Breeding ecology and group dynamics of the apostlebird

Abstract

The breeding biology and social organisation of apostlebirds (Struthidea cinerea, Corcoracidae) were studied over three breeding seasons (August 1998 – January 2001) in southern central New South Wales. Group size ranged from 3 to 17 birds (mean 7.6), and the adult population was marginally male-biased (56.7%). Most new recruits (87.4%) into established groups were philopatric young hatched the previous season. Yearlings were lighter and had shorter wings than older birds. Formation of new groups was uncommon, and usually occurred (3 of 5) when a coalition departed following nest failure. Clutch size ranged from one to eight eggs (mean 3.9), and incubation lasted 15–17 days (mean 16.1). The time from hatching to fledging of the first chick was 17–22 days (mean 18.5). Most broods hatched asynchronously, resulting in a size hierarchy among chicks that was maintained until fledging. Relative weight at fledging was correlated with subsequent mortality. Most nests (80.4%) produced at least one fledgling (range 0–5, mean 2.6). Most groups (81.6%) were double-brooded. Predation resulted in 5 of 11 failures where the cause was known. Partial brood loss was higher among earlier nests, with nestling starvation the major cause. Groups raised 1–9 fledglings annually (mean 4.2), of which 0–7 (mean 2.6) survived to the following season. Group size was a significant determinant of seasonal productivity.