A study of the Abbotsbury Mute Swans

The Mute Swan Cygnus olor population of the Fleet and Radipole Lake, Dorset, have been studied since 1968. The majority of the breeding pairs are in a colony at Abbotsbury on the Fleet. A proportion of their cygnets are reared artificially; few others survive. Regular counts show an influx of birds coming to moult and to winter; their origin is not yet known. Long-term counts show only moderate fluctuations in numbers, the peak having varied from 733 to 1,238 in the course of the study, while the number of breeding pairs has varied from 18-100 at Abbotsbury, and 15-35 at Radipole Lake. The breeding season starts later and the clutch size averages smaller than the Mute Swans elsewhere in southern Britain. Survival of adult birds is higher than elsewhere in Britain, and that of the reared cygnets can be very high. The latter show a correlation between weight and survival. Females start to breed earlier than males, but significant proportions of both sexes do not breed every year despite having achieved breeding status. Comparisons are drawn with colonially breeding swans in Denmark and with populations of territorial birds elsewhere in Britain.