Laying date and clutch size in relation to body weight in the Mute Swan Cygnus olor

Abstract

Mute Swan laying dates and clutch sizes vary with average temperature of the preceding winter, indicating more severe losses of body reserves, or unfavourable feeding conditions, and consequent delayed clutch initiation after cold winters. Weights of females, prior to laying, were lower after a cold winter than after a mild winter, but weights at hatching were similar in both years of study. Individual weights of females allow for prediction of laying date and clutch size. Whereas males hardly change in weight during the breeding season, females put on large body reserves prior to laying, and deplete their reserves almost until the level of starvation during laying and incubation. Some females were unable to fly prior to laying, because of too heavy a wing load; others could not fly at hatching, due to emaciated breast muscles. Female weight loss during egg laying varied with clutch size, and was closely related to total clutch weight. Weight loss during incubation was considerable and fairly constant. It is argued that females may use a threshold weight as a proximate factor to control clutch size. Finally, the differential tasks of the sexes in breeding are discussed against the risks of jeopardizing future reproductive success.