The influence of environmental heterogeneity on winter ranging in the red-backed fairy-wren, Malurus melanocephalus.

Abstract

The decline in food and cover that accompanies the tropical dry season can cause animals to shift and expand their home ranges. However, range expansion is problematic for territorial species. In birds, this can be overcome via seasonal fission-fusion, a pattern in which birds combine individual breeding season territories in the nonbreeding season and travel as a flock over their combined range. Such behavior is present in a variety of birds and may have environmental, trophic and social causes. Here we investigate a potential environmental cause in one such bird, the red-backed fairy-wren (RBFW) Malurus melanocephalus. We used spatial analysis of home ranges, movement logs, and fire history to see whether access to unburned habitat was associated with seasonal fission-fusion. RBFWs showed a strong preference for unburned habitat. However, two other predictions-- that there would be a positive relationship between home range size and sociality and that flock size would be larger in unburned areas--were not supported. Our results suggest that fusion into large flocks did not occur at our site or had not begun in earnest at this point in the dry season. Fusion probably takes off in the late dry season and could potentially be driven by access to unburned habitat. This is suggested by RBFW's preference for unburned habitat, though other environmental and trophic factors are likely to drive group fusion as well.