The island of Mousa, Shetland, is designated as part of the European Natura 2000 reserve network as a Special Protection Area (SPA) for breeding European Storm-petrels Hydrobates pelagicus. In 1996 the population was censused using playback methods and reported as 6,800 (95% CI 4,800–8,800) apparently occupied sites (AOS), c. 26% of the UK breeding population. Re-examination of the 1996 data highlighted some calculation inaccuracies, resulting in a reduced population estimate for 1996 of 5,410 AOS (95% CI 3,932–7,022). Resurvey of the colony in 2008 using identical methods revealed a 118% increase to 11,781 AOS (95% CI 8,100–17,728). A sample of c. 90 nest sites in natural crevices has been monitored annually since 1990 to determine breeding productivity. A deterministic population model, incorporating observed annual productivity estimates, annual survival rates and age of first breeding, also indicates a population increase of similar magnitude (95%) between 1996 and 2008. The similarity in trend estimates from the two independent approaches is perhaps surprising, given the large potential sources of error associated with each, and suggests that both the playback survey method and productivity monitoring from a single annual visit provide reliable demographic estimates. The large increase in population size of European Storm- petrels contrasts starkly with the trends of many other seabird species in the region, and likely results from aspects of Procellariiform foraging and reproductive strategies that confer resilience to unpredictable and scarce food supplies.

Due to their vulnerability to predation, nearly all species of storm-petrels nest on remote islands, visit the colony only under the cover of darkness and nest out of sight in deep crevices. These characteristics severely hinder the estimation of population size, and prior to the Seabird 2000 census (Mitchell et al. 2004) few reliable quantitative survey data were available for any British or Irish colonies. Earlier estimates (e.g. Lloyd et al. 1991) had been based largely on subjective judgement and, in preparation for the Seabird 2000 census, considerable research effort was invested in the development of a reliable, quantitative survey method for storm-petrels. This resulted in the establishment of the diurnal playback technique (Ratcliffe et al. 1998), where responses are elicited from birds occupying nest holes during daylight, in response to playback of recordings of burrow calls. A colony- and year-specific correction factor is required to account for variation in the proportion of birds that respond. Since no previous quantitative estimates of population size existed for most storm-petrel colonies, data collected during Seabird 2000 could not be used to derive population trends. Here we report a playback re-survey, following a 12-year interval, of the largest European Storm-petrel Hydrobates pelagicus colony in the UK, and provide one of the first quantitative estimates of population trend for the species in UK. In addition, we derive a second, independent estimate of population trend from a simple population model parameterised with empirical estimates of annual productivity and age of first breeding from the colony. The extent of similarity between the two estimates of population trend was used to assess the reliability of the playback survey method, which has recently been questioned (e.g. Brown 2006).

We thank Tom Jamieson for provision of boat access to Mousa for the 2008 playback survey and for the ongoing provision of transport to the island for annual nest monitoring. This work was funded by the Royal Society for the Protection of Birds and Scottish Natural Heritage. We thank Iain Nicol, Robert Smith and Ewan Ellis who assisted with the calibration study under the Nuffield School Bursary scheme and Mike Hounsome and Hugh Insley for helpful comments on an earlier draft of the manuscript.

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