Ecological Studies of the Black Swan II*. Colour and Plumage Changes, Growth rates, Sexual Maturation and Timing and Frequency of Breeding

Abstract

Aspects of the biology of the black swan were obtained mainly from studies of captive birds. Details are given on colour and plumage; the distinctive juvenile and adult plumages are described; in adults iris colour differs between the sexes and is related to weight in both sexes and sexual condition in males Growth rates vary significantly according to environment. Fledging was at 108-141 days for captive cygnets but was estimated to extend to 170 days in wild birds. Young birds usually moulted into adult plumage between August and November at ages of 8-26 months. Replacement of the wing plumage requires 38-51 days. Periods of 7-36 months elapse between moults of the wing plumage, the period decreasing significantly wlth age. There is no fixed temporal relationship between breeding and moulting, though adults often moult within six months of laying, so that they are flightless during the pre-fledging period of cygnets. Breeding birds tend to moult later in the year and more frequently than birds not breeding. Birds usually attain sexual maturity and breed at ages of 18-24 or 33-36 months, within 1-2 years of attainment of adult plumage. Captive swans bred in all months of the year except December and January, with most in June-August. Frequency of laying was variable and highly dependent on the particular circumstances of captivity. Most swans commenced two or more clutches annually in at least one year of captivity. Within less than 12 months, a single female was capable of commencing eight clutches or rearing three broods.