Playback resurvey of the UK’s largest breeding colony of European Storm-petrels Hydrobates pelagicus at Mousa, Shetland revealed that the substantial population growth achieved during previous decades has not been maintained. The estimated population size in 2015 was 10,778 apparently occupied sites (AOS) (95% confidence limits [CL] 8,857–13,207). The mean nesting density of birds breeding in natural habitat had declined substantially compared with 2008, but the area occupied by most sub-colonies had increased. Comparison with subcolonies surveyed in 2008 indicated a 12.8% decline, though the lack of precision that surrounds both surveys renders the decline statistically nonsignificant. We discuss the possible causes of the observed change in trend. The use of a new playback recording that did not include alarm calls was associated with a substantial increase in response rate compared with previous surveys. Daily response rates from nests of known occupancy status declined during the course of the fieldwork period, associated with increasing absence of adults at the nest during daylight, as chicks acquired thermoregulatory independence and adults remained at sea by day. We therefore use a date-specific calibration factor to estimate AOS density. Methods of data analysis were improved for the current survey to allow estimation of the number of AOS and associated CL for each subcolony separately. This resulted in a 51% reduction in the size of the confidence interval of the colony population estimate relative to the mean, compared with the 2008 survey. Playback surveys of burrow-nesting seabirds are typically characterised by low precision, which hinders statistically robust detection of population change, even when large declines are indicated. We recommend the adoption of a playback recording that does not include alarm calls, which may depress the frequency of responses or their detection by observers. Further, we suggest that for colonies where sub-colonies occur in discrete patches of habitat that are likely to vary in nesting density, the number of AOS should be estimated for each sub-colony separately. Adoption of these small modifications could substantially improve precision of playback surveys and hence the power to detect population change.

European Storm-petrels Hydrobates pelagicus breed on islands of the northeast Atlantic and Mediterranean with the largest populations occurring in the Faroes, Iceland, Ireland and the UK. The global population is estimated at around 1.5 million individuals (Brooke 2004). Although the current status of many colonies is not well known, the species is suspected to be in decline globally owing to predation by invasive species, pollution and development at breeding localities (Birdlife 2016). The first comprehensive, quantitative assessment of the breeding status of European Storm-petrels in Britain and Ireland was conducted during the Seabird 2000 census (Mitchell et al. 2004), which concluded that Britain and Ireland jointly held around 83,000 pairs in 95 surveyed colonies, with other unsurveyed colonies probably supporting several tens of thousands of additional pairs. Together the populations of Britain and Ireland represent 14–54% of the biogeographic population of the subspecies H. p. pelagicus. Since the Seabird 2000 census, rat eradication programmes have led to the establishment of several further colonies in the UK on islands that were formerly unsuitable (Ramsey, Lundy, St Agnes and Gugh). Notwithstanding these successes, resurvey of some of the largest UK colonies in recent years have indicated declines, (e.g. Priest island, Ross and Cromarty, West Scotland; Insley et al. 2014). The largest UK colony at the time of the Seabird 2000 census was Mousa, Shetland, holding 5,410 pairs in 1996 (estimate recalculated by Bolton et al. 2010), representing about 22% of the UK population at that time. Resurvey in 2008 indicated a substantial increase to 11,781 pairs (Bolton et al. 2010). Here we report results of a census conducted in 2015 and consider possible causes of the changes observed since 1996.

We thank Andy Simpkin, Joshua Murfitt, Louis Vandermaes and Newton Harper for help with fieldwork and the former Mousa boat ferry operators Jimmy Fullerton, Garry Sandison and Alan Pottinger for transport of personnel and equipment to the island. We thank Glen Tyler for assistance in provision of equipment and Kate Harding and Andy Stanbury for assistance in preparing figures. An earlier draft of this paper was improved by comments from Dr Allan Perkins, Dr Ana Sanz Aguilar, Helen Moncrieff, Steph Elliott and Natalie Pion.

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