Fairly common breeder in a variety of habitats, both deciduous and mixed forest including spruce bogs. Sings a lot, a 2-part trill, second half lower. On widespread fall migration often in lower vegetation. Pumps/swishes tail. ID: Small and fat with a short narrow tail and big rounded head, a distinctive shape. Blue-gray head contrasts with olive upperparts but it is large round eyering that really pops. From below is yellow but uniquely white on belly (around legs). Other warblers have bold eyerings, but none is so small and chunky. Ad ♂: brightest colors with yellow throat. Ad ♀: duller, often with pale throat. Imm ♀: dullest are olive with little contrast between head and back, underparts pale yellow but still white between legs.