Small dull-colored leaf warbler with pale eyebrows and wingbars. Eastern breeding population “Mandelli’s Warbler” is buffier than western breeders, particularly on the underparts. Very similar in appearance to Yellow-browed Warbler and Greenish Warbler, but averages duller and grayer than those species, and with an overall darker bill. Two-barred Warbler is also extremely similar, but has a more prominent first wingbar than Hume’s. However, as with many other leaf warblers, plumage wear or poor views can make visual identification difficult. Calls are variable, can be a sparrowlike chip or a two-part call that sounds like a drier, duller version of Yellow-browed Warbler. Gives two songs: a repetition of the two-note call and a long dry buzzing trill. Inhabits higher elevations during the breeding season where it occurs in open coniferous forests and shrubby areas; descends lower into the foothills in the winter.