Regularity: Regularly occurring
Type of Residency: Breeding
Regularity: Regularly occurring
Type of Residency: Breeding
Global Range: (>2,500,000 square km (greater than 1,000,000 square miles)) BREEDING: central British Columbia across southern Canada and northern U.S. to New Brunswick, Prince Edward Island, and Nova Scotia, south to southern California, northern Baja California, northern Sonora, southern Arizona and New Mexico, western and central Texas, Arkansas, northern Georgia, and eastern Virginia (AOU 1998). NON-BREEDING: Nayarit and southwestern Oaxaca south to Panama and northwestern Colombia (Stiles and Skutch 1989, AOU 1998).
Subspecies brewsteri: Breeding distribution is west of the Cascades and in the Sierra Nevada from southwestern California to southwestern British Columbia (Sedgwick 2000).
Length: 15 cm
Weight: 14 grams
Generally indistinguishable from the Alder Flycatcher (E. ALNORUM), but tends to lack a conspicuous eye ring (Alder tends to have one), have a slightly longer bill, and is less green above (NGS 1983). Reliably distinguished from the Alder Flycatcher only by voice. Song is a sneezy "fitz-bew," with accent on the first syllable (Alder Flycatcher song is "rrree-BEEa" or "fee-bee-o" with accent on the second syllable) (Kaufman 1990, McCabe 1991). Breeding habitats of the two species differ somewhat, with Willow Flycatcher in more southern and western regions of North America and in more open habitats and Alder Flycatcher a more northern bird, generally breeding in shrub and alder thickets of boreal forests in the eastern U.S., Canada, and Alaska (McCabe 1991).
Habitat and Ecology
Comments: BREEDING: Strongly tied to brushy areas of willow (SALIX spp.) and similar shrubs. Found in thickets, open second growth with brush, swamps, wetlands, streamsides, and open woodland (AOU 1983). Common in mountain meadows and along streams; also in brushy upland pastures (especially hawthorn) and orchards (NGS 1983). The presence of water (running water, pools, or saturated soils) and willow, alder (ALNUS spp), or other deciduous riparian shrubs are essential habitat elements (Sanders and Flett 1989, USDA Forest Service 1994). Occurs in both mesic and drier upland conditions, but apparently reaches highest densities on wet sites (Sedgwick and Knopf 1992). It is associated with dense riparian deciduous shrub cover separated by open areas, but large contiguous willow thickets without openings are typically avoided; it does not occur in dense tree cover but will use scattered trees for song and foraging perches and gleaning substrate (USDA Forest Service 1994). Habitat preferences may overlap with alder (EMPIDONAX ALNORUM) and least flycatchers (EMPIDONAX MINIMUS), to include deciduous woods and thickets, bottomlands and swamps (Griggs 1997). Foraging habitat may overlap with western flycatcher (EMPIDONAX DIFFICILIS; Frakes and Johnson 1982).
In southwestern Ontario, generally occurs in more xeric upland sites, but in some areas uses boggy alder thickets, overlapping with alder flycatcher (Barlow and McGillivray 1983). In the Sierra Nevada of California, broad, flat meadows with willows and water are essential (Sanders and Flett 1989). In the Northern Rockies, is apparently restricted to riparian areas with adequate shrub cover (Hutto and Young 1999).
In Colorado, males and females were found to select for different habitat attributes: female-selected nest sites typically had dense willows and were similar in patch size and bush height, male-selected song perch sites were characterized by large central shrubs and high variability in shrub size. On an increasing scale, breeding sites were respectively characterized by greater willow density, larger willow patches with smaller gaps, and greater percent willow coverage than non-willow coverage (Sedgwick and Knopf 1992).
