) this means that some 40% of the UK and Irish population (150,000 of 370,000 pairs) breed on the Pembrokeshire islands. There is little information on the status of any population; on Skomer one study has attempted to measure changes in population size in a series of fixed plots each year (Taylor et al. 2011).

The whole-island census on Skomer in 1998 (Smith et al. 2001) was very labour- intensive and hence expensive because, in addition to the breeding season census, it also involved whole-island counts of burrows in winter when the vegetation, particularly Bracken Pteridium aquilinum, had died down and the burrows were more easily located. This part of the census took 600 person-hours to complete (149,507 burrows were counted). The number of shearwaters was estimated by revisiting a number of these burrows the following spring and determining the proportion that were occupied. One assumption made at the time was that burrow numbers remained substantially unchanged between the winter burrow count and the spring playback study.

Funding for a repeat census using the 1998 technique was not available, so a less labour-intensive, more cost-efficient system was developed for 2011. Also, in view of the importance of the two islands for Manx Shearwaters it was desirable to design a method that would be applicable to both Skomer and Skokholm, and which could be repeated as required. Furthermore, such a method might have wider application for the census of other burrow-nesting populations of seabirds on islands.

Dr. M. de L. Brooke kindly made the tapes used in playback. Several members of the Edward Grey Institute of Field Ornithology, together with some MSc students from the Zoology Department at Oxford University who were present on the island, completed the remaining census plots: we are grateful for their assistance at such short notice. Prof. Alan Grafen advised on calculating the estimates and Confidence Intervals and Drs Rhys Green and David Gibbons made useful comments on an early draft, while comments by Annabel Knipe, Roddy Mavor, Norman Ratcliffe and the editor improved the submitted manuscript. Costs were borne by the Countryside Council for Wales.

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