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<br />The characteristic territorial call is a "fitz-bew,U most frequently heard in <br />the morning before 10 AM (Tibbitts et al., 1994). The four subspecies may be <br />differentiated by characteristics of this call. <br /> <br />SWWF return to wintering grounds in August and September (Brown 1991b), but <br />neither migration routes nor wintering areas are well known. Birds call and <br />perhaps defend foraging territories in Central America during winter, and <br />winter movement may be tied to water availability (Gorski 1969). Threats to <br />SWWF on the wintering grounds are undocumented, but habitat losses in Latin <br />America may be a major factor in the decline of this specieB. <br /> <br />Life RequiBites <br /> <br />SWWF are highly territorial. Nest building begins in May after breeding <br />territorieB are eBtablished. The nest is placed in a fork or horizontal <br />branch 1-5 meters above ground (TibbettB et al. 1994). A clutch of three or <br />four eggs is laid from late May through July (Unitt 1987), but in Grand Canyon <br />two or three eggs (usually three) are usually laid (Sogge 1995). Breeding <br />extends through July and singing ceases at the end of the breeding season. <br /> <br />After a 12-14 day incubation, nestlings Bpend 12 or 13 days in the nest before <br />fledging (Brown 1988; Tibbetts et al., 1994). The breeding Beason (eggs or <br />young in nest) along the Colorado River extendB from early June to mid-July, <br />but may extend into August. One clutch is typical, however re-nesting has <br />been known to occur if the initial neBt is destroyed or parasitized (Brown <br />1988) . <br /> <br />Riparian modification, destruction and fragmentation provided new foraging <br />habitat for brown-headed cowbirds IMolothrus ater) and populations of brown- <br />headed cowbirds continue to expand (Hanka 1985, HarriB 1991). Brood <br />parasitism iB currently the greatest threat to SWWF and probably many other <br />Neotropical migrants aB well (Bohning-GaeBe et al., 1993; Sogge et al., 1995). <br />Over half the neBts in Brown's study (1988) contained brown-headed cowbird <br />eggs. Cowbirds may remove prey eggs, their eggs hatch earlier, and the larger <br />nestlings are more competitive in 'the nest. Cowbirds fledged from Sierra <br />Nevada SWWF neBtB while SWWF nestlings died shortly after hatching (Flett and <br />Sanders 1987). Brown-headed cowbirds occur extensively around mule corrals on <br />the rim of the canyon and travel down to the Colorado River. <br /> <br />SWWF may remove cowbird eggB or more commonly abandon the neBt if the <br />parasite'S eggs are deposited. The second nesting attempt is energetically <br />expensive, requ~r~ng a new nest to be built (Sogge 1995), although Brown <br />(1988) noted that a SWWF pair covered a cowbird egg with freBh nesting <br />material and laid a new clutch. The second nest, already at a temporal <br />diBadvantage, is often parasitized aB well. Cowbird paraBitism could be <br />largely reBponBible for the absence of SWWF in otherwiBe suitable habitat in <br />the Grand Canyon (Unitt 1987). Bronzed cowbirdB (Mo10thnls BAnus) have <br />recently been reported colonizing the Grand Canyon and represent another <br />threat (Sogge 1995). <br /> <br />The SWWF in Grand Canyon is conBidered a habitat generalist, occupying sites <br />of average vegetation height and density (Brown and Trossett 1989). The SWWF <br /> <br />17 <br />