Least Bell's Vireo (Vireo belli pusillus)
The Least Bell's Vireo (LBVI) is a migratory songbird that occurs arrives in early April and breeds within an increasing number of riparian areas, primarily in Southern California and northern Baja California.
Federally Endangered (USFWS)
CA State Endangered (CDFW)
The most common vocalization emitted from LVBI males is a husky rambled set of phrases that serves as his primary song throughout the breeding season.
Paired males sometimes produce a soft 3-4 note sequence "uh-uh-uh" interspersed with a delivery of the primary song. This call is only given by paired males so is useful in determining breeding status.
Suitable BREEDING Habitat
This species is a breeding riparian habitat obligate and prefers willow-dominated woodland or scrub that typically exists along streams and rivers (Kus 2002). Habitat characteristics that appear to be essential for LBVI include dense cover of one to two meters (3-6 feet) in height and a stratified canopy providing both foraging habitat and song perches for territorial advertisement (Unitt 2004; USFWS 1998).
Endemic to California and Baja California, this migratory species arrives in California in mid March and departs by late September to fly south to wintering grounds near the tip of Baja.
Federal and state recovery permits are not required to conduct presence/absence surveys for LVBI, as long as this protocol is utilized and vocalization tapes are not used. Surveyors should be familiar with LBVI vocalizations and differentiating similar species.
Presence/absence surveys require a series of 8 survey visits, spaced at least 10 days apart, between between April 10 through July 31. In order to perform nest monitoring activities, the surveyor needs to have a state permit letter and federal recovery permit.
This species formally bred in lowland riparian habitat ranging from coastal southern California through the Sacramento and San Joaquin Valleys as far north as Redbluff, and other scattered locations east of the Sierra Nevada (Grinnell and Miller 1944; USFWS 1998). By the time the species was listed by CDFG in 1984 it had been extirpated from much of is former range and was restricted to Southern California south of Santa Barbara with just 300 pairs statewide (Unitt 2004). Declines were caused by wide spread clearing of riparian habitat combined with brood parasitism by Brown-headed Cowbirds, whose increase in California was as dramatic as LBVI decline. LBVI was listed as a federally endangered species on May 2, 1986.
Currently, with restriction of habitat destruction, extensive cowbird trapping and protection from CDFW and USFWS, populations have recovered in some areas of cismontane Southern California and populations are expanding into former ranges; the northernmost sighting being from Stanislaus County (Howell and Dettling 2009; USFWS 2006). San Diego County holds the largest breeding population of LBVI in the state. It is a fairly common breeder in suitable, coastal lowland habitats (Unitt 2004).
Grinnell, J. and Miller, A. H. 1944. The distribution of birds of California. Pac. Coast Avifauna no. 27.
Howell, C. A. andDettling M.D. 2009. Least Bell’s Vireo Monitoring, Nest Predation Threat Assessment, and Cowbird Parasitism Threat Assessment at the San Juaquin River National Wildlife Refuge. 2008 Field Season Final Report. 40pp.
Kus, B. 2002. Least Bell's Vireo (Vireo bellii pusillus). In The Riparian Bird Conservation Plan: a strategy for reversing the decline of riparian-associated birds in California. California Partners in Flight. http://www.prbo.org/calpif/htmldocs/riparian_v-2.html
Kus, Barbara, Steven L. Hopp, R. Roy Johnson and Bryan T. Brown. 2010. Bell's Vireo (Vireo bellii), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/035
Patten, M. A., McCaskie, G., Unitt, P. 2003. Birds of the Salton Sea. Univ. of California Press, Los Angeles.
United States Fish and Wildlife Service (USFWS). 1998. Draft recovery plan for the Least Bell’s Vireo. U.S. Fish and Wildlife Service, Portland, OR. 139pp.
United States Fish and Wildlife Service (USFWS). 2006. Least Bell’s Vireo (Vireo bellii pusillus) 5-Year Review Summary and Evaluation. U.S. Fish and Wildlife Service, Carlsbad Fish and Wildlife Office, Carlsbad CA. 27pp.
Unitt, Philip. 2004. San Diego County Bird Atlas. San Diego Natural History Museum. San Diego, CA.