WSP00248

• File Number: 8220.101.09
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 10/1/1997
• Author: USDOI-BOR
• Title: Biological Assesment of the October 1997 Fall Test Flow from Glen Canyon Dam
• Water Supply Pro - Doc Type: Biological Opinion

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SOUTHWESTERN WILLOW FLYCATCHER

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Tu;oDomy

The southwestern willow flycatcher (TYRANNIDAE: Empidonax traWi extimus) is an
endangered subspecies that nests in post-dam riparian vegetation and occurs along the Colorado
River downstream from Glen Canyon Dam. This is a small (13-15 em) passerine bird with
grayish/greenish upper parts, a whitish throat, an olive grey breast and yellowish belly. Two pale
wingbars are visible, but the eye ring is lacking or indistinct (New Mexico Department of Game
and Fish 1986; U.S. Fish and Wildlife Service 1993). Females tend to be somewhat smaller
(phillips 1948), but as is typical of the Tyrannidae, there is no sexual dimorphism (Seutin 1981).
Willow flycatchers are Neotropical migrants with a broad breeding range, extending from Nova
Scotia to British Columbia and south to Baja California. It is an insectivorous riparian obligate
bird (Hunter et al. 1981), preferring habitat near open water (Gorski 1969; Sogge 1994).

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Th~ taxonomy of the 10 North American Empidonax species has undergone several revisions
(Browning 1993). Willow flycatchers and the more northern alder flycatcher (E. a/norum) were
once considered to be a single species, the Traill's flycatcher (E trail/ii), and some authors have
lumped both into a superspecies, the "trail/ii complex" (U.S. Fish and Wtldlife Service 1993).
However, genetic and behavioral studies have supported the designstion of two reproductively
isolated species (Seutin and Simon 1988).

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Although there is much individual variation (phillips \94&), the southwestern willow flycatcher is
distinguished from the other E trai/lii subspecies by distribution, morphology and color, nesting
ecology, and song dialect (Aldrich \953; King 1955; Sogge 1994). First described by Phillips
(1948), E. t. extimus is one of at least four commonly recognized willow flycatcher subspecies
(U.S. Fish and Wildlife Service 1992). Although there is undoubtedly overlap, E!. traim;
(perhaps E. t. campestris) breed east of the. Rocky mountains; E.!. brewsteri breeds on the west
coasfofNorth America from British Columbia to south Central California; and Et. adastus
breeds from the Cascades and Sierras in California, east to the Rocky Mountains. E. t. extimus is
the southernmost subspecies, breeding from southern California to west Texas (Unitt 1981; U.S.
Fish and Wildlife Service 1992; Browning 1993).

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The characteristic territorial call of E t. extimus is a "fitz-bew," most frequently heard in the
morning before 10 AM (Tibbitts et aI. 1994). The four subspecies possibly may be differentiated
by characteristics of this call. The U.S. Bureau of Reclamation is currently funding the National
Biological Survey to evaluate this hypothesis (Sedgwick 1994). Another voca1i,,"tion, the
"whitt," an alarm or contact cal1, is less frequently heard.

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DistributioD

E. t. extimus is rare in the southwestern United States. Its historic breeding range includes
Arizona, New Mexico, southern California, and southern portions of Nevada, Utah, and perhaps

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