The Southwestern Willow Flycatcher (Empidonax traillii extimus) is a small bird that breeds in dense riparian habitats across the southwestern United States. It is one of four subspecies of the widespread Willow Flycatcher and was listed as an endangered species by the U.S. Fish and Wildlife Service (USFWS) in 1995. The Southwestern Willow Flycatcher has declined dramatically since the early 20th century due to loss and fragmentation of its breeding habitat. Protection and restoration of riparian habitats are critical for the flycatcher’s recovery. The USFWS leads conservation efforts for the Southwestern Willow Flycatcher in cooperation with other federal agencies, states, tribes, local governments, businesses, and private landowners.
Taxonomy and Naming
The Southwestern Willow Flycatcher is one of four subspecies of the widespread Willow Flycatcher (Empidonax traillii) that breeds across much of North America. The Willow Flycatcher complex was originally described as a single species by John J. Audubon in 1828. Ornithologists later identified up to 15 subspecies across different geographical breeding ranges. Extensive taxonomic study in the late 20th century determined there were four distinct subspecies groups that likely represent separate evolutionary lineages and may warrant designation as separate species.
The Southwestern Willow Flycatcher subspecies (E. t. extimus) was first described in 1930 based on morphological differences and geographical isolation from other flycatcher subspecies. It breeds in Arizona, New Mexico, southern California, southern Nevada, southern Utah, western Texas, southwestern Colorado, and extreme northwestern Mexico. The Southwestern Willow Flycatcher was formally listed as an endangered distinct population segment by USFWS in 1995. This subspecies has a relatively small and fragmented breeding range and population, faces multiple threats, and shows indications of genetic and morphological distinctness. However, its taxonomic status continues to be reviewed based on new research. Other Willow Flycatcher subspecies are not federally listed.
Description
The Southwestern Willow Flycatcher is a small grayish-green passerine bird that blends well into dense riparian vegetation. Adults measure approximately 5.75–6.25 inches (15–16 centimeters) in length and weigh just 0.33–0.49 ounces (10–15 grams). They have a grayish-green back and wings, whitish throat, light grey-green belly, and two pale wingbars. The eye ring is faint or absent. The upper mandible is dark, while the lower mandible is light yellowish. The song is a sneezy “fitz-bew” and call is a repeated “whit”.
Males and females have similar plumage coloration, but males have an olive tinge on the head and back while females have more grayish hoods and backs. Juveniles are duller and buffer overall with diffuse wingbars and a paler mandible. Adult plumage is attained at 1 year old. Compared to related Willow Flycatcher subspecies, the Southwestern subtype generally shows a paler throat and belly and smaller bill, although the coloration and morphology overlap considerably. More study is needed to determine diagnostic physical characteristics.
Distribution and Habitat
The Southwestern Willow Flycatcher breeds in riparian habitats near rivers, streams, or other wetlands in the southwestern United States. Its current breeding range includes southern California, southern Nevada, southern Utah, Arizona, New Mexico, southwestern Colorado, and extreme northwestern Mexico. Areas of greatest abundance are along the Rio Grande in New Mexico, San Pedro and Gila rivers in Arizona, and the Colorado River. The wintering range extends from Mexico to northern South America.
This subspecies inhabits dense riparian habitats dominated by willows, cottonwoods, box elder, tamarisk, Russian olive, buttonbush, arrowweed, and other plants. Nesting areas typically have dense vegetation in the understory and ground cover. Habitat patches as small as 0.25 acres may be used for nesting. Some high-elevation meadows with shrubs are also occupied. Vegetation structure and composition varies across the range. Historical habitat losses have restricted many populations to dense vegetation along regulated river flows.
Life History
The Southwestern Willow Flycatcher is migratory, arriving on breeding grounds from late April to June. Upon arrival, males establish breeding territories and sing to attract females. Nesting peaks from late May through mid-July. Pair bonds are generally confined to a single breeding season.
The open cup nest is built 1-20 feet above ground in a fork or dense clump of live shrub or tree foliage. Nest height is often lower in invasive tamarisk. Materials such as grasses, fibers, spider webs, feathers, and flower petals are woven together. The female lays 3-4 eggs that hatch after 12-15 days. Young fledge from the nest at 12-15 days old and remain dependent on the parents for 2-3 weeks. Pairs may raise one or two broods per season.
