Is the southwestern willow flycatcher endangered?

The southwestern willow flycatcher (Empidonax traillii extimus) is a small migratory bird that breeds in dense riparian habitats across the southwestern United States. This subspecies of the willow flycatcher has declined dramatically in the past few decades and is currently listed as endangered under the Endangered Species Act. Understanding the flycatcher’s endangered status, the reasons for its decline, and ongoing conservation efforts can provide important insights into protecting vulnerable species and habitats.

Quick Facts on the Southwestern Willow Flycatcher

• Scientific name: Empidonax traillii extimus
• Size: 5-6 inches long with a wingspan of 8-9 inches
• Coloration: Olive to gray-green upperparts, whitish throat, light olive breast, two wingbars
• Diet: Insects
• Habitat: Dense riparian vegetation near surface water
• Range: Arizona, New Mexico, southern California, southern Utah, southwestern Colorado, western Texas
• Population: Approximately 1,299 breeding territories as of 2014
• Conservation status:
  • US Fish and Wildlife Service: Endangered (1995)
  • IUCN Red List: Endangered (2004)
• Threats: Habitat loss and degradation, drought, climate change, cowbird parasitism

The southwestern willow flycatcher is a small grayish-green songbird that nests in thickets of trees and shrubs along rivers and streams in the southwestern United States. It has experienced severe population declines since the mid-20th century due to loss and modification of its riparian habitat. Various state and federal agencies consider it a highly endangered species.

Endangered Species Act Listing

The southwestern willow flycatcher was listed as endangered under the U.S. Endangered Species Act in 1995. This listing provides legal protections for the species including:

• Prohibition of “take” – Hunting, harming, harassing, or killing flycatchers is illegal.
• Requirement to designate critical habitat – Areas essential to flycatcher conservation must be identified.
• Development of a recovery plan – Goals and strategies to improve flycatcher populations must be created.
• Authorization of funding for conservation programs – Federal grants help fund habitat restoration and monitoring.

Listing under the Endangered Species Act is based on assessments of extinction risk by the U.S. Fish and Wildlife Service (USFWS). The USFWS listed the southwestern willow flycatcher as endangered based on evidence of:

• Extreme population declines – Estimated 80-90% loss of breeding territories range-wide.
• Habitat destruction – Riparian areas greatly reduced and degraded throughout southwest.
• Small population size – Just 344 known breeding territories in 1993.
• Threats from cowbird parasitism – Brood parasitism impacts nesting success.

These factors led the USFWS to determine that the southwestern willow flycatcher was in danger of extinction throughout all or much of its range. This warranted endangered status and the increased protections offered under the Endangered Species Act.

Population Status and Distribution

The southwestern willow flycatcher has a very limited breeding range concentrated in the southwestern U.S. Arizona, southern California, southern Utah, New Mexico, southwestern Colorado, western Texas, and extreme southern portions of Nevada contain the majority of its remaining habitat.

As of 2014, the known flycatcher population consisted of approximately 1,299 breeding territories across this range. However, population estimates have varied widely over the last few decades as survey efforts have increased. Table 1 summarizes flycatcher population data from 1982-2014:

These numbers reflect known flycatcher breeding territories based on surveys. Total population numbers are likely higher as some flycatchers inevitably go undetected during surveys. However, most experts agree that the species declined dramatically between the 1960s-1990s before stabilizing at a small fraction of historical numbers. Current populations remain critically low compared to historical records.

The flycatcher’s breeding range covers approximately 120,000 square miles. However, its habitat needs limit it to scattered riparian patches totaling less than 2,000 square miles within this larger range. About 75% of known flycatchers breed in Arizona and New Mexico, mostly along the Gila, Rio Grande, Verde, and Salt rivers. Other river basins with significant populations include the Kern, Santa Margarita, San Luis Rey, and San Diego rivers in southern California.

Reasons for Decline

Scientists cite habitat destruction as the primary cause of the southwestern willow flycatcher’s population crash since the mid-1900s. Major factors contributing to this habitat loss and degradation include:

• Urbanization – Housing, commercial, and agricultural development has eliminated or fragmented many riverbottom woodlands.
• Dams and river management – Dams and water diversions have altered natural hydrologic regimes needed to support native riparian communities.
• Livestock grazing – Overgrazing removes the dense understory vegetation required for flycatcher nesting.
• Invasive plants – Introduced species like tamarisk and Russian olive displace native riparian plants.
• Drought and climate change – Reduced water availability stresses riparian habitat and may increase impacts of other threats.

In addition to outright habitat loss, degradation of remaining habitat quality continues to pose challenges. The introduced brown-headed cowbird often outcompetes flycatchers by laying its eggs in their nests. Predators may also exert higher pressures in fragmented habitats. Ongoing droughts may further reduce habitat suitability in some areas.

Habitat loss

The southwestern willow flycatcher evolved to nest in expansive, lush riparian woodlands along free-flowing rivers. Housing and commercial development, dams, water diversions, and flood control projects have eliminated, fragmented, or altered much of this habitat since the early-mid 20th century. Most river systems across the southwest have lost 50-95% of their historical riparian cover. These drastic habitat losses left only small, scattered patches of flycatcher breeding habitat. Small, isolated populations are highly vulnerable to extinction.

