Determinants of the northern and southern range limits of a warbler

Aim To understand the determinants of range limits by studying the behaviour and ecology of a single species (the Hume's leaf warbler, Phylloscopus humei) at both its northern and southern range limits. Locations North-west India (Himachal Pradesh) and central India (Maharashtra). Methods We worked at seven locations primarily in the winter of 1997-98. Five locations spanned the northern range limit of the species and two were close to the southern range limit. We censused arthropod and bird abundance (including both the Hume's leaf warbler and other ecologically similar species) at each site. We also studied the foraging behaviour of the common warbler species. Results Across the whole range of P. humei the abundance of arthropods is tightly correlated with the abundance of all warbler species. The northern range limit of P. humei coincides with the disappearance of arthropod food from its habitat (tree crowns), which is attributed to leaf loss associated with cold temperatures. Two closely related species, P. proregulus and Seicercus xanthoschistos have a northerly range limit beyond that of P. humei. They occupy the understory, which retains leaves and arthropods further north than do the tree crowns. The southern range limit of P. humei is associated 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 decline of P. humei in the south to competition from P. trochiloides. Because 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 results are discussed in the light of current theories for the way range limits are ultimately determined.