Small brown bush warbler with a bright rufous crown, gray belly, and striking pale eyebrow. Frosty-pale throat contrasts with darker neck and breast. Paler gray upperparts, cleaner-looking white eyebrow, and smaller size distinguish this species from Chestnut-crowned Bush Warbler. Other brown bush warblers are smaller or duller, lacking the rufous crown. Gray-sided is generally fairly skulking, foraging in or on the ground in dense vegetation, but occasionally emerges out into the open, particularly to sing. Breeds in forest edges, clearings, and open shrubby areas in montane areas up to the treeline; winters in bushy edge areas in lower foothills. Song is a brief spurt of tinkling high-pitched notes followed by a buzzy whine. Call notes are high-pitched “tseep” and “tzzi” notes.