Empidonax traillii extimus

This subspecies was omitted from the 1957 AOU checklist but has been recognized in all taxonomic studies of E. traillii since its original description (Unitt 1987). Genetic studies (Paxton et al. 2007) have been used to define the taxonomic/geographic scope of this subspecies to the extent possible (USFWS 2013).

Has declined greatly in area of occupancy and abundance in riparian areas of the American Southwest, primarily because of habitat loss and degradation; cowbird parasitism is also a problem in some areas; an estimated 1,300 pairs remain; few populations include more than 50 pairs.

Breeding range includes the southwestern United States (southern California, southern Nevada, southernmost Utah, southernmost Colorado, Arizona, New Mexico, and western Texas) and extreme northwestern Mexico (see USFWS 2013). Most of the population (75 percent of known territories) is in New Mexico and Arizona (Durst et al. 2008). The northern distributional limit of this subspecies cannot be precisely defined (see USFWS 2013). Elevational range extends from near sea level to over 2,600 meters; the majority of territories are below 1,600 meters (Durst et al. 2008).

Winter range extends probably from central Mexico to northwestern Colombia (Stiles and Skutch 1989). In southern California, these flycatchers migrate through desert regions and sometimes along the coast and onto the Channel Islands (Biosystems Analysis 1989).

This flycatcher is represented by many distinct occurrences (subpopulations) and locations (as defined by IUCN). In the mid to late 2000s, it was represented by about 1,300 territories in 173 sites; 97 of 173 sites had 5 or fewer territories (Durst et al. 2008) Some of these sites can be regarded as parts of larger, single occurrences or metapopulations.

HABITAT LOSS/DEGRADATION: Decline is due primarily to destruction and degradation of cottonwood-willow and structurally similar riparian habitats. The causes of habitat loss and change are water impoundment, water diversion and groundwater pumping, channelization and bank stabilization, riparian vegetation control, livestock grazing, off-road vehicle and other recreational uses, increased fires, urban and agricultural development, and hydrological changes resulting from these and other land uses. Tamarisk has replaced native riparian vegetation in many areas, with varying effects on flycatcher populations. Native riparian plant communities probably have a greater recovery value for flycatchers, but currently occupied and suitable tamarisk habitat should be maintained (USFWS 2002). Increased irrigated agriculture and livestock grazing have also resulted in increased range and abundance of brown-headed cowbirds; and, in some areas, heavy brood parasitism by cowbirds has contributed to the decline (Harris 1991, Brown 1988). Proposed reservoirs threaten the habitat of some populations. Ongoing and projected climate change likely will reduce the range-wide availability of suitable nesting habitat for willow flycatchers (USFWS 2013).

Wintering habitat limitations are unknown, but the amount of lowland wet habitat within its wintering range has declined substantially in the last century (Koronkiewicz et al. 1998). See USFWS (1995, 2002) for further details on threats.

DEMOGRAPHIC and HABITAT FRAGMENTATION EFFECTS: This flycatcher exists in small, fragmented populations, with only 6 populations having greater than 50 nesting pairs. The persistence of the smaller populations is dependent on immigration from nearby populations, and their isolated nature increases the risk of local extirpation (USFWS 2002). The vulnerability of the few relatively large populations (e.g. to fire, inundation) makes the above threats particularly acute (USFWS 2002).

PESTICIDES: Also of concern is the intensive use of pesticides both in agricultural areas adjacent to nesting grounds and on the migrating and wintering grounds (USFWS 2002).