Southwestern willow flycatcher (E. T. EXTIMUS) breeds only in dense riparian vegetation near water or saturated soil. Habitat typically contains dense vegetation in the patch interior, often interspersed with small openings, sparser vegetation, or open water that creates a habitat mosaic of variable density. It nests in shrub and tree thickets 4-7 meters tall, with dense foliage 0-4 meters above the ground, and usually a high canopy coverage (USFWS 1995). The dominant plant species, size and shape of habitat patch, canopy structure and other habitat variables vary from monotypic to mixed-species stands and from simple to complex vegetation structures (Sogge et al. 1997). Habitats include dense high-elevation willow; native broadleaf shrubs and trees composed of willow, cottonwood (POPULUS spp.), boxelder (ACER NEGUNDO), ash (FRAXINUS spp.), alder, or buttonbush (CEPHALANTHUS OCCIDENTALIS); monotypic closed-canopy stands of tamarisk (TAMARIX spp.) or Russian olive (ELAEAGNUS ANGUSTIFOLIA); or a mix of native shrubs and exotic species (Sogge et al. 1997). Along the Virgin River, Utah, is restricted to shrub communities with shrub densities ranging from 70 percent to 100 percent (Whitmore 1977).
NEST SITE: Nests primarily near slow streams, standing water or seeps, swampy thickets, especially of willow and buttonbush (AOU 1983, USDA Forest Service 1994), also dogwood (CORNUS spp.), elderberry, hawthorn, rose, tamarisk, and others; in fork or on horizontal limb of shrub, usually 1-3 meters above ground (see Harris 1991). In montane habitats, nests are usually in willows at least 2 meters high with foliage density of 50-70 percent and about 1 meter of cover above the nest (Sanders and Flett 1989). Also see Sedgwick and Knopf (1992) for information on nest sites and song perches in northcentral Colorado.
Historically, southwestern willow flycatcher primarily in willows, buttonbush, and BACCHARIS spp. with a scattered cottonwood overstory. With changes in riparian plant communities, non-native tamarisk and Russian olive provide nesting habitat in some areas (Brown 1988, USFWS 1995). Along the Colorado in the Grand Canyon, for example, the flycatcher nests in tall tamarisk within 30 meters of water (Brown 1988, Sogge et al. 1997); however it is not known if nesting success differs in tamarisk compared to native vegetation (USFWS 1996). Where E. T. EXTIMUS nests in tamarisk, the tamarisk are usually taller (more than 5 meters) and denser (90 percent canopy closure) than in tamarisk-dominated areas where the flycatcher has been extirpated, and broadleaf shrubs may also be an important part of the community (Sogge et al. 1997).
NON-BREEDING: Uses same types of habitats during migration and winter as breeding season (McCabe 1991). Occurs in dense scrub, deciduous broadleaf forest, streamside gallery forest, and freshwater wetlands (Rappole et al. 1995). In western Mexico and Central America, found in humid to semi-arid scrubby fields with hedges, fences woodland and edge, plantations; frequents low to mid-vegetation levels and often comes into open (Howell and Webb 1995).
Non-Migrant: No. All populations of this species make significant seasonal migrations.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: Yes. At least some populations of this species make annual migrations of over 200 km.
Usually arrives on U.S. nesting grounds by May-June (Terres 1980). Present in California from late April to September (Biosystems Analysis 1989). Migrates through southern Arizona mainly in the first half of June and August-September (Phillips et al. 1964). Arrives in Washington in late May or early June. Fairly common migrant in Costa Rica, mid-August to late October (peak late September) and mid-March to late May (Stiles and Skutch 1989). In Ontario, tends to have an earlier and longer migration period in spring than Alder flycatcher (EMPIDONAX ALNORUM), and migrates significantly earlier in fall (Hussell 1991a, 1991b).
Comments: Eats mainly insects caught in flight; occasionally berries. Bent (1942) states that 96 percent of diet is animal matter, most of which is flying insects. Once fledglings are able to forage for themselves, are less dependent on a localized, concentrated food source (USDA Forest Service 1994).
