Distribution: There are 4 subspecies of Bell's vireo, Vireo bellii, with each isolated from each other throughout the year (Hamilton 1962). The subspecies are: (1) Least Bell's vireo V. b. pusillus occurring in southwestern California (principally San Diego, Orange, Riverside, and Santa Barbara Counties) and northwestern Baja California; (2) V. b. bellii, occurring in the Midwestern United States, from eastern Colorado to western Tennessee, north to southern Michigan, southeastern Minnesota, southwestern Wisconsin, and south-central North Dakota; (3) V. b. medius, occurring in south-western Texas, south to Durango and Coahuila, Mexico; and (4) V. b. arizonae, occurring in the southwestern United States along the Colorado River and its tributaries (northwestern and central Arizona, southeastern California, southeastern Nevada, southwestern Utah and Sonora, Mexico (A.O.U.1997; Brown 1993).
The V. b. pusillus subspecies winters in southern Baja California. The remaining subspecies winter in thorn scrub and riparian corridors along the west coast of Mexico, and in tropical deciduous forest and arid tropical scrub of southern Mexico, Guatemala, El Salvador, Honduras, and Nicaragua (Wilbur 1980; Hands et al. 1989; Brown 1993; Howell and Webb 1995).
In Utah, V. b. arizonae breeds in the southwestern corner of the state (Washington and western Kane Counties), specifically the Virgin River drainage and Beaver Dam Wash, and Zion National Park (Hayward et al. 1976; Behle et al. 1985; Wauer 1997).
Ecology: Along the lower Colorado River Valley, V. b. arizonae occurs from early March through late September (Rosenberg et al. 1991). In Nevada, it occurs from 20 Apr-20 September (Alcorn 1988). In Nebraska along the Platte River, peak migrations occur 6-20 May and 25 Aug-17 September (Faanes and Lingle 1995).
Nesting areas for V. b. pusillus in southern California and V. b. arizonae in Arizona, Nevada, and Utah are nearly always associated with cottonwood-willow (Populus spp.-Salix spp.) dominated riparian habitats (Alcorn 1988; Rosenberg et al. 1991; Brown 1993; Wauer 1997). Trees 5-10 years old with a mean canopy height of 7.8 m [26 ft] are a key component of breeding habitat (Franzreb 1989; Hands et al. 1989; Brown 1993). Another critical structural habitat component at nesting sites appears to be a dense shrub layer, variable from ground up to 4 m [13 ft] high (Franzreb 1989; Hands et al. 1989; Brown 1993).
Within preferred riparian habitats noted above, the cup-shaped nest is typically suspended in lateral or terminal forks of sapling-sized trees (2.4-3.0 m [8-10 ft] high), 0.3-1.5 m (1-5 ft) above the ground (Nolan 1960; Barlow 1962; Brown 1993). In California, nests are frequently less than 1 m (3 ft) off the ground (Franzreb 1989). Alternatively, nests may be placed in shrubs (Johnsgard 1979). Nests are always overhung with foliage from taller trees/shrubs. Nests are intricately woven with a variety of dried grasses and shredded bark, supplemented with spider web or cocoon silk (Johnsgard 1979; Baicich and Harrison 1997). Nests are built in about 4-5 days (Nolan 1960; Barlow 1962).
From 3-5 eggs (mean = 3.4) are laid; eggs are oval and white with brown specks (Barlow 1962; Baicich and Harrison 1997). Both adults incubate for about 14 days. Young fledge from the nest in about 11 days. Both sexes participate in care and feeding of young through post-fledging (Bent 1950; Brown 1993). This species is usually single brooded but second broods are documented, especially in California and along the lower Colorado River Valley (Barlow 1962; Rosenberg et al. 1991; Brown 1993).
Bell's vireo is primarily insectivorous (99.3%), with little plant matter (0.7%) being consumed (Chapin 1925). The most common foraging method is gleaning less than 3.5 m (12 ft) above the ground (Salata 1986).
Brown-headed cowbird parasitism is a serious problem for Bell's vireo throughout its range; enough so that local populations in California have been extirpated (Laymon 1987). For California and the southwestern United States, a contributing factor for increased parasitism rates appears to be habitat fragmentation and/or degradation, i.e., impacts due to river channelization, vegetation clearing, and water impoundments (Franzreb 1989). In Kansas, brood parasitism accounted for 65% of 79 eggs (Barlow 1962). Other studies discussed by Hands et al. (1989) and Franzreb (1989) indicated parasitism rates of 30-71%, with rates over 50% typical.
Cowbirds parasitize nests during the vireo's egg-laying period, normally depositing 1 or 2 (rarely 3) eggs (Brown 1993) at 1 egg/day intervals (Brown 1993). If cowbird eggs are deposited prior to vireo egg-laying, nests are usually abandoned (Brown 1993). Also, if vireo eggs are removed by cowbirds, the nest may be abandoned (Hands et al. 1989). If nests are abandoned, renesting efforts typically occur within close proximity to old nests (Hands et al. 1989). This behavior may facilitate continued parasitism (JWM).
Habitat Requirements: Regardless of subspecies or location, Bell's vireo is associated with dense shrubby vegetation, typically early successional stages, specifically second-growth forests or woodlands, scrub oak, coastal chaparral, or mesquite brush lands. In California and the southwestern United States, Bell's vireo may be considered a riparian-obligate species, preferring dense under story vegetation (0-4.0 m [0-13 ft]), especially in seepwillow (Baccharis spp.) and mesquite, and in more recently introduced salt cedar along the lower Colorado River (Franzreb 1989; Hands et al. 1989; Brown 1993). Eastern and arid-land subspecies may use uplands distant from riparian corridors, but often nests near water (Brown 1993). Habitat requirements for Bell's vireo during migration and wintering areas are unknown.