Social organisation and foraging ecology of the cooperatively breeding Chestnut-crowned Babbler (Pomatostomus ruficeps) Dean J. Portelli A , Holly Barclay B , Deborah J. F. Russell C , Simon C. Griffith D and Andrew F. Russell C,D,E A Department of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia. B Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK. C Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK. D Centre for the Integrative Study of Animal Behaviour, Macquarie University, Sydney, NSW 2109, Australia. E Corresponding author. Email: a.f.russell@sheffield.ac.uk Abstract. An individual’s fitness is assumed to be maximised through early dispersal and independent breeding. However, offspring across a diversity of taxonomic groups delay dispersal and remain with at least one of their parents after reaching sexual maturity. Delayed dispersal and resulting family living are expected to arise when constraints exist on independent reproduction and where offspring benefit by remaining philopatric. A first step to elucidating the nature of such constraints and benefits for a given species is to have an understanding of the social organisation and habitat preferences of a species. The present study examined the social organisation, foraging preferences and characteristics of preferred foraging areas during a breeding season in the cooperatively breeding Chestnut-crowned Babbler (Pomatostomus ruficeps). During the study, groups of Babblers bred plurally in breeding units of two to 13 adults that occupied non-exclusive home-ranges averaging 38 ha, with larger groups occupying larger ranges. Babblers spent most of the day foraging, mostly on the ground, and preferred to forage within drainage zones. The preference for such zones probably arose because they offered both greater vegetative cover from aerial predators and biomasses of potential prey. These findings lead to the prediction that the availability of drainage zones within a group’s range will influence offspring dispersal decisions in Chestnut-crowned Babblers at the site studied. Additional keywords: benefits of philopatry, ecological constraints, home-range, plural breeding. Introduction Dispersing to find a breeding opportunity soon after reaching independence is expected to enhance an individual’s fitness (Williams 1966). Nevertheless, extended families, wherein post-independent individuals delay dispersal and breeding and remain with their parents, occur in a diversity of taxonomic groups, including birds (Brown 1987; Koenig et al. 1992). While the incidence of family living has a strong phylogenetic component (Cockburn 2003), variation in the occurrence of family living within taxonomic groups suggests additional demographic or ecological influences (Koenig et al. 1992; Arnold and Owens 1999; Hatchwell and Komdeur 2000; Du Plessis 2004; Ekman et al. 2004). The aim currently is to determine the demographic and ecological factors that select for delayed dispersal and family living in animals (Ekman et al. 2004). Two approaches have led to significant advances in our understanding of the factors that promote delayed dispersal in birds. Using an experimental approach, Du Plessis (1992) and Walters et al. (1992) demonstrated, through providing roosting or nesting holes, that a lack of tree-holes constrains offspring dispersal in Green Woodhoopoes (Phoenicerus purpureus) and Red-cockaded Woodpeckers (Picoides borealis). Komdeur et al. (1995) found that the availability of food on natal territories influenced dispersal decisions of offspring following translocation experiments in Seychelles Warbler (Acrocephalus seychellensis). Furthermore, Dickinson and McGowan (2005) showed that removal of important sources of food led to offspring dispersal in Western Bluebirds (Sialia mexicana). While many broad-scale comparative approaches have failed to identify common ecological correlates of family living (Hatchwell and Komdeur 2000), using a more fine-scale within-family approach (Russell 2004), Rubenstein and Lovette (2007) found across 45 species of African starlings (family Sturnidae) that cooperative breeding is more common in species occupying areas of highly unpredictable rainfall. The reason provided was that in such environments helpers are required for successful breeding when conditions are poor and allow breeders to take advantage of conditions when they are favourable. Consequently, evidence CSIRO PUBLISHING www.publish.csiro.au/journals/emu Emu, 2009, 109, 153–162 Ó Royal Australasian Ornithologists Union 2009 10.1071/MU08065 0158-4197/09/020153