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Literature Review

Southwestern Willow Flycatcher

Empidonax traillii extimus

USFWS: Endangered
CDFG: Endangered

Background

Distribution, Abundance and Trends. The southwestern willow flycatcher is restricted to dense riparian woodlands and forests along the river and stream systems of southern California, primarily in Kern, San Diego, San Bernardino, and Riverside counties. Their breeding range also includes southern Nevada, Arizona, New Mexico, Utah, western Texas, and possibly southwestern Colorado. They are reported as breeding birds in Mexico, in extreme northern Baja California and Sonora. They winter in Mexico, Central America, and northern South America. This flycatcher can be found at sites where a dense growth of willows (Salix sp.), Baccharis, arrowweed (Pluchea sp.), or other plants occurs in thickets. These thickets are often associated with a scattered overstory of cottonwood (Populus fremontii) and other riparian trees. This species has also been found nesting in southern California in relatively narrow bands of riparian habitat and can utilize extremely small remnant riparian areas (one medium size willow tree) during migration (Theresa Newkirk, pers. comm.).

The historic range of the southwestern willow flycatcher in California included riparian areas throughout the southern third of the state; it was reported as common in the Los Angeles basin, the San Bernardino/Riverside area, and in San Diego County. It was also a common breeder along the lower Colorado River, near Yuma. Currently, stable nesting groups are reported from only two locations, along the South Fork of the Kern River and along the Santa Margarita River on Camp Pendleton. Elsewhere, they exist only in small scattered, remnant and isolated populations. Major causes of the decline are cowbird parasitism and destruction or disturbance in riparian habitats.

The breeding status of the southwestern willow flycatcher within the Plan area is not well known. Of the known locations at which this species has been observed, only one was confirmed as supporting breeding birds, sited by Bob McKernan in Mission Creek. Suitable breeding habitat is present in a number of locations where riparian habitat exists, in Chino Canyon, Andreas, Murray, and Palm Canyons, Millard Canyon, Whitewater Canyon, possibly Stubbe and Cottonwood Canyons. Suitable breeding habitat may also occur at Oasis de Los Osos, along the Whitewater River near the Salton Sea, at the Coachella Valley Preserve, and at Dos Palmas Preserve/ACEC.

Southwestern willow flycatchers also migrate through the Plan area en route to other breeding areas. In migration, they may use desert fan palm oasis woodland, mesquite hummocks, mesquite bosque, arrow weed scrub, desert dry wash woodland, southern sycamore-alder riparian woodland, Sonoran cottonwood-willow riparian forest, and southern arroyo willow riparian forest.

The birds begin to arrive in southern California to breed late in the spring, generally from May 15 on through the summer months, until August. Males establish and defend territories beginning shortly after arrival in mid-May. Most birds begin nesting within one week after pair formation, which occurs 10 to 14 days after their arrival. The young fledge in early July and begin to disperse approximately two weeks after leaving the nest.

They construct their nests in dense thickets of willows, mulefat, and other trees and shrubs approximately 4 to 7 meters in height. They virtually always nest near surface water or saturated soil. They have not been found nesting in habitats where the riparian zone is very narrow, or where distances between willow patches and individual shrubs is great. The southwestern willow flycatcher is an insectivore, foraging within and above dense riparian vegetation, sometimes adjacent to nest sites.

Threats and Limiting Factors. The most significant threats to the southwestern willow flycatcher in the Plan area are extensive loss and modification of riparian habitats upon which they depend and nest parasitism by the brown-headed cowbird. Other factors that have contributed to their decline include disturbance of riparian habitat by cattle, fragmentation of breeding areas, flood control activities, invasion of non-native plants in riparian habitats, degradation of habitat as a result of edge effects related to urbanization and other human activities, and sand/gravel mining. Other localized threats may include changes in fire frequency and concentrated human access within some of the riparian areas. For example, the Whitewater River area near Bonnie Bell appears to be heavily used by people. Brown-headed cowbird parasitism rates of southwestern willow flycatcher nests has been reported as ranging from 50 to 80 percent in California, to 100% in the Grand Canyon. The decline in breeding populations of the southwestern willow flycatcher, along with other small, insectivorous, open-cup nesting birds -- among them the yellow warbler and least Bell's vireo -- is well documented. It has been reported (Unitt 1987) from historical and contemporary records that the southwestern willow flycatcher has declined precipitously throughout its range in the last 50 years. Parent birds in parasitized nests either desert the nest or raise the young cowbird at the expense of their own young. Human activities, including golf courses and agriculture, attract cowbirds, thereby increasing the threat to southwestern willow flycatchers.

Special Considerations. Reduction of cowbird populations in southwestern willow flycatcher habitat has been shown to substantially benefit this species, along with other riparian bird species. The predominance of golf courses and agricultural areas, which both provide habitat for the cowbird may make control of this non-native bird difficult.