Pairing and breeding of mute swans

Abstract

During seven years, 1961 to 1967, the population of Mute Swans Cygnus olor in an area of 550 square miles in the Midlands has been studied intensively by catching and ringing. The present paper deals only with the paired segment of the population. The number of pairs has remained fairly constant, around 95, but there is some evidence of a decrease since 1965. There were annual changes in the proportion of breeding pairs and in the recruitment of new pairs. The swans usually first paired at two and bred at three years old, but there were variations, and the age composition of pairs also had effects on their success. Mortality rates, based on the number of birds found dead, or missing from the area, were around 20% for breeding pairs, 35% for non-breeding pairs. There were other variations in mortality rates related to breeding success as well as to weather. Eighty-five per cent of paired breeders retained the same mate from year to year, if it was still alive. Non-breeding pairs were less stable. Divorce rate was 3% for breeding birds and 9% for non-breeders; some complicated pair histories are described. Movements of paired birds were very circumscribed. Only 58% of nests hatched successfully, human predation or interference being the major cause of loss. Most broods survived in part, with an average size at fledging of 3.5, representing 2.0 young per breeding pair. The production of young from the area varied widely between years, from 116 to 170. The relative attractiveness of different habitats was investigated. Despite the stability of the paired population, much apparently suitable habitat remained unexploited each year.