BREEDING: Conduct most of their activity within their defended territory, but both male and female will also use adjacent areas, especially when feeding young; territory defense declines once young are fledged (USDA Forest Service 1994). In Ontario, territory size ranged from about 0.1 hectares to 0.47 hectares and averaged 0.35 hectares (Prescott and Middleton 1988); in southern Michigan, territories averaged 0.7 hectares (Walkinshaw 1966). In California, territories ranged from 0.1 hectares to 0.9 hectares, and averaged 0.2 hectares in Fresno County and 0.4 hectares on the Truckee River (USDA Forest Service 1994). Where breeding range overlaps with alder flycatchers (EMPIDONAX ALNORUM), may show territorial defense toward the other species (Prescott 1987).
NON-BREEDING: In Panama, winter home range estimated to be about 1100 square meters (Gorski 1969).
Life History and Behavior
Lifespan, longevity, and ageing
A late breeder, eggs usually laid in mid- to late-June and young fledge in August (USDA Forest Service 1994). Clutch size is three to four. Incubation lasts 12-15 days, by female. Young are tended by both parents, leave nest at 12-15 days. Site fidelity strong in both males and females (Walkinshaw 1966). May incur a high rate of cowbird parasitism (e.g., Sedgwick and Knopf 1988, Harris 1991, Brown 1988). Sometimes polygynous and may maintain polygynous trios, possibly a response to narrow habitats with high habitat productivity or other factors (Prescott 1986, Sedgwick and Knopf 1989). Singing, unmated males may be present on breeding grounds, and single pairs may breed in absence of other individuals (USDA Forest Service 1994). On one study in Ohio and Nebraska, 91 nests had 272 eggs from which 99 young fledged (36.4 percent success) and 39.5 percent of nests produced at least one young; 96 eggs and 41 nestlings were depredated (50.4 percent; Holcomb 1972).
Molecular Biology and Genetics
Statistics of barcoding coverage: Empidonax traillii
Public Records: 0
Specimens with Barcodes: 3
Species With Barcodes: 1
IUCN Red List Assessment
Red List Category
Red List Criteria
National NatureServe Conservation Status
Rounded National Status Rank: N5B - Secure
Rounded National Status Rank: N5B - Secure
NatureServe Conservation Status
Rounded Global Status Rank: G5 - Secure
Reasons: Stable or increasing over much of the large range; has declined in the Southwest (see files for subspecies EXTIMUS) and in the Pacific states and British Columbia.
Comments: Threats include factors that destroy or degrade shrubby riparian vegetation. Riparian areas are particularly vulnerable to high-intensity livestock grazing, recreation and development pressure, flooding of nesting sites, and water diversions and flood control that prevent shrub and tree regeneration (Ohmart 1996, Saab and Rich 1997). HABITAT LOSS AND FRAGMENTATION: Habitat loss and alteration is thought to be the principle cause of decline in the West. Riparian habitats, particularly cottonwood and willow (SALIX spp.) communities, have been dramatically reduced and degraded by urban development, roads, off-road vehicle use, recreation, livestock grazing, agriculture, water development projects, channelization, willow control, and encroachment by non-native species (USFWS 1996). In California, high quality habitat has nearly disappeared and remaining habitat is widely dispersed and isolated, mostly as montane meadows in the Sierra Nevada and along a few river courses (USDA Forest Service 1994). In Arizona, as much as 90 percent of lowland riparian habitat has been lost or altered (USFWS 1996). These patterns are continuing throughout the western states. NON-NATIVE VEGETATION: Tamarisk or saltcedar (TAMARIX spp.) and Russian olive (ELAEAGNUS ANGUSTIFOLIA) have invaded riparian areas throughout the West, and particularly the southwest, and are likely factors in the flycatcher's decline. Tamarisk has replaced some riparian communities completely, but is less common in others. Tamarisk replaces the preferred multi-layered shrub community with a monotypic stand with one shrub layer, decreases plant and insect diversity, and can increase the frequency and intensity of fire. Dams and flood control, and irrigation water high in salts also give tamarisk a competitive edge over native vegetation. Although may nest in tamarisk where it provides the right vegetation structure, tamarisk may provide poor quality habitat and some