Identification of Marsh Warblers Acrocephalus palustris and Reed Warblers A. scirpaceus on autumn migration through the eastern Mediterranean.

Published criteria for the separation of Reed Warbler Acrocephalus scirpaceus and Marsh Warbler A palustris in the hand were found to be of limited value in identifying these species in samples captured during autumn migration through the island of Lesvos in the Aegean Sea. This is probably because Reed Warbler morphology varies clinally with birds in eastern Europe (likely to be migrating through Lesvos) being more similar to Marsh Warblers than are populations in western Europe from which existing identification criteria were calculated. However, a modified biometric index did separate a sample of 364 birds captured during August and September 1994–1996 and 1998 into two clear‐cut groups along the morphological gradient which is known to distinguish the two species. We were therefore confident that these two groups comprised Marsh and Reed Warblers. Linear Discriminant Function Analysis confirmed that this index could be used to identify approximately 95% of the birds in our sample. The index also correlates well with independent assessments of bare part colouration (eg leg colour) which are known to be of use in distinguishing the two species. We suggest that this index will be useful for identifying Reed and Marsh Warblers wherever migrating populations of the two species drawn from a wide geographical area occur together.