Bird distribution in riparian vegetation in the extensive natural landscape of Australia’s tropical savanna: a broad-scale survey and analysis of a distributional data base

Aim (a) To characterize the riparian bird assemblage, and its variation, in a large area of northern Australia; (b) to examine the distinctiveness of this assemblage in relation to the broader landscape; (c) to consider the influence of disturbance on this assemblage; (d) to examine temporal variability in the riparian assemblage, and especially evidence for seasonal movements between riparian and non-riparian areas. Location c. 620,000 km2 of the seasonal tropics of the Northern Territory, Australia. Methods (a) Synchronous sampling of birds in riparian and adjacent non-riparian areas at 100 sites stratified across 13 catchments and an extensive rainfall gradient. (b) Repeat visits to 13 of these sites at contrasting seasons. (c) Analysis of a larger distributional database to assess the relative occurrence of records in riparian areas relative to non-riparian areas. Results Species richness and the total abundance of birds was significantly greater in riparian zones than in matched non-riparian areas, especially where the riparian zones contained extensive cover of rain forest plants and Melaleuca. Similarity in bird species composition between riparian zones and adjacent non-riparian areas was generally low, and this distinction was greatest in lower reaches of the rivers and where the riparian zone contained no eucalypts. Bird species composition varied gradationally from riparian zones in high rainfall areas, through riparian zones in low rainfall areas and non-riparian zones in high rainfall areas, to non-riparian zones in low rainfall areas. Many species occurred widely across the riparian sites sampled. Of ninety-four species recorded from more than five sites, forty-five species were significantly more abundant in riparian zones than in matched non-riparian zones, whereas this pattern was reversed for only twelve species. There was little association between foraging group and preference for riparian zones. Species had highly idiosyncratic distributions across the riparian samples, with the most common trend being an association with mean annual rainfall. Many species were significantly more closely associated with riparian zones in lower rainfall areas than in higher rainfall areas. Indeed, many species typical of higher rainfall areas extended into lower rainfall areas only, or mainly, along riparian strips. There was some temporal fluidity in bird species composition of riparian zones, suggesting seasonal movements between riparian zones and the surrounding landscape. There was little evidence that disturbance was a major factor influencing the distribution of riparian birds, probably because other major geographical and environmental gradients probably dwarfed the influence of the relatively minor variation between samples in disturbance. Main conclusions The bird fauna of riparian areas is distinct from that of the surrounding savannas, and especially so in lower rainfall areas. Riparian vegetation allows many species to extend their distributions into lower rainfall areas. The riparian assemblage is loosely structured, in that most species have idiosyncratic distributions. As at least some bird species move seasonally between riparian and non-riparian areas, conservation management must ensure that these connections are maintained.