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I I I I I , I , I I I I I I , , I ,I I BIOLOGICAL OPINION Status of the Species quality, or simply a tendency to move inherent to this species. At other locations on the San Pedro River in Pinal Co., such as Cook's Lake and PZ Ranch, flycatcher breeding group size remained stable until 1996. In 1996 a catastrophic fire destroyed much of the breeding habitat at PZ Ranch resulting in nest loss, abandonment of that site and, perhaps, mortality of adults (Paxton et al. 1996). The data presented above and in Table 5 demonstrate that cowbird parasitism and nest depredation are adversely affecting southwestern willow flycatchers throughout their range. Cowbirds have been documented at more than 90% of sites surveyed (Sogge and Tibbitts 1992, Sogge et al. 1993, Camp Pendleton 1994, Muiznieks et al. 1994, Sogge and Tibbitts 1994, T. Ireland 1994 in lift., Whitfield 1994, C. Tomlinson 1995 in lift., Griffith and Griffith 1995, Holmgren and Collins 1995, Kus 1995, Maynard 1995, McDonald et al. 1995, Sferra et al. 1995, Sogge 1995a, Sogge 1995b, Sogge et al. 1995, Cooper 1996, San Diego Natural History Museum 1995, Stransky 1995, Whitfield and Strong 1995, Griffith and Griffith 1996 in lift" Skaggs 1996, Spencer et al. 1996). Parasitism rates have been highly variable, at the same sites, from one year to the next. Thus, the potential for cowbirds to be a persistent and widespread threat remains high. Cowbird trapping has been demonstrated to be an effective management strategy for increasing reproductive success for the southwestern willow flycatcher, as well as for other endangered Passerines (e.g., least Bell's vireo [Vireo bellU pusillus], black-capped vireo [V, atricapillus], and golden-cheeked warbler [Dendroica chrysoparia]). However, cowbird trapping should not be used alone, without protecting existing habitat and expanding riparian areas to reduce the ultimate problem of habitat fragmentation and the resultant vulnerability to cowbirds. Nest loss due to predation is common among small Passerines. The rates documented for southwestern willow flycatchers are also typical for small Passerines (i.e., rates < 50%). However, even at these "typical" levels, nest loss due to predation is a significant factor contributing to low reproductive success. Nest predation presents a difficult management challenge because of the variety of predator taxa involved and the difficulty in developing an effective management plan for more than one taxon. Until specific predators on southwestern willow flycatcher nests are identified, measures to reduce potential predator populations should focus on reducing human activities that attract predators, such as camping, picnicking, etc. where pets are loose and refuse is concentrated. Distribution and Abundance Unitt (1987) noted that taxonomic confusion between E, trailli and E. alnorum (alder flycatcher) and among other Empidonax species that migrate through the southwestern U.S. probably accounted for the relative lack of research on the southwestern willow flycatcher. The alder and willow flycatchers, formerly known as Traill's flycatcher, were not officially recognized as separate species until the American Ornithologist's Union published its sixth edition Checklist of North American Birds (AOU 1983). The lack of systematic, rangewide collections of E.t. extimus 58