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Animal tracking has afforded insights into patterns of space use in numerous species and thereby informed areabased conservation planning. A crucial consideration when estimating spatial distributions from tracking data is whether the sample of tracked animals is representative of the wider population. However, it may also be important to track animals in multiple years to capture changes in distribution in response to varying environmental conditions. Using GPS-tracking data from 23 seabird species, we assessed the importance of multi-year sampling for identifying important sites for conservation during the chick-rearing period, when seabirds are most spatially constrained. We found a high degree of spatial overlap among distributions from different years in most species. Multi-year sampling often captured a significantly higher portion of reference distributions (based on all data for a population) than sampling in a single year. However, we estimated that data from a single year would on average miss only 5 % less of the full distribution of a population compared to equal-sized samples collected across three years (min: − 0.3 %, max: 17.7 %, n = 23). Our results suggest a key consideration for identifying important sites from tracking data is whether enough individuals were tracked to provide a representative estimate of the population distribution during the sampling period, rather than that tracking necessarily take place in multiple years. By providing an unprecedented multi-species perspective on annual spatial consistency, this work has relevance for the application of tracking data to informing the conservation of seabirds. info:eu-repo/semantics/publishedVersion

The Hywind Scotland Pilot Park located off the east coast of Scotland has been in operation since 2017 and comprises five floating wind turbines with a total generating capacity of 30 MW. Ornithological monitoring is required as part of the Marine Licence for the wind farm and is carried out in conjunction with a large-scale seabird monitoring and research programme in north-western Europe (SEATRACK) involving the deployment of geolocation loggers to obtain seabird location estimates throughout the winter. Of particular concern are two species of auks (common guillemot Uria aalge and razorbill Alca torda) known to be highly vulnerable to displacement and barrier effects from offshore renewable developments. As part of the Hywind Scotland’s ornithological monitoring, data on year-round distribution and movements of guillemots and razorbills at three major colonies along the east coast of Scotland (East Caithness SPA, Buchan Ness to Collieston Coast SPA and Isle of May National Nature Reserve) were collected over two years (2017-18 and 2018-19). In 2017, a total of 190 geolocation loggers were deployed at the three study colonies. During the 2018 breeding season, a total of 82 loggers were retrieved and 193 new data loggers were deployed. During the 2019 breeding season a total of 102 loggers were retrieved, including several deployed in the first year. We obtained reasonable sample sizes in all cases (range 11-28) except razorbills at Buchan Ness to Collieston Coast, where only five loggers were retrieved each year. The geolocation data were processed using a probabilistic method to obtain two locations per day for each bird throughout the non-breeding period. The geolocation data were analysed to determine utilisation distributions for each species at each colony in 2017-18 and 2018-19. Overlap in distributions was quantified and minimum adequate sample size of tracked birds was examined. For both species, the distribution of birds from the three study colonies was similar at a broad spatial scale, with key wintering areas located around the colonies and in the central and southern parts of the North Sea. In guillemots, among-colony spatial similarity in kernel densities was generally high throughout the non-breeding period. In razorbills, spatial similarity among colonies was highest in the post-breeding period and lowest in late winter. Our results indicate that the three populations of guillemots and razorbills had a similar overall non-breeding distribution, with extensive use of the central and southern North Sea and areas around the breeding colonies, and this was broadly consistent between years. However, in both species there were important differences among colonies in the location of hotspots during the non-breeding period. The data provide important insights into the year-round space use of these two key species at three major colonies on the east coast of Scotland and their potential interaction with offshore renewable developments.