Distinguishing between Sakhalin Leaf Warbler Phylloscopus borealoides and Pale-legged Leaf Warbler P. tenellipes on call

The Sakhalin Leaf Warbler Phylloscopus borealoides has only recently been confirmed as a frequent passage migrant and probable winter visitor in South-East Asia, alongside the long-recognised Pale-legged Leaf Warbler P. tenellipes. The great morphological similarity of these two species renders them very hard to distinguish either in the field or in the hand. We sampled both species caught on passage at coastal sites in Thailand, extracting and amplifying the mitochondrial cytochrome c oxidase I (COI) gene from tail-feathers. Call-notes of birds were recorded on release. The results from bioacoustics and genetic analyses in a sample of 33 birds were 100% concordant. We were able to confirm that the call-notes of Pale-legged Leaf Warblers (n = 14 individuals) were consistently higher than those of Sakhalin Leaf Warblers (n = 19 individuals), having mean peak frequencies of 5,729.29 +/- 77.23 Hz and 4,786.58 +/- 191.08 Hz for the two species respectively, with no overlap. This confirms previous assertions that the call-notes, though similar, differ significantly in frequency, so that most individuals that are heard to call may be assigned to one species or the other by experienced observers. This will greatly facilitate future assessment of the relative status of the two species from field observations.