Breeding success of, & habitat use by, eider ducks

Abstract

The study covers several facets of the ecology of the Eider ducks of the Northumberland coast between Hadston Carrs and Craster between 9 May and 1 August 1990. Special attention was given to the breeding success of the Coquet island population. Nesting success is high, in excess of 90%, however fledging success is low (<6%) due to high duckling mortality. Starvation and large gull predation are believed to be the media responsible. The uneven distribution of ducklings along the coast is due to differing food availabilty and the ability of ducklings to take different sized prey items as they mature. It was found that both duckling group size and the number of ducklings per attendant female fell as the ducklings aged, reflecting the over-riding importance of duckling mortality in determining these statistics. Significant relationships were also found between duckling activity and duckling age; duckling activity and sea surface roughness; and duckling activity and time relative to low tide. Adult birds are shown to have a feeding pattern determined by the tidal cycle (feeding was not related to the time of day).The spatial variation in adult bird numbers reflects differing distance from the Coquet Island breeding centre and the relative availability of suitable feeding habitat. It is concluded that the adult birds attending young in Amble harbour are in a sub-optimal feeding environment. Fluctuations in overall population and sex ratio in the study area, are due primarily to the female's role as egg incubator. The differences in the pattern of fluctuation between the northern and southern halves of the study area are a reflection of distance from the breeding centre on Coquet Island. An overall, non-breeding season, sex ratio of 1.17 was found.