studies have documented low breeding densities and low reproductive success in tamarisk (USFWS 1995, 1996; Sogge et al. 1997). LIVESTOCK: Placement of nests near the edges of shrubs makes them vulnerable to direct disturbance or destruction by livestock (Sanders and Flett 1989). Cattle and sheep browse on shrubs in the mid-level preferred by willow flycatchers, and consume or trample young woody plants (Sanders and Flett 1989). Heavy or poorly timed livestock grazing damages deciduous shrubs and can prevent shrub regeneration, reducing flycatcher habitat. Cattle prefer willow and cottonwood shoots to tamarisk and other non-natives, further depleting flycatcher habitat. Streambank trampling and soil compaction also adversely affect the water table, reduce free water, and discourage shrub growth (Flett and Sanders 1987). In Oregon, populations increased after reduction in cattle grazing and cessation of poisoning and removal of riparian willows (Taylor and Littlefield 1986). BROOD PARASITISM: Is a common host to brood parasitism by the brown-headed cowbird (MOLOTHRUS ATER; Bent 1942, King 1955, Walkinshaw 1966, Flett and Sanders 1987, Ehrlich et al. 1988, Sedgwick and Knopf 1988, Sanders and Flett 1989). Brood parasitism may pose a significant threat, particularly in its western range where habitat is limited and fragmented, and where livestock are often present in meadows and riparian habitats. In California, parasitism rates are high in the lowlands (e.g., 13 of 19 nests; Harris 1991), but much less in the Sierra Nevada (e.g., 1 of 22 nests; Sanders and Flett 1989). High parasitism rates (11 of 27 nests) were recorded at high elevations in northcentral Colorado, but cowbird eggs were accepted by the hosts at only 2 of these nests (Sedgwick and Knopf 1988). In Arizona, cowbirds were documented at all 12 known breeding locations and parasitism documented at 50 percent of the sites (USFWS 1996). In the Grand Canyon, cowbirds occur at all flycatcher breeding sites and typically 50 percent of nests are parasitized (Brown 1994, Sogge et al. 1997). In hundreds of monitored nests of southwestern willow flycatcher, brood parasitism caused either nest failure or successful rearing of only cowbird chicks (Sogge et al. 1997). There is, however, evidence of adaptive behavior toward cowbird parasitism in some populations. Parasitized nests may be abandoned or dismantled; renesting may occur in some cases (Sedgwick and Knopf 1988, Harris 1991), although fewer eggs may be laid (Holcomb 1974). Flycatchers will chase female cowbirds near nests, and some may respond to cowbird calls by becoming quieter and less conspicuous (Uyehara and Narins 1995). WATER DEVELOPMENTS: Impoundments, channelization, and water diversions have greatly reduced and modified native riparian habitats. Reservoirs flood native riparian communities, and regulated flows reduce the seasonal flooding that many riparian plants need for regeneration. Non-native plants (such as tamarisk in the southwest) often invade new habitat created along reservoir shorelines, or riparian communities of rivers with controlled or reduced flows from water diversion and dams. Channelization also alters the river system dynamics needed to maintain riparian communities (USFWS 1996). Some remaining populations in Arizona are threatened by fluctuating reservoir levels and flooding (Latta et al. 1999).
Subspecies brewsteri: While there appear to be moderate populations in early-seral upland forest habitats the species is especially threatened by population declines in valley habitats, lower nest success in these valley habitats than in early-seral forest, and continual loss of riparian habitat (Altman 2003).
Restoration Potential: Has become so rare in the southwest and California that extirpation from many remaining breeding areas seems likely without dramatic management efforts and habitat restoration. Elsewhere in the West, riparian and meadow restoration is needed to sustain the species and reverse declining trends. Fortunately, riparian and willow habitats are resilient where natural hydrologic dynamics and native vegetation are restored. Where cowbirds are affecting population viability, measures to reduce cowbird presence during the breeding season will be needed. Less is known about the restoration needs in the eastern and northwestern portions of its range, but these populations currently seem somewhat more abundant and stable.
