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Literature Review

Gray Vireo

Vireo vicinior

USFWS: No Status
CDFG: California Species of Special Concern

Background

Distribution, Abundance and Trends. The gray vireo is a small passerine about the size of a house sparrow that inhabits arid, shrub-covered slopes in pinyon-juniper, juniper, and chamise-redshank chaparral habitats on foothills and mesas. Suitable habitat typically occurs from 2,000 to 6,500 feet (600-2,000 m) (Zeiner et al. 1990). In its preferred habitat it is found in areas with sparse to moderate cover and scattered small trees. While junipers are the dominant tree in gray vireo habitat, oaks may also be common.

The summer range of the gray vireo is from New Mexico, southern Nevada, southern Utah, southern Colorado, western Texas, Arizona, and southeastern California. This species winters primarily south of the Mexican border and in southwestern Arizona. In California, breeding gray vireos are known from the northeastern slopes of the San Bernardino Mountains in the vicinity of Rose Mine and Round Valley, in San Jacinto and Santa Rosa Mountains, from Mountain Center to Pinyon Flat and Sugarloaf Mountain, and on the southern slopes of the Laguna Mountains near Campo and Kitchen Creek. It is also known from the mountains of the eastern Mojave Desert, including the Grapevine, Kinston, Clark and New York Mountains.

The summer range of the gray vireo was formerly more widespread, with breeding birds recorded in the Walker Pass area of Kern County, in Joshua Tree National Park, in the northern and western foothills of the San Gabriel Mountains, and at many additional locations on the desert slopes of San Bernardino, Riverside and San Diego counties. The gray vireo is also known as a migrant in Whitewater Canyon (McCaskie 1963, Garrett and Dunn 1981).

Descriptions by Grinnell and Swarth (1913) indicate that the gray vireo was a common summer resident on the slopes of the Santa Rosa and San Jacinto Mountains. Their observations include on a ridge at 4,200 feet near Potrero Spring and north of Asbestos Mountain, and down to 3,000 feet near the head of Palm Canyon. Along the trail from Vandeventer Flat to Pinon Flat, "many birds" were noted at 3,000 to 4,500 feet, as far east as Omstott Creek, which coincided with the limit of Adenostoma species. Based on known territory size and amount of suitable habitat, they estimated that 480 pairs were present. While it is not known how many birds may still exist in the area, sightings are rare. One pair was present near Pinyon Flats in 1977 (Goldwasser 1978a). One to four pairs were observed south of Highway 74 near the Santa Rosa Peak Road in 1979 and a nesting pair was observed in there in 1981 (McKernan pers. comm.). According to U.S. Forest Service records (Freeman pers. comm.) one individual was seen in Pinyon Flat in July 1997. According to Garrett and Dunn (1981) much fieldwork is needed to document the extent and causes of decline of this formerly more widespread species. Regular surveys for this species have not been conducted in the Plan area.

The gray vireo usually arrives from its wintering areas in Mexico from the end of March to early May. It generally departs by the end of August. The nest of the gray vireo is an open cup of plant fibers, bits of leaves, spider silk, and bark strips, often hung from twigs or a forked branch in a shrub or small tree, usually two to eight feet above ground (Zeiner et al. 1990). Eggs are laid from mid-May to mid-June. Gray vireos feed by gleaning insects and invertebrates from bushes and small trees. In New Mexico, territories encompass 100 acres or more (Schwarz 1991).

Threats and Limiting Factors. The reasons for the decline in gray vireo populations in recent decades are not well understood. One major cause of this decline may be parasitism by the brown-headed cowbird. Remsen (1978) has described that this species is highly susceptible to cowbird parasitism. Human activities, including residential development, golf courses and agriculture, attract cowbirds thereby increasing this potential threat to gray vireos. Another possible cause for their decline could be habitat changes and senescence of the vegetation due to fire suppression activities since the turn of the century.

Special Considerations. Reduction of cowbird populations in gray vireo habitat may substantially benefit this species. Research is needed to determine the causes of this species' decline.