8031

• UCREFRP Catalog Number: 8031
• Author: U.S. Department of the Interior.
• Title: Fish and Wildlife Service Final Biological and Conference Opinion on Lower Colorado River Operations and Maintenance - Lake Mead to Southerly International Boundary.
• USFW Year: 1997.
• USFW - Doc Type: \
• Copyright Material: NO

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BIOLOGICAL OPINION

Status of the Species

or sallying, to capture flying insects. The southwestern willow flycatcher is a riparian obligate,
nesting along rivers, streams, and other wetlands where dense growths of willow (Salix sp.),
Baccharis, buttonbush (Cephalanthus sp.), box elder (Acer negundo), saltcedar (Tamarix sp.) or
other plants are present, often with a scattered overs tory of cottonwood (Populus sp.) or willow
or both.

Empidonax traillii extimus is one of five currently-recognized willow flycatcher subspecies (Phillips
1948, Unitt 1987, Browning 1993). It is a neotropical migratory species that breeds in the
southwestern U.S, and migrates to Mexico, Central America, and possibly northern South America
during the non-breeding season (Phillips 1948, Stiles and Skutch 1989, Peterson 1990, Ridgely and
Tudor 1994, Howell and Webb 1995). The historical range of the southwestern willow flycatcher
included southern California, Arizona, New Mexico, western Texas, southwestern Colorado,
southern Utah, extreme southern Nevada, and extreme northwestern Mexico (Sonora and
Baja)(Unitt 1987).

Life History

The southwestern willow flycatcher is an insectivore, foraging within and above dense riparian
vegetation, taking insects on the wing or gleaning them from foliage (Wheelock 1912, Bent 1960).
No information is available on specific prey species, However, fecal samples containing
identifiable invertebrate body parts were collected during banding operations from more than 70
southwestern willow flycatchers in California, Arizona, and southwestern Colorado (M. Sogge,
pers. comm.); however, analyses of these samples have not yet been conducted.

The southwestern willow flycatcher begins arriving on breeding grounds in late April and May
(Sogge and Tibbitts 1992, Sogge et al. 1993, Sogge and Tibbitts 1994, Muiznieks et al. 1994,
Maynard 1995, Sferra et al. 1995). Migration routes are not completely known. However, willow
flycatchers have been documented migrating through specific locations and drainages in Arizona
that do not currently support breeding populations, including the upper San Pedro River (BLM,
unpubl. data), Colorado River through Grand Canyon National Park (Sogge and Tibbitts 1992,
Sogge et al. 1993, Sogge and Tibbitts 1994), LCR (Muiznieks et al. 1994, Spencer et al. 1996),
Verde River tributaries (Muiznieks et al. 1994), and Cienega Creek (Bureau of Land Management,
in liu,). These observations probably also include subspecies E,t. brewsteri and E.!. adastus.
Empidonax flycatchers rarely sing during fall migration, so distinguishing different subspecies of
migrating Empidonax without specimens in hand is not feasible (Blake 1953, Peterson and Chalif
1973). However, willow flycatchers have been reported to sing and defend winter territories in
Mexico and Central America (Gorski 1969, McCabe 1991).

Southwestern willow flycatchers begin nesting in late May and early June and fledge young from
late June through mid-August (Willard 1912, Ligon 1961, Brown 1988, Whitfield 1990, Sogge and
Tibbitts 1992, Sogge et al. 1993, Muiznieks et al. 1994, Whitfield 1994, Maynard 1995). They

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