The ecology and social behaviour of chowchillas, Orthonyx spaldingii

Chowchillas, Orthonyx spaldingii, are a common inhabitant of the World Heritage-listed Wet Tropics of North Queensland and are confined to the Australian tropical rainforests. They are sedentary and groups of two to five birds defend year-round all-purpose territories. They forage as a group and cooperate in territory defence but do not appear to breed cooperatively. The main focus of this study was to investigate what ecological and other factors could be involved in causing Chowchillas to live in groups but not breed cooperatively. I examined patterns of food availability, foraging strategies, territoriality, social behaviour and group structure in Chowchillas.\ud \ud Food availability over time and between sites was measured by sampling the leaf litter fauna. Food availability varied seasonally and from year-to-year but the variation was small. Food availability in patches was predictable on the basis of readily observed characteristics of the leaf litter. Chowchillas ate most types of small animals occurring in the leaf litter. They were found not to search randomly when foraging, instead choosing patches which were likely to contain more food. This strategy probably involved learning the cues which indicated patch quality.\ud \ud Several birds were fitted with radio-transmitters. Home ranges of two groups were mapped over several months and the home ranges of five neighbouring groups were mapped in one period. Home ranges were found to be stable over time and overlapped to some extent. Each group had a core area within their home range which was rarely encroached upon by other groups and this approximated the defended area. Home range size was larger in larger groups and the area per bird also increased with group size. Groups were stable over time. The reproductive rate was estimated at 0.27 fledged young per group per year and the survival rate was estimated at 86% annually. As a result, few young were produced during the two and a half years of the study and only one dispersal event was witnessed.\ud \ud Songs of groups on and near my site were recorded and analysed to examine differences between and within groups. Chowchillas were found to have song dialects and the boundaries between these dialects were sharp. Thus there is a possibility that dispersal is restricted to within dialect areas and that individuals within the same dialect area are more closely related to each other than to those in other dialect areas.\ud \ud I concluded that Chowchillas may not breed cooperatively because: (a) group members are unrelated so young birds would not increase their inclusive fitness by helping the breeders to raise offspring; and (b) young birds have the option of dispersing and possibly breeding on a nearby territory. However, group-territoriality can occur because: (a) young birds need to forage with experienced birds in order to find good food patches; and (b) older birds do not incur any costs by allowing them to do so and may also benefit by having help in defending the territory.\ud \ud The role of Chowchillas in the rainforest was also investigated. Chowchillas were found to have a major impact on the forest floor litter fauna and to turn over large quantities of leaf litter. Their activities also provide other ground-foraging animals, particularly Musky Rat-Kangaroos, with a profitable foraging niche. Thus Chowchillas are an important component of the ecology of Australia's wet tropical rainforests