Southwestern Willow Flycatcher

Empidonax traillii extimus

Description

The Empidonax traillii extimus (southwestern willow flycatcher) is a small songbird, about 5.8 in (15 cm) in body length. It has a grayish-green back and wings, a whitish throat, light grey-olive breast, and pale yellowish belly. There are two wingbars, and the eye-ring is faint or absent. The upper mandible is dark, and the lower is light. The southwestern willow flycatcher is distinguished from other willow flycatchers by its paler color and morphology (primarily wing formula). Its song dialect is also different, having a more protracted, slurred, "fitza-bew" with a burry "bew" syllable, rather than the crisp, sneezy "fitz-bew" of other subspecies.

Behavior

The southwestern willow flycatcher nests in thickets of trees and shrubs about 13-23 ft (4.0-6.6 m) or more in height, with dense foliage from approximately 13 ft (4.0 m) above ground, and often a dense canopy cover. The diversity of nest-plant species may be low (e.g., willows) or comparatively high (e.g., mixtures of various trees and shrubs). Nest site vegetation may be even or uneven-aged, but is usually dense and structurally homogeneous. The southwestern willow flycatcher nests in native vegetation where available, but is also known to nest in thickets dominated by tamarisk and Russian olive. Song perches selected by male birds exhibited higher variability in shrub size than nest sites, and often includes large central shrubs. Habitats not selected for either nesting or singing are narrower riparian zones, with greater distances between willow patches and individual willow plants.

The southwestern willow flycatcher virtually always nests near surface water or saturated soil. At some sites surface water may be present early in the breeding season but only damp soil later on. Ultimately, riparian vegetation is necessary.

The nest is a compact cup made of fiber, bark, and grass, typically with feathers on the rim, lined with a layer of grass or other fine silky plant material, and often with plant material dangling from the bottom. The nest is constructed in a fork or on a horizontal branch, about 3-15 ft (0.9-4.6 m) above ground in a shrub or small tree, with dense vegetation above and around the nest.

The southwestern willow flycatcher is present and singing on breeding territories by mid-May, although its status may be confused by migrating individuals of northern subspecies passing through. It builds its nest and lays eggs in late May and early June, and fledges its young in early to mid-July.

The southwestern willow flycatcher is an insectivore. It forages within and above dense riparian vegetation, taking insects on the wing or gleaning them from foliage. It also forages in areas adjacent to nest sites, which may be more open.

The migration routes and wintering grounds of the southwestern willow flycatcher are not well known. Empidonax flycatchers rarely sing during fall migration, so that a means of distinguishing sub-species is not available.

Habitat

The southwestern willow flycatcher occurs in dense riparian habitats along rivers, streams, and other wetlands. It occurs in stands where dense growths of willows, Baccharis, arrowweed, button-bush, tamarisk, Russian olive, and other woody plants are present, often with a scattered overstory of cottonwood. Throughout its range of the southwestern willow flycatcher, these riparian habitats tend to be uncommon, widely separated, small and/or linear locales, separated by vast expanses of arid habitat.

Distribution

The breeding range of the southwestern willow flycatcher includes southern California, southern Nevada, southern Utah, Arizona, New Mexico, western Texas, southwestern Colorado, and extreme northwestern Mexico. It most likely winters in Mexico, Central America, and perhaps northern South America.

Threats

The southwestern willow flycatcher is endangered by extensive loss of habitat, brood parasitism by cowbirds, and lack of adequate protective regulations. Large scale losses of southwestern wetlands have occurred, particularly affecting the cottonwood-willow riparian habitats of the southwestern willow flycatcher. Changes in riparian plant communities have resulted in the reduction, degradation, and elimination of nesting habitat for the rare flycatcher. As much as 90% of major lowland riparian habitat has been lost or modified in Arizona, and riparian forests are the most highly modified of the natural landscapes of California.

The destruction and modification of southwestern riparian habitats have occurred because of urban and agricultural development, water diversion and impoundment, channelization, livestock grazing, off-road vehicle and other recreational uses, and hydrological changes resulting from these and other land-use practices. Loss of the cottonwood-willow riparian forests has had widespread impact on the distribution and abundance of bird species associated with that forest type.

Overuse by livestock has been a major factor in the degradation and modification of riparian habitats in the western United States. These effects include changes in plant community structure and species composition, and in the relative abundance of plant species. These changes are often linked to more widespread changes in watershed hydrology and directly affect the habitat characteristics critical to the southwestern willow flycatcher. Livestock grazing in riparian habitats typically results in reduction of plant species diversity and density, especially of palatable broadleaf plants like willows and cottonwood saplings, and is one of the most common causes of riparian degradation.

