WSPC12531 (2) - Laserfiche WebLink

Home Browse Search

032487 morning before 10 AM (Tibbitts et aI., 1994). 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). Breeding extends through July, althought singing will continue in migration and on the bird's winter grounds.. After a 12-14 day incubation, nestlings spend 12 or 13 days in the nest before fledging (Brown 1988; Tibbetts et aI., 1994). The breeding season (eggs or young in nest) along the Colorado River extends from late May, and 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). 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). In addition to habitat destruction, or predation, brood parasitism is a threat to SWWF and probably many other Neotropical migrants as well (Bohning- Gaese et aI., 1993; Sogge et al., 1995). The threat of brood parasitism may be locally greater than other threats. 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. SWWF commonly abandon the nest if the parasite's eggs are deposited. The second nesting attempt is 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. Cowbird parasitism could be largely responsible for the absence of SWWF in otherwise suitable habitat in the Grand Canyon (Unitt 1987). Bronzed cowbirds (Molotbrus aenus) have recently been reported colonizing the Grand Canyon and represent another threat (Sogge 1995). The SWWF in Grand Canyon occupy sites with average vegetation canopy height and density (Brown and Trossett 1989). SWWF breed and forage in dense, multistoried riparian vegetation near surface water or moist soil (Whitmore 1977, Sferra et al., 1995, 1997), along low gradient streams (Sogge 1995). Nesting in the Grand Canyon typically occurs in non-native Tamarix approximately 4-7 m tall (13-23 feet), with a dense volume of foliage 0-4 m from the ground (Tibbetts et al., 1994). SWWF are commonly found nesting in saltcedar in upper Grand Canyon (Brown 1988), and nested in saItcedar in Glen Canyon before completion of the Glen Canyon Dam (Behle and Higgins 1959). Although habitat is not limiting in Grand Canyon (Brown and Trossett 1989), required patch size is not known. The 1997 nesting record from lower Grand Canyon demonstrates that this species can colonize new habitat; however, that habitat is influenced by Lower Basin Lake Mead management and is not within the purview of this Biological Assessment. 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 sally-hovers insects from foliage from conspicuous perches (Stevens pers.comm.). SWWF also forage on sandbars,