Sea-watching at Holme Bird Observatory, Norfolk, 2005 to 2015

ABSTRACT Timed sea-watches were carried out at Holme Bird Observatory, Norfolk, UK, between May 2005 and December 2015. The rate of passage of 20 species was calculated as the number of birds passing the observation point per 10 hours. Analysis of the rate of passage by month enabled the species to be grouped into autumn passage migrants, summer visitors, winter visitors and species present throughout the year. There was no obvious spring passage of seabirds at the site. Wind strength and direction proved to be the most important environmental variables affecting seabird presence. Six autumn passage migrants (Manx Shearwater Puffinus puffinus, Gannet Morus bassanus, Great Skua Stercorarius skua, Arctic Skua Stercorarius parasiticus, Little Auk Alle alle and Kittiwake Rissa tridactyla) were most numerous when the wind was strong and northerly, conditions which favoured birds in the North Sea being blown towards the Norfolk coast. Two summer visitors (Sandwich Tern Thalasseus sandvicensis and Common Tern Sterna hirundo) were most numerous in weak winds and at low tide, conditions which may have favoured short feeding movements. Some species (e.g. Teal Anas crecca and Fulmar Fulmarus glacialis) were more numerous in the morning, but others (e.g. Kittiwake and Little Auk) were more numerous in the afternoon. Systematic observation over a 10-year period provided an enhanced understanding of the seasonal variation of different species seen offshore, and of the environmental factors that influence their movements.