Least Bell's Vireo-Federal and State Endangered Bird

Least Bell's vireo (Vireo bellii pusillus), a subspecies of the Bell's vireo, is a federal and state endangered bird about the size of a sparrow. Its range is within the state of California and into northern Baja. It was first listed as a California endangered bird back in 1980 and then received a federal listing in 1986. In 1994, areas of critical habitat were determined. Just prior to being federally listed as endangered, the population was estimated to be only about 300 breeding pairs, but since that time, the population has grown successfully to about 2,000 breeding pairs.

The territorial range of males can vary between about half an acre to over seven acres. The breeding season can vary slightly depending on weather conditions, but as a rule, its between mid-March and September. Scientists have discovered that many of the vireos prefer to breed in the same area each year, and some even choose the same bush or shrub. They build nests somewhat close to the ground, about a meter or so from ground level. They also prefer to live around riparian areas, those areas surrounding rivers and streams.

Nest size varies between 2-4 eggs in a clutch with a 2 week incubation period. Babies fledge in about 10-12 days after hatching. Dangers to the Least Bell's vireo is primarily due to loss of habitat, encroachment of invasive plant species and fragmentation of their habitat. With development constantly pushing up near to their habitat, especially in southern California, it was essential to declare certain areas as protected critical habitat. Invasive species tend to be more aggressive and opportunistic than native species of plants, and can therefore quickly take over and replace the vireos native habitat. If an area is disturbed by development, invasive plants are able to move into the cleared area.

Due to the current endangered status, Least Bell's vireo cannot be harmed in any way. The term given to disturbance is 'take' and unless a qualified biologist holds a 'take' permit, the Least Bell's vireo can't be disturbed. This includes annoying, removing nests or eggs or creating any other disturbance that may cause it to abandon the nest. During the nesting season, if there is any work or activity to be done within Least Bell's vireo habitat, a pre-construction bird survey must be conducted by qualified biologists. If the area contains active nests, the project will need to be placed on hold until the birds have left the area. Sometimes it might be possible to continue with a project, providing a large buffer zone is placed, and monitored, around the nesting area. This would, however, require a permit from the California Department of Fish and Game, and adherence to the Migratory Bird Treaty Act would also apply.