Nesting behaviour of male and female Whistling Swans and implications of male incubation

Abstract

Whistling Swan Cygnus c. columbianus nesting behaviour was studied in the Colville River Delta, Alaska, from 1981 to 1983, and in captivity. Study objectives were to: 1. quantify male and female nesting behaviour, and identify factors influencing involvement of each sex in incubation, 2. compare male and female ability to heat eggs, and 3. determine the effect of nest attendance on the daily activity budgets of male and female. At a wild nest monitored during laying, the male sat on the nest following laying of the first egg. This pattern continued, and during incubation, wild males consistently sat on the clutch when their mates departed. In captivity, however, the male frequently stood beside the nest but rarely sat. Mean female incubation constancy ranged from 60.4-83.0% and differed significantly among individuals. Nest attentiveness of wild females declined curvilinearly with day of incubation, then increased sharply at hatch. Females were more attentive on days with precipitation and as wind chill increased. Male incubation behaviour primarily appeared to be a protective response against egg predators and appeared linked to aggression and territorial defence. Females provided the eggs better quality care than males. Females pulled down, but males did not, suggesting that females possessed a brood patch. The temperature of an artificial egg rose a mean of 2.4C when the captive female sat on the nest. The egg cooled when the male sat on the nest, but 2.5 times more slowly than when he did not sit. Although the male had lesser ability to heat the egg than his mate, the insulative protection males provided appeared important. Incubation in captivity required 38 days, but 31-31 days in the wild, probably because wild males sat on the eggs during female recesses. Incubating reduced the time females could forage. However, wild females increased foraging during middle and late incubation by taking more frequent and longer recesses. Despite incubating an average 35.8% a day, a wild male foraged for a similar time per day throughout the incubation period as during prelaying. Although male incubation was not essential for successful embryo development, it appeared highly beneficial. Male nest attendance: 1. protects the eggs from predators, 2. may shorten the incubation period by at least 6-7 days, and 3. probably reduces demands placed on females during breeding. Male involvement in incubation may be favoured in anatids when high incubation constancy is essential to reproductive success, and female fasting (necessary to maintain high nest attendance) is energetically prohibitive. Fasting may be prohibitive in the short term because of constraints imposed by body size, or in the long term because it seriously lowers future female reproductive value.