The plumage of the Limestone leaf warbler (Phylloscopus calciatilis) is almost identical to that of the Sulphur-breasted warbler (Phylloscopus ricketti), though the new species appears to have a colder yellow chest and more grey topside and stripped crown.Alstrom explains the divergence by the timing of their evolution:
But the new species is smaller, with shorter wings, rounder wing tips and a proportionately larger bill.
"Its vocalisations, both song and contact call, are markedly different from those of the Sulphur-breasted warbler," says Professor Alstrom.
DNA analyses also suggest that it is more closely related to the Yellow-vented warbler (Phylloscopus cantator) from eastern Himalayas, northern Laos and adjacent part of China, which is quite different in plumage.
"The most likely explanation [for this]," he says, "is that the plumages have not diverged much in the Sulphur-breasted and Limestone warblers since they separated from a common ancestor."
Surprisingly, the Yellow-vented warbler and the Limestone leaf warbler separated from a common ancestor much later, but have diverged much more in plumage.
Unlike many new species discoveries that I read about, the Limestone Leaf Warbler is apparently quite numerous within its range. However, it is restricted to limestone karst habitats, which are difficult to access. This may explain why it went undiscovered for so long.
The article announcing the Limestone Leaf Warbler's discovery is available from the journal Ibis.