SNFPA Draft Supplemental Environmental Impact Statement

June 2003

Chapter 3: Affected Environment

3.2.1.2. Least Bell's Vireo (Vireo bellii pusillus)

The information below was extracted and summarized from the following reference: USDI Fish and Wildlife Service. 1986. Draft Recovery Plan for the Least Bell's Vireo (Vireo bellii pusillus). U.S. Fish and Wildlife Service, Portland, OR. Detailed references can be found in that document. This section supplements the information found in FEIS Volume 3, Chapter 4, Part 4.3. This species was not documented in the FEIS. However, in early surveys for the 2003 breeding season, responses from singing males have been detected on a number of occasions within the planning area, along the South Fork Kern River (T. Benson, pers. comm.)

Life History

Breeding Biology: Least Bell's vireos arrive on the southern California breeding grounds in mid-March to early April, with males arriving in advance of females by several days. Observations of banded birds suggest that returning adult breeders may arrive earlier than first-year birds by a few weeks. Least Bell's vireo are generally present on the breeding grounds until late September, although they may begin departing by late July. Stragglers have been noted in October and November, and occasionally individuals overwinter in California.

Predation is a major cause of nest failure in areas where brown-headed cowbird nest parasitism is infrequent or has been reduced by cowbird trapping programs. Most predation occurs during the egg stage. Predators likely include western scrub-jays (Aphelocoma californica), Cooper's hawks (Accipter cooperii), gopher snakes (Pituophis melanoleucus) and other snake species, raccoons (Procyon lotor), opossums (Didelphis virginiana), coyotes (Canis latrans), long-tailed weasels, dusky-footed woodrat (Neotoma fuscipes), deer mice (Peromyscus maniculatus), rats (Rattus spp.), and domestic cats (Felis domesticus) (Franzreb 1989). Other sources of nest failure are human disturbance (trampling of nest or nest site; clearing of vegetation), ant infestations, rainstorms, and unknown factors.

Least Bell's vireo pairs may attempt as many as five nests in a breeding season, although most fledge young from only one or two nests. The likelihood of renesting depends on the time of season, the pair's previous reproductive effort, the success of previous efforts, and other factors. Few nests are initiated after mid-July.

Productivity is a measure of reproductive performance that represents the total production of offspring over all nesting attempts within a season, and is expressed on a per pair basis. The annual average number of fledglings produced per pair has ranged from 0.9 to 4.5, with long-term averages ranging between 1.8 to 3.2.

An even more encompassing measure of productivity is the number of fledglings produced per egg laid. This measure combines the effort of egg production with the probability of hatching and fledging young from those eggs and hence incorporates the number of nesting attempts made by pairs. Annual averages have ranged from 0.31 to 0.85 fledglings per egg at the various sites with long-term averages of 0.37 to 0.75 fledgling per egg, reflecting the differential intensity of pressures such as egg predation, nestling predation, cowbird parasitism, and other sources of nest failure at those sites.

Life History, Demographics, and Dispersal: The least Bell's vireo is a subtropical migrant, traveling some two thousand miles annually between breeding and wintering grounds. Preliminary results of studies of color-banded birds indicate that least Bell's vireo have a life span ranging to 7 years. A large proportion of the population dies before reaching the age of 1 year, as is typical of small migratory passerines.

The earliest studies of color-banded least Bell's vireos suggested that they were strongly site tenacious; once birds selected a breeding site, they returned to it year after year. Not only do least Bell's vireo return to the same drainage, they return to the same territory and even the same nest tree or shrub, a remarkable feat considering the terrain covered during the course of migration. More recent data obtained at several additional breeding sights suggest that site tenacity in least Bell's vireo may not be as strong as previously believed. Many banded birds are seen for the first time as 2-year olds and sometimes older, indicating that they have changed breeding locations during their first few years. The factors promoting a switch in breeding location are not known at this time. Habitat loss, lack of success in obtaining a mate, or even failure to return to the breeding grounds may be possible causes.

