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southwestern Colorado; the eastern edge extends into western Texas (U.S. Fish and Wildlife Service 1993). It winters from Mexico to Panama, with historical accounts from Colombia (phillips 1948). Southwestern willow flycatchers probably return to their wintering grounds in August and September (Brown 1991). Neither migration routes nor wintering areas are well known. Winter movement may be tied to water availability (Gorski 1969). Birds call and perhaps defend foraging territories in Central America during this season (Gorski 1969). Threats to this species on the wintering grounds are undocumented, but habitat losses in Latin and South American are likely. The southwestern willow flycatcher has been extirpated from much of its former range (Hunter et aI. 1987). Although probably never common, E.t.extimus population declines have been noted for nearly 50 years, corresponding with loss and modification of riparian habitats (phillips 1948). Southwestern riparian ecosystems support a rich avian fauna (Johnson and Haight 1981) and habitat changes have resulted in reduction or extirpation of many species (Hunter et aI. 1987). Modification 'and fragmentation of these systems through development and intensive livestock grazing have resulted in devastating ecological changes to the southwestern willow flycatcher and its habitat. Groundwater withdrawal, flow regulation, and destruction of native willow/cottonwood vegetation has provided opportunity for invasion by exotic species, notably tamarisk (Tamarix spp.). Habitat fragmentation and modification has benefited other bird species, especially cowbirds (M%thrus spp.), which parasitize willow flycatcher nests, contributing to the precipitous population declines. Habitat loss in Central and South America may also be a contributing factor to this population decline, The U.S. Fish and Wildlife Service (1992) estimated that only 300-500 breeding pairs remain in the United States. This includes the largest colonies in California (115 pairs) and approximately 100 pairs in New Mexico (Sogge 1994). Limited information exists for Colorado, Utah, Nevada, and Texas. Southwestern willow flycatcher populations have experienced such sharp reductions since 1950 that it was proposed for listing with critical habitat, under the Endangered Species Act, on 23 July 23 1993 (U.S. Fish and Wtldlife Service 1993). This subspecies is now less common than most other currently listed species (Unitt 1987). It is considered as a species of special concern in Arizona (Arizona Game and Fish Department 1996), and as a state endangered species in New Mexico and California. Arizona has probably !lX]lerienced the greatest reduction inE.t.extimus numbers. It formerly bred throughout the state at high and low elevations. For example, a 1931 record exists from the south rim of the Grand Canyon (Brown et aI. 1984). By 1981, the population was estimated at less than 25 pairs in the state (Unitt 1981). The former range included the lower Colorado River, from which it had been extirpated but is now apparently recolonizing (Robert McKernan, San Bernardino County Museum, personal communication). McKernan (op. cit.) reported numerous territories downstream from Hoover Dam in 1996. In 1995, the Arizona Partners in Flight Program reported 83-81 territories, 62 pairs, 56 nesting attempts and 21 successful nests which produced at least 51 fledged young (Spencer et aI. 1996). 26