Preserve Selection and Design Considerations: Minimum area requirements and patch dynamics are still largely unknown. Small openings in deciduous shrub habitats or adjacent stream edges increase habitat suitability, and large, contiguous willow patches apparently do not support willow flycatchers at the interior of the patch (USDA Forest Service 1994). In California montane meadows, the smallest documented nesting area is a 0.25 hectare meadow in the Sierra Nevada, but other observers found most nesting territories in meadows greater than 8.0 hectares and none in meadows less than 0.4 hectares (USDA Forest Service 1994). For southwestern willow flycatcher (E. T. EXTIMUS), patches as small as 0.5 hectares have been found to support one to two pairs, and habitat patches range from 0.5 to 1.2 hectares (USFWS 1995).
Management Requirements: HABITAT MANAGEMENT: Will benefit from maintaining communities of deciduous shrubs in riparian areas and meadows, with patches of dense shrubs interspersed with openings, and with open water nearby. In areas where populations have declined, existing breeding areas should be protected from habitat loss. It should be noted that unoccupied sites are not necessarily unsuitable, as populations may be dynamic (Harris et al. 1987).
In the Sierra Nevada, needs riparian areas and wet meadows at least 0.25 hectares in size with openings and large, dense patches of deciduous shrubs, and dense foliage at mid-heights (1-2 meters); habitat areas of at least 8 hectares are optimal. Territories contained 5-80 percent willow (SALIX spp.) cover (average 44 percent), 18-78 percent (average 54 percent) foliage density in the 0-1 meter shrub layer, and 45-96 percent (average 69 percent) foliage density in the 1-2 meter shrub layer (Sanders and Flett 1989).
Management recommendations for montane meadows include: (1) maintain riparian deciduous shrubs at least 1-2 meters high in patches greater than 0.1 hectares (100 square meters); (2) maintain more than 40 percent foliage cover density in lower 2 meters of the deciduous shrub layer; (3) maintain shrub patches interspersed with openings, and opening or paths should be at least 2 meters wide to allow aerial foraging (USDA Forest Service 1994).
Critical habitat areas containing the remaining known populations of southwestern willow flycatcher (E. T. EXTIMUS) were identified by the U.S. Fish and Wildlife Service in Southern California, Arizona, and New Mexico (USFWS 1997a, 1997b). In Arizona, habitat recommendations include: (1) establish a "no net loss" policy; increasing suitable riparian habitat and promoting regeneration of native plants; (2) using buffer zones between riparian habitats and adjacent developments; (3) restore connectivity of natural reaches of habitat by restoring degraded segments; (4) establish areas of slow water and backwater; (4) manage for large contiguous habitat blocks rather than isolated fragments (Latta et al. 1999). Also see Deshler et al. (1997) for an extensive bibliography on this subspecies.
WATER MANAGEMENT: The species requires saturated soils, standing water or flowing water near nesting sites. Natural flooding and channel meandering can promote native riparian vegetation communities. Maintaining wetlands and wet meadows will help sustain willow communities. Arizona recommendations include: managing water diversions and groundwater withdrawal to maintain streamside vegetation and mimic natural stream flow regimes, including periodic floods (Latta et al. 1999).
GRAZING: In Oregon, populations dramatically increased after reducing cattle grazing and ceasing the poisoning and removal of riparian willows (Taylor and Littlefield 1986). Reducing or eliminating grazing during the nestling period would reduce direct damage by livestock (USDA Forest Service 1994). Flycatchers will nest near cattle trails (Sanders and Flett 1989), and in some circumstances cattle might be used to create trails and openings in exceptionally dense willow stands (outside the breeding season) to benefit flycatchers. Livestock management recommendations include: (1) eliminate livestock use during the breeding period from early June to mid-August; (2) manage stocking rates and timing to encourage riparian shrub growth and vigor; (3) use exclosures to protect sensitive areas from browsing and soil compaction (USDA Forest Service 1994).
