The southwestern willow flycatcher (Empidonax traillii extimus) is a small neotropical migratory bird that breeds in riparian habitats in the Southwestern U.S. and spends the non-breeding season in southern Mexico, Central America, and northern South America. Widespread loss, modification, and fragmentation of its streamside breeding habitat and documented population declines lead the U.S. Fish and Wildlife Service to list this subspecies as endangered in March of 1995.
Data from survey and monitoring efforts in Arizona, California, Colorado, Nevada, New Mexico, and Utah (>800 historic and new sites surveyed) were compiled to evaluate the current distribution, population size, breeding status, and habitat use of the southwestern willow flycatcher. Small numbers of territorial males have been confirmed at approximately 70 sites on 35 drainages in California, Arizona, and New Mexico.
Flycatchers have been detected in some historically-occupied drainages in Nevada, Utah, and Colorado, but breeding attempts of known E.t. extimus have not been verified. Rangewide, 78% of extant sites are comprised of five or fewer territories. Up to 20% of extant sites are occupied by single, unmated males. Only five known sites are comprised of 20 or more territories (two in California, two in Arizona, and one in New Mexico), and only seven drainages are known to support more than 20 territories (Rio Grande and Gila rivers in New Mexico; San Pedro River, and the confluence of the Salt River and Tonto Creek in Arizona; the San Luis Rey, Kern, and Santa Ynez rivers in California).
Preliminary data from surveys on the lower Colorado River (Lake Mead to Yuma) indicate that this drainage may also support 20 or more territories. The largest known breeding group occurs in southwestern New Mexico where approximately 135 territories have been documented on the Gila River. Overall, a total of 420 territories was documented rangewide during the 1993 to 1995 survey effort. A substantial proportion of those territories were comprised unmated males. Additional survey effort, particularly in California, may discover additional small breeding groups. Rangewide, the E.t. extimus population is probably fluctuating at between 400 and 500 territories with a substantial proportion of individuals remaining unmated.
A variety of continuing threats leave the mostly-small breeding groups vulnerable to extirpation from stochastic events alone (e.g. fires). Several breeding groups monitored intensively in California, Arizona, and New Mexico have declined concomitantly with high rates of brood parasitism by brown-headed cowbirds (Molothrus ater). Nearly all breeding groups monitored are experiencing nest depredation at rates ranging from 30 to 50%.
Exotic plant species, such as saltcedar (Tamarisk sp.), giant reed (Arundo donax), and Russian olive (Elaeagnus angustifolia), have replaced cottonwood-willow habitat throughout much of this sub-species' historic range. While 55% of extant flycatcher sites are still dominated by native broadleaf plants, 33% of all flycatcher sites are comprised of mixtures of native broadleaf and exotic species, and 12% are comprised of pure stands of saltcedar.
Recent fires in Arizona and New Mexico have eliminated significant stands of occupied flycatcher habitat. Fire remains one of the most critical threats to this subspecies. Water development projects, urban and agricultural development in and adjacent to floodplains, and overgrazing by livestock in both riparian areas and upland habitats continue to threaten occupied and potential habitat necessary for the survival and recovery of the southwestern willow flycatcher.
The ultimate need to ensure survival and recovery is to reduce habitat fragmentation so that dispersal patterns and capabilities of this subspecies result in a positive growth rate and population expansion, rather than the negative growth rates that characterize most of the small breeding groups that have been monitored. The combination of small breeding group size, high levels of brood parasitism by cowbirds and nest depredation, and concomitantly low reproductive rates indicate that demographic stochasticity is probably the most critical short-term factor influencing this subspecies.
Therefore, management actions are needed to provide immediate protection to occupied habitat and to increase reproductive output. Habitat protection and enhancement efforts in unoccupied habitat should be targeted at areas adjacent or close to existing breeding groups and at historically-occupied locations.
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