Altered river hydrology

The southwestern willow flycatcher depends on natural variation in river flows and flooding to maintain healthy riparian habitat. Dams, reservoirs, and water diversions often heavily alter hydrologic patterns downstream. This disrupts regeneration of native vegetation communities. Controlled flood regimes and reduced flooding also foster infestations of exotic plants like tamarisk. Such changes to natural river processes have degraded habitat suitability for the flycatcher.

Cowbird parasitism

The brown-headed cowbird lays its eggs in other birds’ nests, leaving hosts to raise cowbird chicks. Cowbird parasitism has been documented in up to 77% of flycatcher nests at some sites. This can significantly decrease flycatcher productivity. Cowbird populations have increased with habitat fragmentation and cattle grazing in riparian areas.

Climate change

Climate models predict increasing aridity in the desert southwest region. Ongoing droughts have already caused Some flycatcher habitat degradation. Hotter, drier conditions could compound other threats by increasing stress on limited riparian habitat. The flycatcher’s small, geographically-restricted population makes it especially vulnerable to future climate shifts.

Conservation Actions

Many government agencies, conservation groups, and landowners are engaged in efforts to protect and restore flycatcher habitat. Major components of southwestern willow flycatcher conservation include:

Habitat protection and management

Key sites are protected through designation as wildlife refuges, private reserves, and other conservation lands. Habitat enhancement projects remove invasive plants, recreate natural river dynamics, plant native vegetation, and reduce threats like grazing and recreation. Cowbird trapping and rodent control can also improve nesting success at some sites.

Monitoring and research

Regular flycatcher surveys track population trends and allow rapid response to new threats. Research on topics like cowbird parasitism, survivorship, dispersal patterns, and genetics inform habitat management. Color banding enables tracking of individual flycatchers.

Critical habitat designation

The USFWS has designated over 200,000 acres of critical habitat across southern California, Arizona, New Mexico, southern Nevada, and southern Utah. This protects essential nesting areas from destruction or adverse modification. Updated critical habitat rules were finalized in 2013 and 2021 to better reflect current habitat use.

Recovery planning

The Southwestern Willow Flycatcher Recovery Plan provides goals and strategies to stabilize populations, protect habitat, and ultimately delist the species. Updated in 2002 and 2021, the plan guides collaborative recovery efforts range-wide.

Enforcement of ESA protections

Regulatory mechanisms like section 7 consultations ensure federal actions do not jeopardize flycatchers. The ESA prohibits killing, collecting, or harming flycatchers without a permit. Habitat loss is also strictly regulated within designated critical habitat. Violations carry potentially severe civil and criminal penalties.

Outreach and partnerships

Outreach to private landowners provides options like safe harbor agreements that encourage voluntary habitat conservation. Cooperative efforts across local, state, tribal, and federal entities help pool resources and expertise. Partnerships with utilities, land managers, and developers promote compatibility between flycatchers and business operations.

Outlook for the Future

Targeted conservation efforts have stabilized flycatcher numbers since the 1990s. However, populations remain very small and vulnerable. Ongoing habitat protection and restoration are essential for the flycatcher’s recovery. Sustained funding to implement conservation actions laid out in the Recovery Plan will be key.

Climate change looms as an emerging long-term threat to flycatcher habitat. Riparian areas may be especially impacted by hotter, drier conditions in the desert southwest. Range shifts northward or to higher elevations are unlikely given the flycatcher’s limited dispersal tendencies. Effects of climate change will need to be closely monitored and addressed.

Preventing Endangered Species Act delisting will require more than simply maintaining current population levels. Significant expansion of occupied habitat and increases in flycatcher numbers will be necessary to downlist or delist the species. Realistically, the southwestern willow flycatcher will depend on active conservation management for the foreseeable future. But with committed, collaborative efforts, reaching recovery goals remains possible. Ongoing protection of this unique bird and its verdant riparian home will contribute to the broader preservation of the Southwest’s natural heritage.

References

• U.S. Fish and Wildlife Service. (2002). Southwestern Willow Flycatcher Recovery Plan. Albuquerque, New Mexico.
• U.S. Fish and Wildlife Service. (2013). Designation of Critical Habitat for Southwestern Willow Flycatcher; Final Rule. Federal Register, Vol. 78 (2).
• U.S. Fish and Wildlife Service. (2021). Endangered and Threatened Wildlife and Plants; Revision of Critical Habitat for Southwestern Willow Flycatcher; Final Rule. Federal Register, Vol. 86 (155).
• Paxton, E.H., Sogge, M.K., & Durst, S.L. (2007). The Southwestern Willow Flycatcher Breeding Site and Territory Summary – 2006. U.S. Geological Survey report to U.S. Bureau of Reclamation.
• Durst, S.L. et al. (2008). Southwestern Willow Flycatcher Breeding Site and Territory Summary – 2007. U.S. Geological Survey report to U.S. Bureau of Reclamation.
• Ellis, L.A. et al. (2008). Southwestern Willow Flycatcher Population Status – 2005. U.S. Geological Survey report to U.S. Bureau of Reclamation.
• Moore, D. & Ahlers, D. (2009). 2009 Southwestern Willow Flycatcher Study Results: Selected Sites Along the Rio Grande From Velarde to Elephant Butte Reservoir, New Mexico. Bureau of Reclamation report.
• American Bird Conservancy. (2015). Flycatchers’ Future Is High and Dry. Bird Conservation, Fall 2015 issue.
• BirdLife International. (2016). Empidonax traillii. The IUCN Red List of Threatened Species 2016.
• Cornell Lab of Ornithology. (2003). Birds of North America: Willow Flycatcher.