Male Incubation and its Effect on Reproductive Success in the Black Swan,Cygnus atratus

Male incubation is very unusual among Anatidae. In the black swan, males spend more time incubating the clutch than females do. We investigated the incubation patterns of males and females in a breeding colony in Vienna over a one-year period. Breeding occurred year round, with a peak in winter. Inattentive periods during incubation were shorter in winter than at other times of the year, probably because the heat loss of eggs is then higher. We tested the importance of the costs of incubation by prolonging the breeding period experimentally. Eggs were exchanged for dummies, which were readily accepted and incubated for up to 342 days. The higher the incubation effort of females, measured as the product of their proportion of incubation and the total length of the incubation phase, the longer was the interval between the end of incubation and laying of a subsequent clutch. This suggests that the incubation effort of females is limiting the reproductive output of a pair. By taking charge of the greater part of incubation, males may increase the productivity of their female partners and thereby their own reproductive success. This amplification of female fecundity is a factor favouring a high share in male incubation which is discussed in polyandrous species (e.g. Jenni 1974) but generally overlooked in monogamous systems. The ultimate cause for male incubation in black swans may lie in the combination of a monogamous mating system and the capability of this species to breed whenever the environmental conditions turn favourable, at any time of the year.