| Summary: | The aims of the study were to examine some population parameters of the Common Guillemot Uria aalge on Skomer Island, Wales, where a decline in numbers has occurred, and to examine a number of different aspects of the birds' breeding biology. The Common Guillemot is one of the most numerous seabirds in the northern hemisphere and breeds at densities unparalleled by any other bird species. Chapter 2 describes some Guillemot population parameters on Skomer. Between 1934 and 1975 a 62% decrease in numbers took place. The population may have stabilized during the course of this study and the population in 1975 was estimated at about 2,200 pairs. Breeding success over three years averaged 70%, adult survival was 93% in 1974 and 88% in 1975. The age at which Common Guillemots start to breed is not known, but is assumed to be similar to other auks, i.e. about 5 years. Survival rates of birds up to five years derived from recoveries of ringed birds varied between 27-41% in different areas. High adult mortality, as in 1975, caused by chronic oil pollution may be limiting the Skomer population at present. Chapter 3 investigates the adaptive nature of high density nesting. Breeding success was highest among those Guillemots nesting at high densities on broad ledges, and was lowest among those on broad ledges at low densities. Breeding success was related to the synchrony of laying; dense groups had a shorter spread of laying than sparse groups, and a short spread of laying minimized the number of birds at the beginning and end of the season whose breeding was out of phase with the rest. Birds which were out of phase suffered higher losses of eggs and chicks, mainly through gull predation. Photographs of Skomer Guillemot colonies taken in 1934 show that at that time most available ledge-space was occupied and birds bred on both broad and narrow ledges. In 1973-1975 relatively few birds nested on broad ledges, most were on narrow ledges at Medium densities. It is suggested that as the population declined the density of birds ...
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