Habitat Relationships

Least Bell's vireos are obligate riparian breeders, typically inhabiting structurally diverse woodlands along watercourses. They occur in a number of riparian habitat types, including cottonwood-willow woodlands/forests, oak woodlands, and mule fat scrub. Several investigators have attempted to identify the habitat requirements of the least Bell's vireo by comparing characteristics of occupied and unoccupied sites and have converged on two features that appear to be essential: (1) the presence of dense cover within 1-2 meters (3-6 feet) of the ground, where nests are typically placed and (2) a dense, stratified canopy for foraging. Although least Bell's vireos typically nest in willow-dominated areas, plant species composition does not appear to be as important a determinant of nesting site selection as habitat structure.

Although least Bell's vireos are tied to riparian habitat for nesting, they have been observed extending their activities into adjacent upland habitats. Least Bell's vireos along the edges of riparian corridors maintain territories that incorporate both habitat types. One study found that least Bell's vireos along the Sweetwater River in San Diego County traveled 3-61 meters (9-183 feet) from the riparian edge to reach upland areas. Upland habitat was used primarily by foraging adults and adults foraging with fledglings; however, 35 percent of the pairs whose territories included nonriparian habitat placed at least one nest there. Researchers speculated that upland vegetation, in particular laurel sumac (Malosma laurina) and elderberry may have provided important supplemental food resources for birds in marginal habitat. Use of upland vegetation has also been observed early in the spring when floodwaters inundates adjacent riparian habitat. Under such conditions, least Bell's vireos may nest exclusively in the nonriparian habitat.

Little is known about the least Bell's vireo's wintering habitat requirements. It is known that least Bell's vireos are not exclusively dependent on riparian habitat on the winter grounds.

Historical and Current Distribution

Historically, the least Bell's vireo was widespread and abundant, ranging form interior northern California near Red Bluff (Tehama County), south through the Sacramento-San Joaquin Valleys and Sierra Nevada foothills, in the Coast Ranges from Santa Clara County south to approximately San Fernando, Baja California, Mexico. Populations also were found in the Owens Valley, Death Valley, and at scattered oases and canyons throughout the Mojave Desert.

In the decades following 1940, extensive habitat loss coupled with brood parasitism by the brown-headed cowbird decimated least Bell's vireo populations rangewide, and the decline has been well documented. By the early 1980's, the least Bell's vireo had been extirpated from the Sacramento and San Joaquin Valleys, once the center of its breeding range. Breeding populations in northern Baja California apparently underwent similar declines during the same period. By the time the least Bell's vireo was federally listed in 1984, the statewide population was estimated at 300 pairs, with the majority concentrated in San Diego County.

Since the least Bell's vireo was federally listed in 1986 and intensive cowbird removal programs initiated, the species has undergone an increase almost as dramatic as its decline. While a few populations surviving the decline have generally stabilized is size (e.g. Sweetwater, San Diego and Santa Ynez River populations), most have undergone tremendous growth. For example, available census data indicate that the least Bell's vireo population in southern California increased from an estimated 300 pairs in 1986 to an estimated 1346 in 1996.

In addition to population size increases, observations also indicate that least Bell's vireos are also expanding their range and recolonizing site unoccupied for years or decades. Expansion is occurring both eastward in San Diego County as birds become reestablished is the more inland reaches of the coastal valleys and northward as birds disperse into Riverside and Ventura Counties. As populations continue to grow and least Bell's vireos disperse northward, it is anticipated they could reestablish in the central and northern portions of their historical breeding range.

Risk Factors

There are two main risk factors influencing least Bell's vireo populations. Grinnell and Miller (1944) considered the least Bell's vireo still "common, even locally abundant under favorable conditions of habitat." However, they noted that in the "last fifteen years a noticeable decline has occurred in parts of southern California and in the Sacramento-San Joaquin Valley." That decline has been reported to have continued for four more decades, the combined result of habitat loss and degradation and nest parasitism by the brown-headed cowbird.

USDA Forest Service · Pacific Southwest Region