Foraging Segregation of Australian Warblers (Acanthizidae) in Open Forest near Sydney, New South Wales

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Abstract
Nine species of Australian warblers (Acanthizidae) occur together in open eucalypt forests near Sydney, New South Wales. The ecological segregation of six of these species, which foraged primarily in shrub and tree vegetation and are similar sized (5-8 g), was studied during 1984. Seasonal data were obtained for five species; Weebill Smicrornis brevirostris. White-throated Warbler or Gerygone Gerygone olivacea, Striated Thornbill Acanthiza lineata, Little Thornbill A. nana and Buff-rumped Thornbill A. reguloides. The sixth species, Brown Thornbill A. pusilla, was uncommon and seasonal comparisons were not possible. As has been reported for similar species associations in the northern hemisphere, the species studied differed in prey attack behaviour, foraging heights, use of plant species and selection of foraging substrates. There were significant seasonal differences in the use of foraging resources. For example, bark was more important as a foraging substrate in autumn and winter than in spring and summer. A. reguloides foraged more on the ground outside the breeding season than when nesting. The increased abundance of flying insects during summer correlated with an increased use of hawking and snatching as prey attack behaviours by all species. Despite their differences, species broadly overlapped in their use of resources and had similar niche breadths. Overlaps in the use of resources were greatest during spring, when prey resources were probably most abundant, and least in autumn when prey decreased in abundance. The forest in which the study was conducted was dominated by two species of eucalypts. Birds selected between these species but used the same foraging behaviours in each. Despite the selection between eucalypt species, the limited array of plant species had little effect on species richness. Comparison of niche breadths for the same species at other sites where there were different numbers of co-existing species suggests that niche breadth and overlap may be primarily determined by the availability of resources (e.g. substrates, plant species) and not by competitive interactions between the species themselves.