The effects of vegetation structure on the birds in a tropical savanna woodland in north-eastern Australia

Management of the dynamics of woody vegetation in Australia's tropical savannas is a vexing issue for both pastoralists and conservation biologists. In savanna regions around the world, increasing density of woody vegetation contributes to declines in pastoral productivity, but its effects on native fauna are largely unknown. In this paper we examine the avifauna in savanna woodlands of varying structure in the Desert Uplands bioregion, Queensland. Vegetation cover maps derived from aerial photographs were used to choose 60 sites, across 4 cattle stations. We sampled sites mapped at 30-45% and 45-60% foliage cover, and areas which previously had these levels of cover but had been mechanically modified, both by broad scale clearing and selective thinning. Between May and June 2004, we measured a range of habitat variables and sampled the birds at each site. Bird species composition varied significantly between treatments. Bird richness and frequency was greatest in intact vegetation. Thirteen species of birds were most frequently encountered in sites with 30-45% canopy cover, compared with 10 species in the 45-60% cover sites, 4 species in the thinned sites and 7 in the cleared sites. Our results suggest that increasing density of woody vegetation in savanna woodland may be to the advantage of some savanna bird species. Mechanical modi. cation to reduce woody vegetation appears to also benefit some common, widespread species, but has a generally negative overall effect on bird species richness.