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Breeding extends through July and may conclude in August, and parent bird singing ceases at the end of the breeding season. Population surveys must begin in late May/early June and include a repeat visit later in June to early July to distinguish migrants from residents, as well as to determine breeding status and success (Tibbitts et aI. 1994). The Department of the Interior Grand Canyon Research and Monitoring Program is currently funding additional research and population surveys along the river. Willow flycatchers are highly territorial. Nest building begins after breeding territories are established. The nest is an open cup of bark and grass, often lined with feathers (Johnson 1989). The outside diameter of the nest is approximately 7.1 em (3 in) wide and deep placed in a fork or horizontal branch 1-5 meters above ground (Tibbetts et aI. 1994). A clutch of three or four eggs is laid from late May through July (Unitt 1987). The beige eggs are lightly mottled with brown flecks at the larger end. Two to three eggs, generally three, complete the clutch in the Grand Canyon (Sogge 1994). After approximately 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 early June to mid-July, but may extend into August. One clutch is typical, however renesting has been known to occur if the initial nest is destroyed (Brown 1988). Vegetation composition of nest sites is variable. Shrubs, such as willows (SoJa sp.) are common, with or without a cottonwood (Populus sp.) overstory. Russian olive and tamarisk stands are also used. Southwestern willow flycatchers in Grand Canyon nest in tamarisk, the dominant species at 75% of the nest sites (Brown 1988). E.t extimus also used tamarisk before completion of the Glen Canyon Dam (Behle and Higgins 1959). These stands are mixed tamarisk.willow or monotypic. Along the Colorado River, the southwestern willow flycatcher can be regarded as a habitat generalist, occupying sites of average height and density (Brown and Trossett 1989). Historically, Grand Canyon southwestern willow flycatchers have nested in relatively broad stands of riparian saltcedar (Tamarl% ramosissima) in wide reaches of the Colorado River. Four areas have been consistently occupied by territorial birds there over the past two decades: Miles 50.5L (1993 to 1995), 51.5L (1991 to 1995), 65.3L (1994 to 1995) and Cardenas Marsh at Mile 71.0L (1911 to 1994; U.S. Bureau of Reclamation 1996b). These four areas all lie in wide, low gradient reaches (Schmidt and Grafl990), with abundant lower riparian zone vegetation dominated by tamarisk and coyote willow (SoJa exigua) on upper bar surfaces, and with associated fluvial marshes in reattaclunent bar settings (Stevens et aI. 1995). Fluvial marsh vegetation at those sites consisted of common reed (Phragmites austraJis), horsetail (Equisetum spp.), cattail (Typha spp.) and other wetland species. In the past decade, nests have been placed 2 to 5 m above the ground in dense groves of tamarisk, occasionally with a scattered overstory of taller trees (Brown and Trossett 1989, Tibbitts et al. 1994, Petterson and Sogge 1996), and usually with nearby exposed sand bars or fluvial marshes. The recently reported nests in upper Lake Mead occurred in Sa/a gooddingii, a native species which, with,S. exigua, has become a co-dominant on the delta since 28