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.

Monitoring during the non-breeding season is difficult for many seabirds since individuals often spread out widely (e.g. Frederiksen et al. 2012) and into areas where they are not easily observed. Paucity of data leads to a lack of conservation management outside of the breeding season, even though the non-breeding period is an important time for survival and acquisition of body reserves for breeding (Calvert et al. 2009). Telemetry has facilitated efforts to better understand the non-breeding season of migratory seabirds; however, it is difficult to apply this method to large numbers of individuals within a population as tags are frequently lost (Fort et al. 2013). Although the timing of breeding for most seabirds is documented in detail, little is known about the proportion of the year species spend visiting breeding colonies.

The Common Guillemot (Uria aalge; hereafter Guillemot) is an abundant colonialbreeding seabird in the northern North Atlantic and North Pacific. In the northern parts of its range, the species is strictly seasonal in its attendance at the breeding colonies. At the end of the breeding season adults disperse away from the colonies and undertake their main moult of the year during which they are flightless, and return to the colonies 4–6 weeks prior to the first eggs being laid (Gaston & Jones 1998). However, at the southeastern-most colonies in both the North Pacific (e.g. Farallon Islands, California) and North Atlantic (e.g. Britain), Guillemots visit breeding sites during the late autumn and the winter (Greenwood 1972; Taylor & Reid 1981; Boekelheide et al. 1990). In extreme cases, birds can be seen ashore during the day at the breeding ledges for 10 months of the year (Harris et al. 2006).

In Britain, autumn and winter attendance of Guillemots during the day has been well documented at the Isle of May (Firth of Forth), St Abb’s Head (Berwickshire), Fowlsheugh (Kincardineshire), and Iresgoe and An Dun (Moray Firth) (Harris 1984; Mudge et al. 1987). Although attendance varies both between and within years, there is a clear pattern of daily attendance. Harris (1984) monitored the time Guillemots spent at Fowlsheugh, where the first eggs are typically laid in late April and the last chicks fledge in early August, from October 1981 to March 1982 and found that birds arrived at the colony just before dawn and typically left well before dusk. In contrast, Mudge et al. (1987) followed attendance at colonies in the Moray Firth between September and April 1983–85 and reported less frequent attendance. Again, birds were usually present at dawn, although during February and March, birds sometimes came ashore later in the day, and remained on land for less than a few hours, but occasionally were present until dark. These studies suggested that Guillemots are not present at the colonies overnight. However, the above studies used outdated Kodak ‘Analyst’ time-lapse film cameras and slow Kodachrome 40 film, so would not have been able to detect birds present overnight.

In early 2015, we monitored the attendance of Guillemots at a colony in Shetland for the three months prior to the breeding season using a more sensitive time-lapse camera setup than had been available for earlier studies. Here, we present evidence to show, for the first time, that Guillemots are often present at the breeding site throughout the night prior to the breeding season and that the likelihood of attendance at the colony increased as the breeding season approached.

We thank Glen Tyler, Kate Thompson, Alison Phillips and Helen Moncrieff for help in field, Adam Cross for statistical advice, Neville Robbins for help with camera housing construction and Roddy Mavor and Martin Heubeck for their advice on plot selection and unpublished data. We also thank the two anonymous reviewers who helped improve the presentation of the manuscript. This study was funded by Scottish Natural Heritage.

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