The southwestern willow flycatcher 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 2,640 ft at the South Fork Kern River and 3,000 ft at upper San Luis Rey River in California and to over 8,530 ft in Arizona, southwestern Colorado, and north-central New Mexico.

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.

Saltcedar (Tamarix ramosissima) is an invasive species found throughout the Southwest and has replaced essential vegetation, by outcompeting native species, in riparian areas where the Southwest Willow flycatcher is found, which could be a contributing factor in this species decline. In two sites, one in Arizona and the other in New Mexico, native trees were able to replace patches of tamarix, 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. However, it's important to note that because willow flycatchers can and do breed, in some locations, within Saltcedar habitat it can serve as vital habitat in the recovery of this species.

The San Pedro River Preserve was purchased by the Nature Conservancy to preserve habitat for this subspecies. 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. This subspecies was described by A.R. Phillips in 1948.

Source
Wikipedia