An insectivorous, Neotropical migrant that breeds in the southwestern United States and winters in western Mexico, Lucy's Warbler is one of the smallest of the New World (wood) warblers. This species was first discovered along the lower Colorado River by J. G. Cooper (Cooper 1861) at Ft. Mojave, Arizona, near the junction of Arizona, California, and Nevada. The often remote mesquite bosques in which it breeds were rarely visited by early researchers, and this small, inconspicuous bird was overlooked by the few ornithologists who first worked in the Southwest.
Lucy's Warbler nests in close association with riparian mesquite (Prosopis spp.). It breeds in some of the densest concentrations of any noncolonial nesting species in North America, building a miniature nest usually well concealed in a cavity or similar site. It also breeds in lowland cottonwood-willow (Populus-Salix) riparian gallery forests and, less often, in midelevation, sycamore-ash-live oak (Platanus-Fraxinus-Quercus). These riparian ecosystems have been greatly reduced locally throughout much of the southwestern United States, extirpating many breeding populations of Lucy's Warblers. Brood parasitism by Brown-headed Cowbirds (Molothrus ater) may also have affected some already dwindling local populations.
This species nests almost entirely in the hot lower Sonoran desert of the southwestern United States and northwestern Mexico. It occupies the driest habitat of the 4 southwestern lowland-breeding warblers and has been called the “mesquite warbler” (Gilman 1909b) and “desert warbler” (Monson 1979). The other 3 desert-nesting warblers—Yellow Warbler (Setophaga petechia), Common Yellowthroat (Geothlypis trichas), and Yellow-breasted Chat (Icteria virens)—generally occupy wetter habitats.
Numerous professional and amateur ornithologists have published accounts of this species (Chapman 1907b, Gilman 1909b, Monson 1979, Harrison 1984d, Curson et al. 1994), but aside from the review by Bent (Bent 1953b), a complete life history study has never been conducted. Complete information is lacking on this species' breeding ecology—e.g., mate selection and copulation, nest-building, incubation, and parental care—as well as information on demographics, causes of population fluctuations, and general life history and ecology.