Empidonax traillii

Sometimes treated as E. brewsteri, a junior synonym. Formerly regarded as conspecific with E. alnorum as E. traillii, Traill's Flycatcher (AOU 1998). See Phillips (1948) for a review of geographic variation in morphology, with the original descriptions of subspecies alascensis and extimus. Unitt (1987) reviewed infraspecific variation and concluded that four subspecies (brewsteri, extimus, adastus, and traillii) are recognizable. Paxton (2000) concluded that E. t. extimus is genetically distinct from other subspecies. Sedgwick (2001) demonstrated that E. t. adastus and E. t. extimus each have distinctive songs and used vocal signatures to determine distributional limits of the two subspecies. The two song types seem to be largely allopatric, separated by latitude and/or elevation. The two groups appear to be evolving independently of one another and warrant at least subspecific status.

This species has a large range and large population; however, it has declined over the short and long-term rangewide, especially in the Southwest (see subspecies extimus) and in the Pacific states and British Columbia.

Breeding: central British Columbia across southern Canada and northern U.S. to New Brunswick, Prince Edward Island, and Nova Scotia, south to southern California, northern Baja California, northern Sonora, southern Arizona and New Mexico, western and central Texas, Arkansas, northern Georgia, and eastern Virginia (AOU 1998).
Non-breeding: Nayarit and southwestern Oaxaca, Mexico, south to Panama and northwestern Colombia (Stiles and Skutch 1989, AOU 1998).

Subspecies brewsteri: Breeding distribution is west of the Cascades and in the Sierra Nevada from southwestern California to southwestern British Columbia (Sedgwick 2000).

There are hundreds if not thousands of occurrences of this species throughout its breeding range

Threats include factors that destroy or degrade shrubby riparian vegetation. Riparian areas are particularly vulnerable to high-intensity livestock grazing, recreation and development pressure, flooding of nesting sites, and water diversions and flood control that prevent shrub and tree regeneration (Ohmart 1996, Saab and Rich 1997).

Habitat Loss and Fragmentation: Habitat loss and alteration is thought to be the principle cause of decline in the West. Riparian habitats, particularly cottonwood and willow (Salix spp.) communities, have been dramatically reduced and degraded by urban development, roads, off-road vehicle use, recreation, livestock grazing, agriculture, water development projects, channelization, willow control, and encroachment by non-native species (USFWS 1996). In California, high quality habitat has nearly disappeared and remaining habitat is widely dispersed and isolated, mostly as montane meadows in the Sierra Nevada and along a few river courses (USDA Forest Service 1994). In Arizona, as much as 90 percent of lowland riparian habitat has been lost or altered (USFWS 1996). These patterns are continuing throughout the western states.

Non-native Vegetation: Tamarisk or saltcedar (Tamarix spp.) and Russian olive (Elaeagnus angustifolia) have invaded riparian areas throughout the West, and particularly the southwest, and are likely factors in the flycatcher's decline. Tamarisk has replaced some riparian communities completely, but is less common in others. Tamarisk replaces the preferred multi-layered shrub community with a monotypic stand with one shrub layer, decreases plant and insect diversity, and can increase the frequency and intensity of fire. Dams and flood control, and irrigation water high in salts also give tamarisk a competitive edge over native vegetation. Although may nest in tamarisk where it provides the right vegetation structure, tamarisk may provide poor quality habitat and some studies have documented low breeding densities and low reproductive success in tamarisk (USFWS 1995, 1996; Sogge et al. 1997).

Livestock: Placement of nests near the edges of shrubs makes them vulnerable to direct disturbance or destruction by livestock (Sanders and Flett 1989). Cattle and sheep browse on shrubs in the mid-level preferred by willow flycatchers, and consume or trample young woody plants (Sanders and Flett 1989). Heavy or poorly timed livestock grazing damages deciduous shrubs and can prevent shrub regeneration, reducing flycatcher habitat. Cattle prefer willow and cottonwood shoots to tamarisk and other non-natives, further depleting flycatcher habitat. Streambank trampling and soil compaction also adversely affect the water table, reduce free water, and discourage shrub growth (Flett and Sanders 1987). In Oregon, populations increased after reduction in cattle grazing and cessation of poisoning and removal of riparian willows (Taylor and Littlefield 1986).

Brood Parasitism: This species is a common host to brood parasitism by the brown-headed cowbird (Molothrus ater; Bent 1942, King 1955, Walkinshaw 1966, Flett and Sanders 1987, Ehrlich et al. 1988, Sedgwick and Knopf 1988, Sanders and Flett 1989). Brood parasitism may pose a significant threat, particularly in its western range where habitat is limited and fragmented, and where livestock are often present in meadows and riparian habitats. In California, parasitism rates are high in the lowlands (e.g., 13 of 19 nests; Harris 1991), but much less in the Sierra Nevada (e.g., 1 of 22 nests; Sanders and Flett 1989). High parasitism rates (11 of 27 nests) were recorded at high elevations in north-central Colorado, but cowbird eggs were accepted by the hosts at only 2 of these nests (Sedgwick and Knopf 1988). In Arizona, cowbirds were documented at all 12 known breeding locations and parasitism documented at 50 percent of the sites (USFWS 1996). In the Grand Canyon, cowbirds occur at all flycatcher breeding sites and typically 50 percent of nests are parasitized (Brown 1994, Sogge et al. 1997). In hundreds of monitored nests of southwestern willow flycatcher, brood parasitism caused either nest failure or successful rearing of only cowbird chicks (Sogge et al. 1997). There is, however, evidence of adaptive behavior toward cowbird parasitism in some populations. Parasitized nests may be abandoned or dismantled; renesting may occur in some cases (Sedgwick and Knopf 1988, Harris 1991), although fewer eggs may be laid (Holcomb 1974). Flycatchers will chase female cowbirds near nests, and some may respond to cowbird calls by becoming quieter and less conspicuous (Uyehara and Narins 1995).

Water Development: Impoundments, channelization, and water diversions have greatly reduced and modified native riparian habitats. Reservoirs flood native riparian communities, and regulated flows reduce the seasonal flooding that many riparian plants need for regeneration. Non-native plants (such as tamarisk in the southwest) often invade new habitat created along reservoir shorelines, or riparian communities of rivers with controlled or reduced flows from water diversion and dams. Channelization also alters the river system dynamics needed to maintain riparian communities (USFWS 1996). Some remaining populations in Arizona are threatened by fluctuating reservoir levels and flooding (Latta et al. 1999).

Subspecies brewsteri: While there appear to be moderate populations in early-seral upland forest habitats the species is especially threatened by population declines in valley habitats, lower nest success in these valley habitats than in early-seral forest, and continual loss of riparian habitat (Altman 2003).