BROOD PARASITES: Eliminating livestock in breeding habitat during the breeding season can help alleviate cowbird brood parasitism. Cowbird trapping and control is intensive, expensive, and must be ongoing to be effective. However, control efforts on two sites in California in the early 1990s helped conserve isolated populations, stabilizing numbers at one site and increasing nest success at the other (USFWS 1996). Management recommendations to reduce the impacts of cowbirds include: (1) manage livestock to prevent aggregations near breeding habitat during nest building, egg-laying and incubation, late May to late July; (2) ideally, corrals, pack stations, and other facilities that concentrate stock should be 5 to 10 kilometers from important habitat areas, and at least 1 kilometer from nest sites; (3) implement cowbird trapping if necessary in coordination with state fish and wildlife agencies (USDA Forest Service 1994). In Arizona, recommendations include: reducing parasitism rates to less than 20 percent; monitoring nests; implementing trapping programs where parasitism rates exceed 20 percent (Latta et al. 1999).
Management Research Needs: Details of habitat and management needs in the eastern extent of its range are largely unknown. Where populations are depleted in the West, annual population and productivity monitoring is needed as well as surveys to determine presence/absence, distribution, and habitat relationships. Information is needed on landscape relationships, minimum area requirements, patch dynamics; also effect of breeding habitat isolation and connectivity. Further study is needed of habitat preferences throughout the species range, particularly in relation to productivity, use of non-native vegetation, relationship to land management activities, and response of flycatchers to habitat restoration. More information is needed on parasitism rates, productivity of parasitized nests, response to parasitism, and activities and habitat that promote the incidence of cowbirds. Effects of pesticides on the species are unknown. Information on winter habitat use, winter ecology, and threats on the wintering grounds are almost entirely lacking.
Biological Research Needs: Many aspects of the taxonomy, biology and ecology remain unstudied. Diet composition, winter habits and ecology are unknown. Genetic studies of subspecies are needed. Migration patterns and the location of wintering sites for different subspecies remain unknown, largely due to the difficulty of distinguishing the species outside the breeding season. Demographics (productivity and survivorship) need further study, especially in relation to habitat quality and to landscape variables at different scales. Variation in songs and calls across the species range and among subspecies need study.
Relevance to Humans and Ecosystems
Stewardship Overview: Found in shrubby deciduous habitats, especially riparian areas and meadows with shrubby patches dominated by willows (SALIX spp.) or alder (ALNUS spp.). Has undergone dramatic population declines in the past 30 years, particularly throughout its western range where its preferred riparian and willow habitats have been lost and degraded on a broad scale. The bulk of recent research and management focus on the species has been in California and the Southwest where populations are critically low. Much further study of habitat requirements and response to land management activities and restoration is needed.
The willow flycatcher (Empidonax traillii) is a small insect-eating, neotropical migrant bird of the tyrant flycatcher family. There are four subspecies of the willow flycatcher currently recognized, all of which breed in North America (including three subspecies which breed in California). Empidonax flycatchers are almost impossible to tell apart in the field so biologists use their songs to distinguish between them. The binomial commemorates the Scottish zoologist Thomas Stewart Traill.
Description and ecology
Adults have brown-olive upperparts, darker on the wings and tail, with whitish underparts; they have an indistinct white eye ring, white wing bars and a small bill. The breast is washed with olive-gray. The upper part of the bill is gray; the lower part is orangish. At one time, this bird and the alder flycatcher (Empidonax alnorum) were considered to be a single species, Traill's flycatcher. The willow and alder flycatchers were considered the same species until the 1970s. Their song is the only reliable method to tell them apart in the field.
|length||130–150 mm (5.2–6 in)|
|weight||13.5 g (0.48 oz)|
|wingspan||220 mm (8.5 in)|
|wing||68.7–75.6 mm (2.70–2.98 in)|
|tail||54–64.5 mm (2.13–2.54 in)|
|culmen||10.5–12.3 mm (0.41–0.48 in)|
|tarsus||15.5–18.0 mm (0.61–0.71 in)|
These neotropical birds migrate to Mexico and Central America, and in small numbers as far south as Ecuador in South America, often selecting winter habitat near water. Willow flycatchers travel approximately 1,500–8,000 km (930–4,970 mi) each way between wintering and breeding areas.
This bird's song is a sneezed fitz-bew. The call is a dry whit.
