The behavioural ecology and conservation of a mulga-dependent, cooperatively-breeding Australian passerine: the Hall's babbler (Pomotostomus halli)

A project undertaken at the School of Biological, Earth and Environmental Sciences, The University of New South Wales, and supervised by Richard Kingsford and Dean Portelli

Our knowledge of the behaviour, ecology and conservation status of Australian birds is strongly biased toward species that live close to metropolitan areas and the coastal fringe of Australia. This is clearly evident among the Australo-Papuan babblers which have been the focus of considerable research in avian behaviour and ecology. Grey-crowned and white-browed babblers have been well studied in eastern and south-western Australia, respectively, but little is known of the biology of chestnut-crowned and Hall's babblers, which are confined to semi-arid and arid regions of eastern Australia.

Hall’s babbler was discovered relatively recently in 1963 and since this time only two short-term studies of its behavioural ecology have been undertaken. Concerted research on the species is desirable since it is listed as Vulnerable in New South Wales and its closest relative, the grey-crowned babbler, has declined markedly in south-eastern Australia. The broad aim of this research was to substantially increase our knowledge of the biology of Hall's babbler, particularly its habitat use, social organisation and life-history.

We investigated habitat use of Hall’s babbler at micro- and macrohabitat scales and throughout its geographic range, using direct field sampling techniques and spatial data analysis within a geographic information system. The species is a habitat specialist of vegetation communities dominated by Acacia species, predominantly mulga (Acacia aneura) but also several species that occur on residual soils of tablelands, mesas and their associated slopes. This conspicuous habitat specialisation was apparent at all spatial scales examined; indeed the geographic range of the species closely matches the distribution of mulga and related vegetation communities across eastern Australia, suggesting its habitat specialisation is the primary factor delineating the geographic range boundary. Despite the lack of historical data on the distribution of Hall's babbler, we found no evidence that the geographic range has contracted markedly since European settlement, unlike what has occurred in the grey-crowned babbler. Data is lacking to examine changes in abundance or the incidence of local extinction, which may result from habitat modification resulting from pastoralism. Nonetheless, we found detection rates of the species in Birds Australia atlas surveys were considerably lower in western New South Wales. Although we have increased our understanding of the habitat requirements and geographic range of Hall's babbler, its conservation status in New South Wales remains unclear.

Hall’s babbler, like other Australo-Papuan babblers, breeds cooperatively with individuals in addition to the mated pair helping to raise offspring. Hall's babblers live year-round in cohesive breeding units, which consist of a socially monogamous pair and a small number of helpers; though lone pairs are common. Breeding units typically form through delayed dispersal of male offspring, which contribute to raising subsequent broods of the pair while they queue for a breeding position. Female offspring on the other hand disperse within their first or second year to find a breeding position. The cooperative-breeding system of Hall’s babbler can be classified as a ‘plural breeding’ system since breeding units form larger social groups during the non-breeding season.

The life-history of Hall's babbler is similar to that of Australo-Papuan babblers and typical of the life history strategy of other 'old endemic' Australian passerines and passerines of the southern hemisphere and tropics more generally. Clutch size was small (usually 2 or 3 eggs), the species was multi-brooded, the breeding period was long (Jun–Feb) but variable among years, incubation and nestling periods were protracted, juvenile and adult survival were high, and fledging success and annual productivity were low.

Thanks to support from the Hermon Slade Foundation we achieved our aim, greatly increasing our knowledge of the behaviour and ecology of Hall's babbler, a species that was previously poorly known.

Captions to Figures

Figure 1. Adult Hall's babbler (Photo: D. Portelli).

Figure 2. Close-up of adult Hall's babbler with distinctive plumage and down-curved probing bill (Photo: D. Portelli).

Figure 3. Three 13-day-old chicks temporarily removed from the nest for banding (Photo: D. Portelli).

Figure 4. Typical Hall's babbler habitat, dominated by mulga (Acacia aneura), at Bowra, SW Queensland (Photo: D. Portelli).

Figure 5. Colour-banded Hall’s Babbler. Each babbler in the study population is banded with an individually-unique combination of three colour bands permitting identification of individuals with binoculars.

Figure 6. A recently fledged Hall’s Babbler.

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