The feeding and breeding biology of the sacred ibis Threskiornis aethiopicus in southern Victoria

Abstract

This study aimed to elucidate the major selective factors affecting the biology, and particularly the breeding ecology of the Sacred Ibis (Threskiornis aethiopicus) in Australia. A comparative study of the breeding ecology of the Family Plataleidae in Australia was undertaken to define the ecological niche of the Sacred Ibis. The Australian ibis and spoonbills have very similar breeding ecology to their conspecifics and congeners elsewhere in the world. However, the breeding success of Australian birds appears to be limited by food supply whereas, in African members of the Family, for example, breeding success is limited by predation. In Australia, Sacred Ibis exhibit two broad responses to their environment: some populations showed adaptations in their breeding to an unpredictable environment; and, other populations have developed complex breeding strategies that are dependent on a highly predictable set of environmental conditions. In unpredictable environments, Sacred Ibis do not breed regularly but rather breeding is closely tied to the variable environmental conditions; the ibis move away from the breeding site when the area dries out. In unpredictable environments, clutch size is adjusted to the conditions and the ibis may employ multiple brooding. Where the environment is much more predictable, Sacred Ibis breed on a very regular cycle. In these environments, ibis are highly sedentary and clutch and brood size and multiple brooding are adjusted to maximise seasonal reproductive success, that is, Sacred Ibis raise fewer offspring per brood than the apparent maximum number possible as a trade-off against raising more broods in each breeding season.

The timing and duration of the breeding attempts has had a major effect on the mating system. Sacred Ibis are essentially monogamous but one case of apparent polygyny was found. The mating system is also characterised by a high rate of promiscuous copulation and changes of mate between breeding attempts. The roles of male and female in post-fledging care of young and costs/benefits to life-time fitness are discussed. The selective factors maintaining colonial nesting are reviewed and the single most important evolutionary pressure affecting the breeding ecology of the Sacred Ibis is finding food; predation was shown to be unimportant.