Limestone leaf warbler

The limestone leaf warbler (Phylloscopus calciatilis) is a species of warbler in the family Phylloscopidae. When this species was first seen, beginning in 1994, it was mistaken for the similar sulphur-breasted warbler. It is smaller than the sulphur-breasted warbler, and has more rounded wings. The plumage is almost identical, with comparisons showing only a slightly colder yellow below and a greyer tinge above. Although smaller, the bill is proportionally larger than that of the sulphur-breasted warbler. Accurate measurements are not available; the holotype has a wing length of 5.2 cm (2.0 in); the paratype a tail length of 3.7 cm (1.5 in) and a bill length of 1.39 cm (0.55 in). The species is known to occur in northern Vietnam and Laos, and potentially also occurs in southern China as well. The species name, calciatilis, means "dwelling on limestone", which along with its common name is a reference to its natural habitat, which is broadleaved evergreen and semi-evergreen forest growing around limestone karst mountains. The bare-faced bulbul, described in 2009, was found in the karst of the same region.

The limestone leaf warbler (Phylloscopus calciatilis) is smaller than the sulphur-breasted warbler (Phylloscopus ricketti) and has more rounded wings and a proportionately elongated bill. Although a smaller species, the bill is proportionately larger than that of the sulphur-breasted warbler. The plumage is nearly identical, with only a slightly lighter yellow below a grey tinge. Comparisons between these two species suggest it is doubtful that they can be reliably distinguished by plumage alone. The holotype has a wing length of 5.2 cm; in the paratype a tail length of 3.7 cm and a bill length of 1.39 cm.

The limestone leaf warbler is greenish-olive in colour with a yellow breast and striped crown. Although it looks similar to other warblers, it can be distinguished by being smaller, having shorter wings, and a larger bill. It is also distinguished by its black lateral crown-stripes, which are paler and more diffuse near the bill. The median crown stripe is greenish-yellow with the supercilium being yellow with a faint greenish tinge. The eye-stripe on lores and upper ear-coverts are a well defined black colour with a green tinge. The mantle, scapulars, back, rump, lesser and uppertail-coverts are a bright grey-green colour, with the throat, breast and belly a bright yellow. The sides of the breast have a green tinge. The remiges, rectrices, alula, medium, greater and primary coverts are a dark grey colour with a brown tinge, with bright greyish-green outer edges. The juvenile resembles the adults, but has looser, fluffier plumage and, possibly as a result of this, slightly less yellow underparts. It has marginally whiter and more clear-cut pale tips to the greater coverts, and probably slightly more green on the anterior part of the lateral crown-stripes.

“In plumage, the limestone leaf warbler appears to be largely indistinguishable from P. ricketti, although as a result of the small number of specimens available for the limestone leaf warbler and their poor quality, detailed comparisons are difficult to make. The only two specimens of the limestone leaf warbler, which have been directly compared with a series of P. ricketti are marginally colder yellow below a more greyish tinge above, and show marginally greyer lateral crown-stripes than P. ricketti. The limestone leaf warbler is easily separable from P. cantator by its yellow belly”.