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. . of a BHBF on SWWF habitat in lower Grand Canyon. Results of Hualapai Indian Tribal analyses on riparian resources in lower Grand Canyon indicate that the impacts of the 1996 BHBF extended no farther than Mile 255 (Christensen 1997). This point lies approximately 10 miles upstream from recently reported SWWF nesting areas on upper Lake Mead. Therefore, this, and the shorter duration of the proposed 1998 BHBF, suggest that a planned high flow should have no effect on potential SWWF habitat in lower Grand Canyon. SWWF are highly territorial. Nest building begins in May after breeding territories are established. The nest is placed in a fork or horizontal branch 1-5 meters above ground (Tibbetts et al. 1994). A clutch of three or four eggs is laid from late May through July (Unitt 1987), but in Grand Canyon two or three eggs (usually three) are usually laid (Sogge 1995). After a 12-14 day incubation, nestlings spend 12 or 13 days in the nest before fledgling (Brown 1988; Tibbetts et aI., 1994). The breeding season (eggs or young in nest) along the Colorado River extends from late May to mid-July, but may extend into August. One clutch is typical, however re-nesting has been known to occur if the initial nest is destroyed or parasitized (Brown 1988). Cowbird parasitism could be largely responsible for the absence of SWWF in otherwise suitable habitat in the Grand Canyon (Unitt 1987). Riparian modification, destruction and fragmentation provided new foraging habitat for brown-headed cowbirds (Molothrus ater) and populations of brown-headed cowbirds continue to expand (Hanka 1985, Harris 1991). Brood parasitism, habitat destruction and predation are all major threats to SWWF (McCarthey, et al. 1997). Over half 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. Cowbirds fledged from Sierra Nevada SWWF nests while SWWF nestlings died shortly after hatching (Flett and Sanders 1987). Brown-headed cowbirds occur extensively around mule corrals on the rim of the canyon and travel down to the Colorado River. Bronzed cowbirds (Molothrus aenus) have recently been reported colonizing the Grand Canyon and represent another threat (Sogge 1995). Second nesting attempts are energetically expensive, requiring a new nest to be built (Sogge 1995), although Brown (1988) noted that a SWWF pair covered a cowbird egg with fresh nesting material and laid a new clutch. The second nest, already at a temporal disadvantage, is often parasitized as well. Proximity to water is necessary and is correlated with food supplies. Little is known of SWWF food preferences but it is probably a generalist feeder. It typically hovers and gleans insects from foliage, or flycatchers from conspicuous perches (Stevens personal communication). SWWF also forage on sandbars, backwaters, and at the waters edge in the Grand Canyon (Tibbetts et aI., 1994). 1998 GCD Beach/Habitat Building Flow 21 Biological Assessment