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<br />W AIS Document Retrieval
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<br />002691
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<br />southern portions of Nevada and Utah, and extreme northwestern Mexico
<br />(Hubbard 1987, Unitt 1987, Wilbur 1987). During the breeding season,
<br />the species occurs in riparian habitats along rivers, streams, open
<br />water, cienegas, marshy seeps, or saturated soil where dense growths of
<br />willows (Salix sp.), Baccharis, arrowweed (Pluchea sp.), tamarisk
<br />(Tamarix sp.) or other plants are present, sometimes with a scattered
<br />overstory of cottonwood (Populus sp.) (Gri~ell and Miller 1944,
<br />Phillips 1948, Zimmerman 1970, Whitmore 1977, Hubbard 1987, Unitt 1987,
<br />whitfield 1990, Brown and Trosset 1989, Brown 1991, Sogge et al. 1997).
<br />These riparian communities, which tend to be rare and widely separated,
<br />provide nesting, foraging, and migratory habitat for the southwestern
<br />willow flycatcher. Empidonax traillii extimus is an insectivore that
<br />forages within and occasionally above dense riparian vegetation, taking
<br />insects on the wing and gleaning them from foliage (Wheelock 1912, Bent
<br />1960) .
<br />Empidonax traillii extimus nests in dense riparian vegetation
<br />approximately 4-7 meters (m) (13-23 feet) tall, often with a high
<br />percentage of canopy cover. Historically, E. t. extimus nested
<br />primarily in willows, with a scattered overstory of cottonwood
<br />(Grinnell and Miller 1944, Phillips 1948, Whitmore 1977, Unitt 1987,
<br />Sogge et al. 1997). In addition to nesting in riparian woodland
<br />vegetation consisting of willows, arrowweed, tamarisk "or other
<br />species' I, southwestern willow flycatchers nest almost exclusively in
<br />coast live oaks (Quercus agrifolia) on the Upper San Luis Rey River in
<br />San Diego County, California, which may be defined as an oak "riparian
<br />woodland. " Following modern changes in riparian plant communities in
<br />the southwest; E. t. extimus still nests in willows where available but
<br />is also known to nest in areas dominated by tamarisk and Russian olive
<br />(Zimmerman 1970, Hubbard 1987, Brown 1988). Sedgewick and Knopf (1992)
<br />found that sites selected as song perches by male willow flycatchers
<br />exhibited higher variability in shrub size than did nest sites and
<br />often included large central shrubs. Habitats not selected for either
<br />nesting or singing were narrower riparian zones, with greater distances
<br />between willow patches and individual willow plants.
<br />Large scale losses of southwestern wetlands have occurred,
<br />particularly the cottonwood-willow riparian habitat of the southwestern
<br />willow flycatcher (Phillips et al. 1964, Johnson and Haight 1984,
<br />Katibah 1984, Johnson et al. 1987, Unitt 1987, General Accounting
<br />Office 1988, Dahl 1990, State of Arizona 1990). Changes in the riparian
<br />plant community have reduced, degraded and eliminated nesting habitat
<br />for the willow flycatcher, curtailing its
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<br />distribution and numbers (Serena 1982, Cannon and Knopf 1984, Taylor
<br />and Littlefield 1986, Unitt 1987, Schlorff 1990). Habitat losses and
<br />changes have occurred (and continue to occur) because of urban,
<br />recreational and agricultural development, fires, water diversion and
<br />impoundment, channelization, livestock grazing, and replacement of
<br />native habitats by introduced plant species (see 58 FR 39495 and
<br />Tibbitts et al. 1994 for detailed discussions of threats and impacts) .
<br />Brood parasitism by the brown-headed cowbird (Molothrus ater) is
<br />another significant and widespread threat to the southwestern willow
<br />flycatcher (Rowley 1930, Garret and Dunn 1981, Unitt 1987, Sogge 1995a
<br />and 1995b, Whitfield and Strong 1995, Sferra et al. 1997). Although
<br />some host species seem capable of simultaneously raising both cowbirds
<br />and their own chicks, such is not the case with southwestern willow
<br />flycatchers. Of all the nests monitored throughout the southwest
<br />between 1988 and 1996, there are only two cases known where
<br />southwestern willow flycatchers successfully fledged both flycatchers
<br />and cowbirds. In all other cases, parasitism caused complete nest
<br />failure or the successful rearing of only cowbird chicks (Brown 1988,
<br />Whitfield 1990, Whitfield and Strong 1995, Sogge 1995a and 1995b,
<br />Maynard 1995, Sferra et al. 1997).
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