The southwestern willow flycatcher, Empidonax trailli extimus, breeds in the southwestern United States (and possibly northern Mexico), and winters in Central America and southern Mexico. It is rare in southern Utah during the summer. The southwestern willow flycatcher is found most frequently in riparian habitats, especially in areas of dense willow. This flycatcher is difficult to distinguish from other related species, though its territorial song is distinctive.
The southwestern willow flycatcher eats insects, seeds, and berries. Breeding occurs during late spring or early summer, with peak breeding activity occurring in June. A nest is constructed in a vertical fork of a willow or other riparian tree. The female then lays and incubates two to four eggs; the young hatch after twelve or thirteen days. The hatchlings are tended by both parents and leave the nest after about two weeks.
The southwestern willow flycatcher is currently very rare throughout its range. It is consequently Federally listed as endangered. The major factor in the decline of the southwestern willow flycatcher is likely the alteration/loss of the riparian habitat necessary for the species.