Structure and dynamics of a Mute Swan population

Abstract

In 1971-1974, the Uists and Benbecula contained up to about 750 full-grown Mute Swans Cygnus olor in August, about 500-550 in spring and autumn, and about 400 in December. These figures were remarkably similar each year. On average, about 20-22% of the spring stock produced were young. Non-breeders were numerous, and were found mainly in flocks on saline lochs. The numbers of non-breeders were augmented each July-September by about 200 birds which came to the islands to moult, mostly on Loch Bee. Breeding and non-breeding swans occurred on three main types of loch, eutrophic (machair), mesotrophic (other farm land) and saline (connected with the sea). They were absent from the numerous oligotrophic lochs. Loch Bee in S. Uist and the Loch an Duin complex in N. Uist were especially important saline areas for flocked swans. Constancy in the numbers of dispersed pairs was attributable to territorial behaviour, but constant numbers of flocked birds in given months in different years must be due to other behaviour, possibly involving some other adjustment to food. Among pairs that produced young, the ratio of young to old was similar on all three types of loch. Overall, the ratio of young produced per successful breeding pair varied in different years between 1.02-1.074 per breeding adult.