Habitat selection by ground-foraging robins in Eastern Australia: Scale dependence and hierarchical selection

Abstract

Throughout the world, the continued clearing and subsequent fragmentation of native vegetation has resulted in an unprecedented loss of biodiversity. While some species adapt to such habitat change, many others respond unfavorably; declining in distribution and abundance. To understand reasons for this response in such species requires an understanding of the processes governing habitat selection, which operate at multiple spatial and temporal scales. In birds, habitat selection is generally assumed to be hierarchical in nature. An inevitable consequence of such a hierarchical structure is that habitat selection at a given spatial scale is constrained by habitat selection at other spatial scales. Unfortunately, habitat selection studies are often limited in spatial scope, focusing on management units operating at the landscape scale. The problem with this approach is that it assumes a 'top-down' model of habitat selection, and focuses on species responses at the coarsest spatial scales (e.g., landscape and remnant scale), while underestimating the importance of responses at fine spatial scales (e.g., foraging microhabitat). Furthermore, it also underestimates the potential for fine scale selection to constrain selection at coarser spatial scales, represented by a 'bottom-up' model of habitat selection. In the present study, I examined habitat selection at four spatial scales in two species of sedentary, ground-foraging birds in northern New South Wales; the Eastern Yellow Robin ('Eopsaltria australis') and the Scarlet Robin ('Petroica multicolor'). I assessed habitat selection in the two species at the foraging microplot scale (pounce site, 0.3 m x 0.3 m), foraging mesoplot scale (foraging area surrounding pounce site, 5 m x 5 m), territory scale (represented by 100 m x 5 m transects), and landscape scale (10 km x 10 km). In addition, in an attempt to provide information that might explain why particular foraging microhabitat was selected by the two species, I also investigated the association of microhabitat structure with the abundance of the invertebrate orders that constitute the majority of the robins' diet.