Determinants of the northern and southern range limits of a warbler

Aim To understand the determinants of range limits by studying the behaviou r and ecology of a single species (the Hume's leaf warbler, Phylloscopus hu mei) at both its northern and southern range limits. Locations North-west India (Himachal Pradesh) and central India (Maharashtr a). Methods We worked at seven locations primarily in the winter of 1997-98. Fi ve locations spanned the northern range limit of the species and two were c lose to the southern range limit. We censused arthropod and bird abundance (including both the Hume's leaf warbler and other ecologically similar spec ies) at each site. We also studied the foraging behaviour of the common war bler species. Results Across the whole range of P. humei the abundance of arthropods is t ightly correlated with the abundance of all warbler species. The northern r ange limit of P. humei coincides with the disappearance of arthropod food f rom its habitat (tree crowns), which is attributed to leaf loss associated with cold temperatures. Two closely related species, P. proregulus and Seic ercus xanthoschistos have a northerly range limit beyond that of P. humei. They occupy the understory, which retains leaves and arthropods further nor th than do the tree crowns. The southern range limit of P. humei is associa ted with high food levels in its habitat, but also increasing numbers of a potential competitor, P. trochiloides, which is very similar in habits and also occupies the tree crowns, but is about 40% heavier. We attribute the d ecline of P. humei in the south to competition from P. trochiloides. Becaus e food levels are high in the south, the mechanism of competition is likely to be interference, rather than exploitative. Main conclusion Competition for food is likely involved in setting both the northern and southern range limits, but in very different ways. The result s are discussed in the light of current theories for the way range limits a re ultimately determined.