Studies of ibises in Victoria, III: Sexual dimorphism, sex ratio, breeding season, reproductive success, mating patterns, site fidelity, movements and survival of Australian White Ibis at three locations in southern Victoria

Abstract

Wild, free-ranging Australian White Ibises Threskiornis molucca were studied between 1978 and 1984 at three breeding sites in southern Victoria: Rhyll Swamp and Coolart Lagoon (where the Ibises fed in ‘natural’ sites including wetlands, pastures and intertidal mudflats) and at Healesville Sanctuary (where they fed predominantly on food provided to captive animals and in urban fringe areas). Clutch size, breeding success, mating patterns and site fidelity of individually recognisable birds were studied over three breeding seasons at Healesville. There was frequent change of mates, and breeding pairs that stayed together over the three breeding seasons were more successful than those that changed mates. Adults had high site fidelity at all sites. Recoveries of birds banded as nestlings showed variation in dispersal and philopatry between the sites and provided rates of recruitment into the breeding population. The data presented add substantially to knowledge of breeding atnatural sites. Comparisons of population-level data with previous studies show that the breeding season was later and shorter, breeding success was lower, and there was more juvenile dispersal at natural than at ‘urban’ sites. Sexual size dimorphism is documented in bill length, which showed no overlap between males and females, and therefore allows reliable identification of sex. The novel combination of detailed study of breeding by individuals and at a population level over multiple seasons provides deeper insights into the species’ ecology than have previously been possible.