Abstract

The Black-necked Swan (Cygnus melanocoryphus) is the only species of the genus inhabiting South America. Although most of the members have been extensively studied, the reproductive biology of C. melanocoryphus, has not been described yet. In this study, we evaluate the starting and extension of the breeding season and clutch size number, using data collected at Carlos Anwandter Sanctuary (CAS), Valdivia, Chile, corresponding to 18 reproductive seasons. We also studied male and female behaviour during incubation period. We monitored time budget and the distance relative to the nest of 6 breeding pairs during one reproductive season. Nesting period can extend from June to January and is influenced by the population number and weather conditions. Clutch size is constant across seasons with a mode of three eggs (mean of 3.13 ± 0.017 eggs; n = 5897). Both pair members cooperate in duties, such as vigilance and nest maintenance; however, incubation is an exclusive activity of females while the active defense of the nest is performed by males. Therefore, we propose that both extension of breeding season and number of breeding pairs are associated with density-dependent processes while clutch size seems to be highly conservative. Finally, we observed a clear separation in parental tasks for each sex during incubation period.