Morphological and molecular variation across a migratory divide in willow warblers, Phylloscopus trochilus
A migratory divide is a narrow region in which two populations showing different migratory directions meet and presumably also mate and hybridize. Banding of willow warblers, Phylloscopus trochilus, in Europe has demonstrated a migratory divide latitudinally across central Scandinavia. In autumn, southern birds migrate southwest to tropical West Africa, whereas northern birds migrate southeast to