Interspecific variation in non-breeding aggregation: a multi-colony tracking study of two sympatric seabirds

Abstract

**Abstract:** Migration is a widespread strategy for escaping unfavourable conditions during winter, but the extent to which populations that segregate during the breeding season aggregate during the non-breeding season is poorly understood. Whilst segregation may be associated with higher overall likelihood of overlap with threats, but fewer populations affected, aggregation may result in a lower probability of exposure to threats, but higher overall severity. We investigated non-breeding distributions and extent of population aggregation in two sympatrically breeding auks. We deployed geolocators-immersion loggers on common guillemots Uria aalge and razorbills Alca torda at eleven colonies around the north of the UK and tracked their movements across two non-breeding seasons (2017-18; 2018-19). Using 290 guillemot and 135 razorbill tracks, we mapped population distributions of each species and compared population aggregation during key periods of the non-breeding season: post-breeding moult and mid-winter, observing clear interspecific differences. Razorbills were largely distributed in the North Sea, whereas guillemot distributions were spread throughout Scottish coastal waters and the North, Norwegian and Barents Seas. We found high levels of aggregation in razorbills and a strong tendency for colony-specific distributions in guillemots. Therefore, razorbills are predicted to have a lower likelihood of exposure to marine threats, but more severe potential impact due to the larger number of colonies affected. This interspecific difference may result in divergent population trajectories, despite the species sharing protection at their breeding sites. We highlight the importance of taking whole-year distribution into account in spatial planning to adequately protect migratory species. **Authors:** Lila Buckingham¹, Maria Bogdanova¹, Jonathan Green², Bob Furness³, Francis Daunt¹ ¹UK Centre for Ecology and Hydrology, ²University of Liverpool, ³MacArthur Green