LEAST BELL’S VIREO } Vireo bellii pusillus
DESCRIPTION: Least Bell’s vireos are small songbirds, generally no more than five inches in length, with a seven-inch wingspan. They have dull, ashy-grey heads; wings and tail are somewhat darker. Their undersides are predominately buffy white. Least Bell’s vireos have narrow, white eye-rings and faint wingbars. Both sexes look similar, with short, rounded wings, a relatively long tail, dark brown eyes, a grayish-black bill, and legs and feet that are dark grayish-blue to black.
HABITAT: A riparian species, least Bell’s vireos depend on dense, low-growing thickets of willows, mulefat, mugwort, and California wild rose. Vireos inhabit areas where an overstory of taller willows, cottonwoods, and sycamores is also present. During the winter, they are known to occur in mesquite scrub vegetation. Foraging sometimes takes place in adjacent chaparral and coastal sage scrub.
RANGE: The least Bell’s vireo was once widespread with a summer range from northern California all the way to Baja California, Mexico, extending as far east as Death Valley. The vireo can today be found in a handful of locations from Santa Barbara to San Diego counties.
MIGRATION: The least Bell’s vireo is a neotropical migrant, traveling 2,000 miles annually between breeding and wintering grounds.
BREEDING: Males arrive in breeding areas ahead of females and use their song to establish territories up to three-quarters of an acre in size. After a mate is selected and a low-lying, open-cup nest is constructed by the pair, egg laying begins within a couple of days. Typically, three or four eggs are laid. Both parents share in the two-week incubation and care for the chicks until fledging. Breeding lasts from mid-March until late September.
LIFE CYCLE: Least Bell’s vireos have a lifespan of about seven years.
FEEDING: Least Bell’s vireos prey on bugs, beetles, grasshoppers, moths, spiders, and caterpillars. They glean insects from leaves, twigs, and branches by hovering and picking prey off these stationary objects. They also utilize aerial pursuit, or flycatching.
THREATS: The least Bell’s vireo has suffered habitat loss due to urbanization, agricultural development, the damming and canalization of rivers and streams, invasion of nonnative plants, pesticides, road construction, and sand and gravel mining. Brood parasitism by the brown-headed cowbird is another serious threat.
POPULATION TREND: Once described as one of the most abundant birds in the state of California, the least Bell’s vireo suffered precipitous declines between 1930 and 1985. By 1986, only an estimated 300 pairs remained. Following the species’ listing as endangered, the population increased to 1,300 pairs by 1996 and 2,500 pairs in 2004.