Stable isotope ratios in winter-grown feathers of Great Reed Warblers Acrocephalus arundinaceus, Clamorous Reed Warblers A. stentoreus and their hybrids in a sympatric breeding population in Kazakhstan

Analyses of the stable isotope composition of feathers can provide significant insight into the spatial structure of bird migration. We collected feathers from Great Reed Warblers Acrocephalus arundinaceus, Clamorous Reed Warblers A. stentoreus and a small sample of their hybrids in a sympatric breeding population in Kazakhstan to assess natural variation in stable isotope signatures and delineate wintering sites. The Great Reed Warbler is a long-distance migrant that overwinters in sub-Saharan Africa, whereas the Clamorous Reed Warbler performs a short-distance migration to the Indian sub-continent. Carbon (dC), nitrogen (dN) and deuterium (dD) isotope signatures were obtained from winter-grown feathers of adult birds. There were highly significant differences in dD and less significant differences in dC between Great and Clamorous Reed Warblers. Thus, our results show that the stable isotope technique, and in particular the deuterium (dD) signal, resolves continental variation in winter distribution between these closely related Acrocephalus species with sympatric natal origin. The isotope signatures of hybrid Great · Clamorous Reed Warblers clustered with those of the Great Reed Warblers. Hence, a parsimonious suggestion is that the hybrids undergo moult in Afrotropical wintering grounds, as do the Great Reed Warblers. The observed dD values fell within the range of expected values based on available precipitation data collected at precipitation stations across the wintering continents of each species. However, the power to predict the winter origin of birds in our study system using these data was weak as the expected values ranged widely at this broad continental scale.