Vireo bellii

See Johnson et al. (1988) and Murray et al. (1994) for analyses of the phylogenetic relationships among vireos. Four subspecies recognized: V. b. bellii breeds from Colorado to Illinois south through e. Texas to Tamaulipas, Mexico; V. b. medius breeds from sw. Texas to c. Mexico; V. b. arizonae breeds from se. California to sw. New Mexico south to Sonora and Chihuahua, Mexico; and V. b. pusillus breeds in c. and sw. California south to Baja California, Mexico (AOU 1957, Brown 1993).

BREEDING: southern California, southern Nevada, southwestern Utah, Arizona, southern New Mexico, northeastern Colorado, Nebraska, South Dakota, western North Dakota, southeastern Minnesota, southern Wisconsin, northeastern Illinois, northwestern Indiana, and southwestern Michigan south to northern Baja California, southern Sonora, southern Durango, Zacatecas, southern Tamaulipas, southern Texas, north-central Louisiana, Arkansas, and southwestern Tennessee, southwestern Kentucky, southern Indiana, and western Ohio (Brown 1993, AOU 1998). NON-BREEDING: southern Baja California and southern Sonora south to Honduras, primarily on Pacific slope; casually north to California, Arizona, Texas, Louisiana, and southern Florida, and south to Nicaragua (Brown 1993, AOU 1998).

Several populations have been reduced or extirpated (Brown 1993). Arizona Bell's Vireo (V. B. ARIZONAE) has expanded range and increased in abundance along the Colorado River through Grand Canyon, Arizona as a result of increased breeding habitat created by flood control at Glen Canyon Dam; a breeding range expansion of at least 219 km was observed over 11 years (Brown et al. 1983).

HABITAT: Declines may be related to loss of riparian habitat (USFWS 1988), particularly in western portions of range. Urban development, water diversion, flood control projects, grazing, and the spread of agriculture have destroyed much western nesting habitat. The Central Valley of California, where the species was once a common breeding bird but is now extirpated, has lost 95 percent of its riparian vegetation in this century to agriculture and other anthropogenic factors (Smith 1977). The species also once bred in the Owens River Valley, where today water diversion to supply urban areas has killed most of the once lush riparian vegetation (Robinson et al. 1995). Overgrazing suppresses shrub growth and reduces available nest sites and vireo density (by 50 percent in Oklahoma; Overmire 1963). BROOD PARASITISM: Observed rates of brood parasitism by Brown-headed Cowbirds (MOLOTHRUS ATER) vary geographically, ranging from 6 percent in Grand Canyon, Arizona, to 69 percent in Kansas. Based on a model developed for Least Bell's Vireos (V. B. PUSILLUS) in California, cowbird parasitism rates exceeding 30 percent lead to unstable populations that could be extirpated by stochastic events, while rates of 48 percent and 69 percent lead to population extinction in 18 and 8 years respectively (Brown 1993, Laymon 1987). In one California study, parasitism rates were reduced from 47 percent to 10 percent, increasing fledglings per pair from 2.08 to 2.86; the latter productivity rate, but not the former, should allow the population to persist and experience moderate growth (Robinson et al. 1995). Elsewhere, nonparasitized nests successfully fledge more young than parasitized nests (Barlow 1962, Brown 1993). Birds forced into fragmented habitat or marginal nesting areas are more vulnerable to parasitism. There is reason to believe, however, that large vireo populations in suitable habitat can maintain themselves in the face of cowbird parasitism (Robinson et al. 1995). In addition, different subspecies may naturally have different vulnerability to cowbird parasitism due to the timing of migration and nest initiation. In Arizona, V. B. ARIZONAE initiates nesting in early April before cowbirds become common, whereas V. B. PUSILLUS initiates nesting during a later period that coincides with the seasonal arrival of cowbirds; this reduces the relative rate of brood parasitism experienced by the earlier nesting subspecies (Corman, pers. comm.). PREDATION: Because often nests near the ground, brooding adults and young are commonly depredated by various mammals and reptiles, including domestic cat (FELIS DOMESTICUS), raccoon (PROCYON LOTOR), opossum (DIDELPHIS VIRGINIANA), coyote (CANIS LATRANS), long-tailed weasel (MUSTELA FRENATA), dusky-footed woodrat (NEOTOMA FUSCIPES), deer mouse (PEROMYSCUS MANICULATUS), house mouse (MUS MUSCULUS), rat (RATTUS RATTUS; Brown 1993, Bent 1950), and various snakes (Cink 1977, Nolan 1960). Suspected or confirmed avian predators include greater roadrunner (GEOCOCCYX CALIFORNIANUS), American crow (CORVUS BRACHYRHYNCHOS), and scrub jay (APHELOCOMA CALIFORNICA; Collins et al. 1989). POPULATION ISOLATION: One isolated population with relatively low rates of brood parasitism and intact habitat has declined; parent-offspring matings were documented, so inbreeding depression may be responsible (Robinson et al. 1995).