The effects of habitat fragmentation on dispersal, population structure and reproductive success in cooperatively breeding white-winged choughs
A project undertaken at the Centre for Resource and Environmental Studies, Australian National University, and supervised by N Beck and R Heinsohn
Habitat loss and fragmentation are amongst the most serious environmental threats affecting the persistence of animal species throughout the world. In Australia, loss and fragmentation of woodland habitats has resulted in the widespread decline of many bird species in cropping and livestock-grazing regions. White-winged choughs (Corcorax melanorhamphos) have recently been identified as sensitive to habitat fragmentation (Watson 2000), and are one of a large group of woodland bird species that are thought to be on the decline (Birds Australia 2002). However, the unusual aspects of chough biology may render the species more sensitive to modifications of the landscape than currently thought.
White-winged choughs are highly social, cooperatively breeding birds that range throughout the eucalypt woodlands of south-eastern Australia. They are unusual in being 'obligate' cooperative breeders; groups of less than four adults are incapable of breeding successfully and only groups of seven or more succeed in raising more than one young per year (Heinsohn 1992). Related to this distinctive breeding system is the unusual dispersal behaviour of choughs. Dispersal by individuals is not known to occur. Instead, once groups become very large (over 12 individuals) they may break up into smaller groups of between 4 and 6 that then disperse and reform with birds from other groups (Heinsohn et al. 2000). Such dispersal events may be triggered by the death of one of the breeding pair (Rowley 1978). However, dispersal of chough groups on a large scale is rare, having only been observed once in over ten years of study, and may be precipitated by severe El Nino induced drought conditions (Heinsohn et al. 2000).
An understanding of dispersal behaviour is fundamental to understanding the true impact of habitat fragmentation. Due to its rare and sporadic nature, chough dispersal is poorly understood. This project is using both field studies and genetic analysis to investigate dispersal behaviour and the effects of habitat fragmentation on white-winged chough populations.
The severe drought conditions experienced by southern Australia in the second half of 2002 and early 2003 appear to have triggered widespread disruption to the groups in the local Canberra chough population. Therefore, this field season, our efforts have been concentrated on marking the local population and monitoring the movements of individuals and changes to group composition. This will provide us with the unusual opportunity to verify the results of genetic population analyses with observational data before extending our genetic analysis further. To this end we are in the process of developing a microsatellite genotyping system, with six variable loci already isolated and developed.
This study has broad significance as Australia has an unusually high proportion of cooperatively breeding bird species whose social systems may be affected by habitat loss and fragmentation. Therefore this project aims to use choughs as a model for the effects of habitat fragmentation on Australia’s large number of highly social species.
Birds Australia (2002). Atlas Newsletter 9 – February 2002. www.birdsaustralia.com.au/atlas
Heinsohn, R. G. (1992). Cooperative enhancement of reproductive success in white-winged choughs. Evolutionary Ecology 6: 97-114
Heinsohn, R., Dunn, P., Legge, S. & Double, M. (2000). Coalitions of relatives and reproductive skew in cooperatively breeding white-winged choughs. Proceedings of the Royal Society of London. Series B. 267: 243-249
Rowley, I. (1978). Communal activities among white-winged choughs, Corcorax melanorhamphus. Ibis 120: 178-196
Watson, J., Freudenberger, D. & Paull, D. (2000). An assessment of the focal-species approach for conserving birds in variegated landscapes in southeastern Australia. Conservation Biology 15: 1364-1373