The flycatcher is primarily insectivorous, hawking from a perch to catch flying insects in mid-air or gleaning from foliage. It mainly eats bees, wasps, flies, butterflies, moths, beetles, and spiders. Foraging occurs within and above dense riparian vegetation. Before migration, flycatchers feed more on berries and seeds to accumulate fat reserves.
Migration to wintering grounds begins in August and continues through September. The Southwestern Willow Flycatcher winters in the tropical and subtropical lowlands of Central America and northern South America. It returns north between April and early June to the same nesting sites, if suitable habitat conditions persist. Average lifespan in the wild is estimated between 2-3 years.
Population Status
The Southwestern Willow Flycatcher has experienced severe population declines and habitat losses since the early 1900s. Originally estimated at several thousand pairs, as low as 300-500 breeding pairs remained when it was federally listed as endangered in 1995. The most recent population estimate is approximately 1,299 territories, though the population fluctuates annually.
Major factors in the decline include habitat loss and modification from river management, urban and agricultural development, livestock grazing, and replacement by invasive plant species. Nest parasitism by Brown-headed Cowbirds (Molothrus ater) has also lowered reproductive success. Ongoing threats include changes in riparian plant communities, wildfire, drought, and climate change impacts. Population growth is constrained by high nest predation and brood parasitism rates in many areas. Conservation and recovery of the Southwestern Willow Flycatcher depends on protecting and restoring riparian plant communities throughout its breeding range.
USFWS Conservation Efforts
The U.S. Fish and Wildlife Service (USFWS) is the primary federal agency responsible for conserving and recovering endangered species like the Southwestern Willow Flycatcher under the Endangered Species Act (ESA). USFWS works closely with partners through the Southwestern Willow Flycatcher Recovery Team to address conservation threats, survey and monitor populations, restore habitat, and enhance breeding productivity. Ongoing recovery efforts focus on five key strategies:
Protect and manage habitat
USFWS designates critical habitat, develops habitat conservation plans, and works with public and private land managers to conserve and improve flycatcher breeding habitat. Conservation activities include restoring natural river flows, controlling invasive species, protecting habitat from impacts, and preventing habitat losses.
Survey and monitor populations
Coordinated, range-wide surveys are conducted each year to track flycatcher population sizes and distribution. Banding helps assess survival and dispersal. Demographic monitoring provides data on reproduction, survival, and causes of mortality to guide management.
Reduce parasitism and predation
Programs to trap and relocate Brown-headed Cowbirds or discourage nest parasitism near flycatcher habitat can improve productivity. Controlling predators like snakes, jays, and squirrels may also increase nesting success.
Captive breeding and release
Experimental captive breeding and release programs have attempted to establish new flycatcher populations and augment small ones. However, wild-bred birds show much higher survival than captive-reared birds. Captive breeding is not currently a major recovery strategy.
Increase public awareness
Education and outreach aim to build public support for flycatcher conservation and engage stakeholders in recovery efforts. USFWS also continues to study flycatcher biology to improve management strategies.
Full recovery and delisting of the Southwestern Willow Flycatcher depends on establishing multiple large, stable, and self-sustaining populations throughout its range. USFWS works proactively with many partners across the government, conservation, academic, business, and private sectors to conserve flycatcher habitat and aid the species’ recovery. Public engagement and support for endangered species conservation is vital for the flycatcher’s long-term success.
Conclusion
The Southwestern Willow Flycatcher is an endangered bird that has declined due to riparian habitat loss across the southwestern U.S. Intense conservation efforts since the 1990s have stabilized some populations but recovery is an ongoing process. The small grey-green flycatcher depends on dense riparian habitat that is rare and fragmented across its range. USFWS leads continuing efforts to survey, monitor, and restore habitat, increase productivity, and build partnerships to conserve the species. Achieving lasting recovery will require commitment from public and private stakeholders to protect remaining habitat and reverse historical habitat losses. The Southwestern Willow Flycatcher serves as an indicator of the overall health of southwestern riparian ecosystems. Its protection benefits many species that share its specialized habitat. Ongoing conservation of this unique subspecies aims to prevent its extinction and restore self-sustaining populations.