Willow flycatcher

The subspecies are best distinguished from each other by their songs. In addition, the four subspecies have significant genetic differences based on mitochondrial DNA analysis. Their winter ranges have been elucidated using mitochondrial DNA genetic studies of 172 birds sampled in winter combined with plumage coloration and morphological differences.

The four subspecies of the willow flycatcher are:

The little willow flycatcher (E.t. brewsteri) is the Pacific Slope subspecies of the willow flycatcher. Described by Oberholser in 1918, it breeds in California from Tulare County north along the western side of the Sierra Nevada, and in Oregon and Washington west of the Cascade range.

The Great Basin/Northern Rockies subspecies of the willow flycatcher (E. t. adastus) breeds in California east of the Sierra/Cascade axis, from the Oregon border into Modoc County and possibly into northern Inyo County. Populations at high elevation just east of the Sierra Nevada crest but south of Modoc County are assumed to be E. t. brewsteri. There has been very little study of E. t. adastus in California. It was described by Oberholser in 1932.

The southwestern willow flycatcher (E. t. extimus) is a federally endangered subspecies and it is known to be found in Arizona, California, Colorado, Nevada, New Mexico, Texas, and Utah. It was listed in 1995, at which time it was known to breed at only about 75 sites in riparian areas throughout the American southwest. The known breeding population was estimated at between 300 and 500 pairs. Breeding occurs from near sea level on the Santa Margarita River to 800 m (2,640 ft) at the South Fork Kern River and 910 m (3,000 ft) at upper San Luis Rey River in California and to over 2,600 m (8,530 ft) in Arizona, southwestern Colorado, and north-central New Mexico. This subspecies was described by A.R. Phillips in 1948.

The largest remaining population in California is on the South Fork Kern River, Kern County. In southern California, this subspecies breeds on the San Luis Rey River, at Camp Pendleton, the Santa Margarita River and Pilgrim, De Luz, French, and Las Flores creeks; as well as on the Santa Ynez River. In 1996, breeding was confirmed along the Arizona side of the lower Colorado River at Lake Mead Delta and at Topock Marsh. Examination of museum specimens of 578 migrating and wintering E. t. extimus indicating that Guatemala to Costa Rica constitutes the main winter range.

This species is experiencing population declines throughout the Southwest due to habitat loss/alteration and invasive species of grass. One of these is saltcedar (Tamarix ramosissima), found throughout the Southwest, where it has replaced essential vegetation by outcompeting native species in riparian areas where the southwestern willow flycatcher is found.[5] In two sites, one in Arizona and the other in New Mexico, native trees were able to replace patches of saltcedar and populations of willow flycatchers increased. It was documented that in these sites 90% of the willow flycatcher's nests were found in native vegetation, only 10% were in mixed vegetation (native species and saltcedar) and few were in areas dominated by saltcedar.[6] However, it is important to note that because willow flycatchers can and do breed, in some locations, within saltcedar habitat, it can occasionally serve as vital habitat in the recovery of this species.[7]

The San Pedro River Preserve was purchased by the Nature Conservancy to preserve habitat for this subspecies. NatureServe considers the subspecies Imperiled.[8] North American beavers (Castor canadensis) are thought to play a critical role in widening riparian width, openings in dense vegetation, and retention of surface water through the willow flycatcher breeding season.

The eastern nominate subspecies of the willow flycatcher (E. t. traillii) was described by Audubon in 1828. It breeds from the eastern coast of the United States to the western Rocky Mountains.