Night rafting behaviour in Great Northern Divers Gavia immer and its potential use in monitoring wintering numbers

Abstract

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.