Another factor in the loss and modification of southwestern willow flycatcher habitat is invasion by the exotic tamarisk (or saltcedar). Tamarisk was introduced into western North America in the late 1800s as an ornamental windbreak and for erosion control. It has spread rapidly along southwestern watercourses, typically at the expense of native plants, especially those of cottonwood/willow communities. Although tamarisk is present in nearly every southwestern riparian community, its dominance varies. It has replaced some native communities entirely, but occurs at a low frequency in others. Further, tamarisk establishment often results in a self-perpetuating regime of periodic fires, which were uncommon in native riparian woodlands. The rapid spread of tamarisk has coincided with the decline of the southwestern willow flycatcher. Although the southwestern willow flycatcher will nest in tamarisk, it is not known whether its reproductive success differs from that when nesting in native vegetation. However, ornithologists believe that success is lower in tamarisk-dominated habitats.

Water developments have also likely reduced and modified southwestern willow flycatcher habitat. The series of dams along most southwestern rivers (the Colorado, Gila, Salt, Verde, Rio Grande, Kern, San Diegito, and Mojave Rivers) have altered riparian habitats downstream through hydrological changes, vegetational changes, and have inundated habitats upstream. Logging in the upper watersheds of southwestern rivers may constitute another potential threat to the southwestern willow flycatcher by disturbing the nesting habitat.

Its habitat rarity and small, isolated populations make the remaining flycatchers susceptible to local extirpation through stresses such as floods, fire, brood parasitism, predation, depredation, and land development. In early 1993, catastrophic floods in southern California and Arizona affected much of the remaining breeding habitat. Historically, these floods have always destroyed habitat, but they were also important in regenerating the cottonwood-willow community. However, with little southwestern willow flycatcher habitat remaining, widespread events like those of 1993 could destroy virtually all the remaining habitat of the rare bird. Further, regeneration by natural vegetation after floods may be inhibited if the area is subjected to overgrazing by domestic livestock.

Predation of southwestern willow flycatchers may be a significant threat and may be increasing with habitat fragmentation. In the lower Colorado River valley, the great-tailed grackle (Quiscalus mexicanus ) preys on the eggs and young of other birds. Predation increases with decreasing distance from nests to thicket edges, suggesting that habitat fragmentation may increase the threat of predation.

Brood parasitism by the brown-headed cowbird (Molothrus ater ) also threatens the southwestern willow flycatcher. Cowbirds lay their eggs in the nests of other, usually smaller, songbirds. The host birds incubate the cowbird eggs, which typically hatch prior to those of the host. In most cases, the host has little or no reproductive success. Cowbirds have increased greatly in the range of the southwestern willow flycatchers, mostly because of the spread of cattle, and this is a further stress for the rare flycatcher.

Conservation and Recovery

No conservation plans or habitat restoration projects specific to the southwestern willow flycatcher exist on lands managed by the U.S. Fish and Wildlife Service (FWS), Forest Service, Bureau of Land Management (BLM), Bureau of Reclamation, Indian Nations, or State agencies. The FWS and BLM have focused some attention on modifying livestock grazing practices in recent years, particularly as they affect riparian ecosystems. Reclamation and restoration of riparian habitat has also been undertaken as a mitigation for damage caused by development projects within the range of the southwestern willow flycatcher, including some historical nesting locations. The BLM currently manages about 40 mi (64 km) of the upper San Pedro River in Arizona (including historic nest sites) as a Riparian National Conservation Area. Riparian habitat rehabilitation is also underway at several National Wildlife Refuges managed by FWS in the breeding range of the southwestern willow flycatcher. The Nature Conservancy manages one of the largest remaining flycatcher populations, as well as several other areas with high recovery potential. The U.S. Marines have maintained a cowbird control program near the Santa Margarita River to benefit the least Bell's vireo. This program has also benefited southwestern willow flycatchers nesting there. Grand Canyon National Park has instituted a seasonal recreation closure at the remaining site with nesting willow flycatchers in the Grand Canyon, and has begun a cowbird monitoring program.

The designation of critical habitat for the southwestern willow flycatcher, originally encompassing about 640 mi (1,024 km) of riparian habitat in California, Arizona, and New Mexico, has been deferred while FWS gathers further data.

Contacts

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Building
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
P.O. Box 1306
Albuquerque, New Mexico 87103-1306

Telephone: (505) 248-6911
Fax: (505) 248-6915
http://southwest.fws.gov/

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
P.O. Box 25486
Denver Federal Center
Denver, Colorado 80225
http://www.r6.fws.gov/

Reference

U.S. Fish and Wildlife Service. 1995. "Determination of Endangered Status for the Southwestern Willow Flycatcher." Federal Register 60 (38): 10693-10715.