Editing Willow flycatcher (section)

== Taxonomy ==
There are four subspecies recognized, all of which breed in North America (including three subspecies that breed in California).<ref name="Paxton1" /> At one time, this bird and the [[alder flycatcher]] (''Empidonax alnorum'') were considered to be a single species, '''Traill's flycatcher'''. Their song is the only reliable method to tell them apart in the field.<ref name="FWS" /><ref name="AAB" /> The [[binomial name|binomial]] commemorates the Scottish zoologist [[Thomas Stewart Traill]].

=== Subspecies ===
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.<ref name="Paxton1" /> 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:

==== ''E. t. brewsteri'' – Little willow flycatcher ====
The little willow flycatcher (''E.t. brewsteri'') is the Pacific Slope subspecies of the willow flycatcher. Described by [[Harry C. Oberholser|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.

==== ''E. t. adastus'' ====
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, California|Modoc County]] and possibly into northern [[Inyo County, California|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 [[Harry C. Oberholser|Oberholser]] in 1932.

==== ''E. t. extimus'' – Southwestern willow flycatcher ====
The southwestern willow flycatcher (''E. t. extimus'') is a federally [[Endangered species|endangered]] subspecies found in Arizona, California, Colorado, Nevada, New Mexico, Texas, and Utah. It was listed in 1995, when it was known to breed at only about 75 sites in riparian areas throughout the American southwest. The breeding population was estimated at between 300 and 500 pairs. Breeding occurs from near sea level on the [[Santa Margarita River]] to {{convert|2640|ft|m|abbr=on|disp=flip}} at the [[South Fork Kern River]] and {{convert|3000|ft|m|abbr=on|disp=flip}} at upper [[San Luis Rey River]] in California and to over {{convert|8530|ft|m|abbr=on|disp=flip}} 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, California|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|Lake Mead Delta]] and at [[Havasu National Wildlife Refuge|Topock Marsh]]. Examination of museum specimens of 578 migrating and wintering ''E. t. extimus'' indicate that [[Guatemala]] to [[Costa Rica]] constitutes the main winter range. 

This species is experiencing population declines throughout the Southwest due to [[Habitat loss|habitat loss/alteration]] and invasive grass species. 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.<ref>{{Cite journal |last1=DeLoach |first1=CJ |last2=Dudley |first2=Tom |date=2004 |title=Saltcedar (Tamarix spp.), endangered species, and biological weed control - Can they Mix? |url=https://naldc-legacy.nal.usda.gov/naldc/download.xhtml?id=39820&content=PDF |journal=Weed Technology |volume=18 |pages=1542–1551 |doi=10.1614/0890-037X(2004)018[1542:STSESA]2.0.CO;2 |s2cid=35498749|url-access=subscription }}</ref> 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. 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.<ref>{{Cite journal |last1=Bean |first1=Daniel |last2=Dudley |first2=Tom |date=2012 |title=Tamarisk biocontrol, endangered species risk and resolution of conflict through riparian restoration |journal=BioControl |volume=57 |issue=2 |pages=331 |doi=10.1007/s10526-011-9436-9 |bibcode=2012BioCo..57..331D |s2cid=16497604}}</ref> However, because willow flycatchers can and do breed in some locations within saltcedar habitat, it occasionally serves as vital habitat in the recovery of this species.<ref>{{Cite journal |last1=Sferra |first1=Susan |last2=Paxton |first2=Eben |last3=Sogge |first3=Mark |date=2008 |title=Tamarix as Habitat for Birds: Implications for Riparian Restoration in the Southwestern United States |url=https://zenodo.org/record/1230659 |journal=Restoration Ecology |volume=16 |issue=1 |pages=146–154 |doi=10.1111/j.1526-100X.2008.00357.x |bibcode=2008ResEc..16..146S |s2cid=56042221}}</ref>

The [[San Pedro River Preserve]] was purchased by the [[Nature Conservancy]] to preserve habitat for this subspecies. [[NatureServe]] considers the subspecies ''Imperiled.''<ref>{{Cite web |title=''Empidonax traillii extimus''. NatureServe Explorer 2.0 |url=https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.105260/Empidonax_traillii_extimus |access-date=2023-06-23 |website=explorer.natureserve.org}}</ref> [[North American beaver|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.

==== ''E. t. traillii'' ====
The eastern [[Nominate subspecies#Nominotypical subspecies and subspecies autonyms|nominate]] subspecies of the willow flycatcher (''E. t. traillii'') was described by [[John James Audubon|Audubon]] in 1828. It breeds from the eastern coast of the United States to the western [[Rocky Mountains]].