About Phylloscopus trochiloides (Sundevall, 1837)
This species is a typical leaf warbler in appearance, with grayish-green plumage on the upper body and off-white plumage on the lower body. Southern and western populations have a single wing bar, which distinguishes them from most similar species except the Arctic warbler Phylloscopus borealis. This species is slightly smaller than the Arctic warbler, has a thinner bill, and lacks a dark tip on the lower mandible. A latitude-based analysis of wintering birds found that more northerly P. trochiloides individuals are smaller, meaning this species does not appear to follow Bergmann's rule. The species' song is a high, jerky trill, and in some populations it includes a sequence of down-slurred notes, and more rarely up-slurred notes. It breeds in lowland deciduous or mixed forest; non-breeding individuals in the warmer parts of its range may move to montane habitat during summer. For example, individuals from southeast of the Himalayas are quite often seen in Bhutan during the hot months, typically in humid Bhutan Fir (Abies densa) forest up to about 3,800 meters above sea level or higher, but they do not breed here. They return to the adjacent subtropical lowlands in winter. This species builds its nest on the ground within low shrub. Like its close relatives, this small passerine bird is insectivorous.
