Seasonal variation in foraging ecology of three species of overwintering Leaf Warblers (genus Phylloscopus) in the Himalayan foothills

We studied a guild of overwintering Phylloscopus warblers (Hume’s Warbler, Grey-hooded Warbler, Lemon-rumped Warbler) in the Himalayan foothills to gain an understanding of the manner in which they respond to changes in their habitat structure and resource availability from early to late winter (2006–2007). The relative abundances of birds and vegetation attributes were quantified at 91 point count stations. Arthropod abundance was measured in both the tree and shrub layers. Foraging behaviour was characterized in terms of the following dimensions: foraging height, foraging tree species, foraging manoeuvre and foraging substrate. From early to late winter, we observed a significant decline in vegetation cover in the canopy layer and arthropod abundance in both the tree and shrub layers, but did not detect any associated decline in the relative abundance of the three species. While a general shift to lower foraging heights was observed in late winter, no significant seasonal variation could be detected at the species level in the other dimensions. Furthermore, though interspecific segregation by foraging heights remained conserved across winter, foraging segregation in terms of the use of foraging trees and manoeuvres emerged only in late winter. Our results suggest that a shift in foraging patterns enabled the guild to adjust to changes in habitat structure and decline in food availability in winter.