The Lucy’s warbler is a small, pale songbird with a light gray head and upperparts, and whitish underparts. It has a small, pointed beak, white eyerings and black legs. Most distinctive are the coppery-brown patches on its crown and rump.
The Lucy’s warbler’s song is a bright, fast trill of chit chit chit chit sweet, che-che-che that rapidly rises and falls in pitch.
The Lucy’s warbler breeds in the hot, dry climate of the Sonoran Desert in Arizona and New Mexico. It can also be found in parts of surrounding states and the tips of northwestern Mexico. It likes to inhabit mesquite and brush particularly around streams and dry washes. It can also sometimes be found in oak or sycamore groves in desert canyons and foothills. Of all the New World warblers, the Lucy’s warbler prefers the driest habitat. It migrates down to south where it stays along the western edge of Mexico, namely the Pacific coast.
The Lucy’s warbler feeds primarily on insects, including caterpillars, wasps, beetles, and leafhoppers. It likely forages by hopping around the mesquites and other desert brush, gleaning insects off twigs and foliage. It occasionally flies out to catch insects midair. Before swallowing caterpillars, it has been observed shaking them violently to skin off hairs.
The Lucy’s warbler approach to nesting is unique, as only one other North American warbler species uses a cavity. It builds its nest in hollows in mesquites, abandoned woodpecker holes, behind loose bark on a tree, or in the cavities of a cactus. The Lucy’s warbler does not nest in man-made boxes. Both sexes fill the cavity with a woven nest of twigs, straw, and leaf stems before lining it with fine plant fibers, feathers, and hair. The female lays 3 to 7 eggs that are white with reddish-brown spots at the large end. The female likely incubates the nest but the duration is unknown. Both parents feed the young before they leave the nest after an estimated 11 days.