Phylloscopus collybita
Olive green upperparts, yellowish chest with gradual transition to off-white belly. Very similar to Willow Warbler, and most field characters are subtle and often hard to see. Distinguished by (usually) black, or dark-brown legs, only faint dark framing to ear-coverts, less marked supercilium. Shorter primary projection (1/2 to 2/3'rds the lenght of tertials), with regularly spaced tips (visible when wing folded). Wings more fan-shaped and less pointed than in Willow Warbler. Juveniles much less yellow below than in W.Warbler. Generally leaves a duller and greyer impression than W. Warbler, but plumage very variable. Frequently flicks tail, even sideways. Generally also more active when moving among the foliage.
Contact/alarm call a soft, plaintive ascending "hooeet". Similar to W.Warbler, but shorter with a monosyllabic feel. Song a very distinct: "chiff-chaff-chiff-chiff-chaff-chiff", in a regular clock-like rhythm. Each syllable at seemingly random pitch, but no large intervals tonally. Sometimes "get stuck" at one note.
Wikipedia: map (se also Xeno-canto below)
Birdlife ecology
