Morphology and foraging behaviour of Siberian Phylloscopus warblers

There are seven species of Phylloscopus warblers breeding sympatrically in Central Siberia. In general they are very similar in morphology and behaviour. Slight differences in morphology, however, are often interpreted as having arisen through adaptation to distinct foraging ecologies, thereby mediating species coexistence. We studied the morphology and foraging behaviour of these warblers, taking into account phylogenetic relationships using Felsenstein's method of independent contrasts. Striking correlations were found between morphology and foraging techniques, as well as between morphology and microhabitat selection. Species with large hind limbs and short wings foraged in dense vegetation near the ground using gleaning techniques. In contrast, species with long wings and large bills favoured open vegetation, and foraged using sallying manoeuvres. Independently of these trends, small species foraged at higher levels in the vegetation, and used hoverflight more often than did larger species. Foraging methods were highly correlated with vegetation structure. Coniferous trees were exploited using hoverflight when they had short needles and gleaning when needles were long. Sallying predominated in light deciduous vegetation whereas gleaning was used most in dense, deciduous bushes. After controlling for the effects of vegetation structure on foraging behaviour, species differences became less pronounced, but were still significant. It is therefore concluded that morphology may be regarded as having adapted to enable more efficient exploitation of certain microhabitats. A preference for these microhabitats then further increases the observed behavioural differences between species. Morphology may therefore partly determine a species' ability to colonise new habitats.