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1987 (L.E. Stevens, Applied Technology Associates, Inc., personal communication). ~: F ~, '" ~ t ~ ~:- ~ ~ ~ Structural complexity may be correlated with temperature regulation (Hunter et aI. 1987). Heat stress can affect egg survival. Tamarisk stands in Grand Canyon afford some thermal protection, although probably not to the same extent as broadleafed plants. Monotypic tamarisk stands in the Grand Canyon, which lack structural complexity, have had low nesting success. Predation also may be a factor as tamarisk may offer less visual protection than broadleafed vegetation. However, there are no data to support these suppositions. . ~"I r. ,. [.'1 .' \~ Although habitat may not be limiting (Brown and Trossett 1989), patch size is not weU known. Territories from 2.8 to 3.1 ha in area are typical (Brown 1991), but territory sizes as low as 0.5 ,ha have been noted in Grand Canyon (Sogge 1994, 1995), and E. t. extimus have been observed defending territories as small as 0.11 ha in both breeding and wintering ranges (Gorski 1969). The importance of habitat fragmentation cannot be underestimated: larger patches are more likely to support willow flycatchers (Sedgwick and Knopf 1992). The width of the riparian zone cannot be extremely narrow; Et.extimus is not found along high gradient streams (Sogge 1994). Patch width may be correlated to quality, i.e., narrower in high quality habitats. However, there is no correlation between stream width and flycatchers (Sedgwick and Knopf 1992). ~}: i.... (I ;~:r: I ';''0'' 't';', ~ ~I I Nests are located in close proximity to water, a characteristic possibly correlated with edge effects and food supplies. Although intuitively it would appear monotypic tamarisk stands do not support the insect density of a more diverse broadleaf community, there is evidence to the ,; contrary (Stevens 1985; Cohan et al. 1918 in Hunter et aI. 1988). It appears that food is not limiting to birds in saltcedar, but this has not been tested. Although invertebrate biomass may be substantial, low species richness may reduce the overall food base. ~ ~. '" i.l" ,~!t.~ t',!;, ~'.l;!; ~ Although little is known of southwestern willow flycatcher food preferences, the birds are probably generalists and opportunistic feeders. Empidonax flycatchers hover and glean insects from foliage (King 1955). On the wing, northern willow flycatchers forage more in the open and less in trees than the closely related alder flycatcher (Barlow and McGillivray 1983). Southwestern willow flycatchers also forage on sandbars, backwaters, and at waters edge in the Grand Canyon (Tibbetts et aI. 1994). t::; ~~---I ,,.", V ;:.-:. r" ,.-,t. t,:-<;. r~';\ ~"i ., ~:':~: ~~, !"if;" '. Riparian modification, destrUCtion and fragmentation accompanying western expansion provided new foraging habitat for brown-headed cowbirds (Molothrus ater) and populations of this species continue to expand (Hanka 1985; Harris 1991; U.S. Fish and Wildlife Service 1993). Brood parasitism is currently a substantial threat to southwestern willow flycatchers in the United States, and probably to many other neotropical migrants as well (Bohning-Gaese et aI. 1993). The open- cupped nest of the willow flycatcher may be readily visible to brood parasites. Over one half of the nests in Brown's study (1988) contained brown-headed cowbird eggs. Cowbirds may remove prey eggs. their eggs hatch earlier, and the larger nestlings are more competitive in the nest. A cowbird fledged from a Sierra Nevada nest in which three willow flycatcher nestmates had died a few days after hatching (Flett and Sanders 1981). R~ h~ ,~, ~~~~. ~ '",:-- '~r i,' . 29