Night roosts (rafts) of Great Northern Divers Gavia immer were searched for and located around the coast of Mull, western Scotland. Observations on the location, formation and size of roosts were made between late September and late May in three successive non-breeding seasons. The locations of roosts in four sea lochs remained constant between months and years, and numbers of birds involved in early and late winter was broadly similar between years, as was their behaviour. Comparisons made between day-time and evening roost counts in the same sea lochs suggest the latter are a more accurate and time-efficient method of monitoring numbers in defined areas.

The preferred coastal areas in the British Isles for wintering Great Northern Divers Gavia immer are in and around the bays and sea lochs of western Ireland, the west coast of Scotland and the Northern Isles (Parrack 1986). There have been rather few published studies on the behaviour of Great Northern Divers Gavia immer (hereafter ‘divers’) during winter. In the USA, McIntyre (1978) described how birds that defended inshore feeding territories during the day at a location in Virginia rafted together over deeper water at night, but no evidence of such rafting was found at a site in Rhode Island (Daub 1989; Ford & Gieg 1995). More recently, night rafting has been recognised among wintering divers in Morro Bay, California (Morro Coast Audubon Society website), but it was not observed during a 5-year behavioural study in southwest Norway (Byrkjedal 2011). In Britain, communal night rafting of Great Northern Divers during winter has only been reported from Shetland and the Inner Hebridean island of Skye (Suddaby 1992; McMillan 2008). I observed the phenomenon of night rafting while conducting counts of Great Northern Divers around the coast of Mull, Inner Hebrides. This paper describes this behaviour, and discusses it in terms of monitoring wintering numbers of Great Northern Divers.

I thank Irene Boston, Nick Corley, Dave O’Hara and Ruth Walker for assistance with roost counts, Bob McMillan for advice on finding roosts, and Bob McGowan and Ilka Win whose comments greatly improved the manuscript.

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