Nocturnal colony attendance by Common Guillemots Uria aalge at colony in Shetland during the pre-breeding season

Abstract

Time-lapse photography was used to describe daily and seasonal trends in attendance by Common Guillemot Uria aalge at a colony in Shetland prior to the breeding season, including detection of nocturnal presence. A camera took a photo every 30 minutes from 30 January until 21 April 2015. A total of 3,435 photos were analysed, of which 3,232 photos allowed birds to either be accurately counted (2,552) or estimated (680) within a representative plot. High quality moonlit shots showed that large numbers of Common Guillemots were present ashore at Sumburgh throughout the night, while manipulated non-moonlight night photos revealed attendance at the colony, even when counting was not possible. Clear cycles of attendance at the colony were apparent with day-time peaks of 90–120 birds occurring on average every 7 days over the study period. Pre-breeding attendance is described, as is the nocturnal presence of Common Guillemots in the three months prior to breeding.