This bird competes for habitat with the alder flycatcher where their ranges overlap.
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.
E. t. brewsteri – The little willow flycatcher
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 Cascades, extending to the coast in northern California.
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.
E. t. extimus – The southwestern willow flycatcher
The southwestern willow flycatcher (E t. extimus) is a federally endangered subspecies. 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 (800 m) at the South Fork Kern River and 3,000 ft (910 m) at upper San Luis Rey River in California and to over 8,530 ft (2,600 m) in Arizona and southwestern Colorado. 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. The population is declining due to habitat loss. 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.
E. t. traillii
- BirdLife International (2012). "Empidonax traillii". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
- Paxton, Eben H.; Unitt, Philip; Sogge, Mark K.; Whitfield, Mary; Keim, Paul (2011). "Winter Distribution of Willow Flycatcher Subspecies". The Condor 113 (3): 608–618. doi:10.1525/cond.2011.090200. Retrieved 17 February 2013.
- "Southwestern Willow Flycatcher (Empidonax traillii extimus)". U.S. Fish and Wildlife Service Nevada Office. Retrieved 16 February 2013.
- "Willow flycatcher". All About Birds. Cornell Lab of Ornithology. Retrieved 16 February 2013.
- Godfrey, W. Earl (1966). The Birds of Canada. Ottawa: National Museum of Canada. p. 255.
- Sibley, David Allen (2000). The Sibley Guide to Birds. New York: Knopf. p. 326. ISBN 0-679-45122-6.
- Sogge, Mark K.; Marshall, Robert M.; Sferra, Susan J.; Tibbitts, Timothy J. (May 1997). A Southwestern Willow Flycatcher Natural History Summary and Survey Protocol: Technical Report NPS/NAUCPRS/NRTR-97/12 (Report). National Park Service and Northern Arizona University. p. 37. Retrieved 17 February 2012.
- Sedgwick, James A. (2001). "Geographic Variation in the Song of Willow Flycatchers: Differentiation between Empidonax traillii adastus and E. t. extimus". The Auk 118 (2): 366–379. doi:10.1642/0004-8038(2001)118[0366:GVITSO]2.0.CO;2. Retrieved 17 February 2012.
- Paxton, E.H. (2000). Molecular genetic structuring and demographic history of the Willow Flycatcher (M.Sc.). Flagstaff, Arizona: Northern Arizona University. Retrieved 17 February 2012.
- Gill, F.; Donsker, D., eds. (2014). "IOC World Bird List (v 4.4)". doi:10.14344/IOC.ML.4.4. Retrieved 14 November 2014.
- (S. Laymon, pers. comm.)
- Craig, Diane; Williams, Pamela L. (1998). Willow Flycatcher (Empidonax traillii). The Riparian Bird Conservation Plan: a strategy for reversing the decline of riparian-associated birds in California. California Partners in Flight. (Report) (PRBO Conservation Science). Retrieved 16 February 2013.
- Finch, Deborah M.; Stoleson, Scott H. (2000). Status, ecology, and conservation of the Southwestern Willow Flycatcher. Gen. Tech. Rep. RMRS-GTR-60. (Report). Ogden, Utah: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. p. 131. Retrieved 17 February 2012.
Names and Taxonomy
Comments: Sometimes treated as E. brewsteri, a junior synonym. Formerly regarded as conspecific with E. alnorum as E. traillii, Traill's Flycatcher (AOU 1998). See Phillips (1948) for a review of geographic variation in morphology, with the original descriptions of subspecies alascensis and extimus. Unitt (1987) reviewed infraspecific variation and concluded that four subspecies (brewsteri, extimus, adastus, and traillii) are recognizable. Paxton (2000) concluded that E. t. extimus is genetically distinct from other subspecies. Sedgwick (2001) demonstrated that E. t. adastus and E. t. extimus each have distinctive songs and used vocal signatures to determine distributional limits of the two subspecies. The two song types seem to be largely allopatric, separated by latitude and/or elevation. The two groups appear to be evolving independently of one another and warrant at least subspecific status.