Aspects of the Nesting Biology of the Magpie Goose Anseranas semipalmata: Incubation Period, Hatching Synchrony and Patterns of Nest Attendance and Defence

Abstract

The period between clutch initiation and departure of family groups from successful Magpie Goose nests ranged from 31 to 33 d. Incubation period was 25 days and family groups remained at the nest up to 4 d after the first egg hatched. Hatching was poorly synchronised relative to many other waterfowl, probably due to variable but often elevated egg temperatures (up to 36.6°C) during the 5-10 d laying period. During daytime visits by observers in a motorised airboat, many birds performed displays of various kinds including elaborate decoys, threat postures involving approach to the observer, and less frequently attacks with bill, wings and feet. Males were more often encountered at nests than females. Males more often engaged in aggressive, presumably higher-risk behaviours. Responses involving close approach to the observer were more frequently observed during later phases of incubation, when birds that did not display also permitted closer approach and retreated shorter distances. Females were more likely to be encountered at the nest during the laying period and again around the time of hatching. Consistent with the patterns of daytime encounter, continuous remotely logged (radio telemetry) observations of nest attendance of two birds suggest that males incubate principally during daylight and females nocturnally, but this temporal segregation weakens near hatching. No significant association was found between frequency of nest protecting behaviours (decoy, threat or attack) among attending birds and ultimate nest success. A number of observations at nests attended by marked birds indicated that some nests were incubated or defended by more than one male, confirming observations in semi-captive flocks.