California Partners in Flight Riparian Bird Conservation Plan

Least Bell's Vireo (Vireo bellii pusillus)

Photo by James Gallagher, Sea and Sage Audubon


Prepared by: Barbara Kus (

USGS Western Ecological Research Center
San Diego Field Station, 5745 Kearny Villa Road, Suite M
San Diego, CA 92123


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.


range map



The Least Bell's Vireo, Vireo bellii pusillus, is one of four subspecies of Bell's Vireo recognized by the American Ornithologist's Union (AOU 1957). It is the western-most subspecies, breeding entirely within California and northern Baja California. A second subspecies, V. bellii arizonae, has a limited distribution in California along the lower Colorado River, but occurs primarily throughout Arizona, Utah, Nevada, and Sonora, Mexico. The subspecies are believed to be isolated from one another during both the breeding and wintering seasons (Hamilton 1962).

MANAGEMENT STATUS: The Least Bell's Vireo was listed as a state endangered species by the California Fish and Game Commission in 1980, and as a federally endangered species in 1986. Critical habitat for the species was designated in 1994.



Historically, the Least Bell's Vireo was a common to locally abundant species in lowland riparian habitat, ranging from coastal southern California through the Sacramento and San Joaquin Valleys as far north as Red Bluff (Tehama County). Populations also occurred in the foothill streams of the Sierra Nevada and Coast Ranges, and in Owens Valley, Death Valley, and scattered locations in the Mojave Desert (Cooper 1861; Baird et al. 1874; Belding 1878; Fisher 1893, Anthony 1893, 1895; Grinnell and Swarth 1913; Grinnell and Storer 1924; Grinnell et al. 1930, Grinnell and Miller 1944). Grinnell and Miller (1944) reported elevational extremes of -54 m (-175 ft.) in Death Valley to 1,260 m (4,100 ft.) at Bishop, Inyo County.


By the time the species was listed by the U.S. Fish and Wildlife Service in 1986, it had been extirpated from most of its historic range, and numbered just 300 pairs statewide. Populations were confined to eight counties south of Santa Barbara, with the majority of birds occurring in San Diego County. In the decade since listing, Least Bell's Vireo numbers have increased 6-fold, and the species is expanding into its historic range. In 1998, the population size was estimated at 2,000 pairs (L. Hays, USFWS, pers. comm.). Nesting vireos have recolonized the Santa Clara River (Ventura County) to the north, where 67 pairs nested in 1998 (J. Greaves, pers. comm.), and the Mojave River (San Bernardino County) to the northeast (Kus and Beck 1998). The northernmost reported sighting in recent years is of a nesting pair of vireos near Gilroy (Santa Clara County) in 1997 (Roberson et al. 1997). Roughly half of the current vireo population occurs on drainages within Marine Corps Base Camp Pendleton in San Diego County (Fish and Wildlife Service, 1998).


Least Bell's Vireos winter in southern Baja California, Mexico. Unlike during the breeding season, they are not limited in winter to willow-dominated riparian areas, but occupy a variety of habitats including mesquite scrub within arroyos, palm groves, and hedgerows bordering agricultural and residential areas (Kus, unpubl. data).



Males establish and defend territories through counter-singing, chase and sometimes physical combat with neighboring males. Territory size ranges from 0.5 to 7.5 acres; some averages are: Tijuana River, 1991: 2.5 ± 1.2 acres, Kus 1991e; 1992: 2.7 ± 1.4 acres, Kus1992c; 1993: 1.8 ± 0.8 acres, Kus 1993d; Sweetwater River, 1996: 1.9 ± 0.8 acres, RECON 1989; Prado Basin (Santa Ana River): 1987: 1.9 ± 0.9 acres, Hays 1987; 1988: 1.6 ± 0.9 acres, Hays 1988; San Diego River: 1987: 2.1 ± 1.0 acres, Kus 1989a; 1988: 1.7 ± 0.9 acres, Kus 1989a. Newman (1992) investigated the relationship between territory size, vegetation characteristics, and reproductive success for populations of vireos at the San Diego and Sweetwater Rivers, but found no significant factors which could account for the variability in territory size observed at his sites.


Arrival dates on breeding grounds: Birds begin returning to southern California breeding sites in mid- to late-March; Grinnell and Miller (1944) reported later arrival (early April) for historic northern California populations. Males arrive in advance of females by several days, and observations of banded birds suggest that returning adult breeders may arrive earlier than first-year birds by several weeks (Kus, unpubl. data).

Departure dates from breeding grounds:Vireos 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 (McCaskie and Pugh 1965; McCaskie 1969; K. Miner, pers. comm.; J. Newman, pers. comm.).

Spring migration period: Vireos usually arrive in California during mid- to late-March. Two least Bell's vireo were seen as early as March 17, 1972 at Old Mission Damn in San Diego (McCaskie 1972).

Fall Migration Period: Vireos usually leave their breeding grounds by September. Some extreme dates are September 23 (1977, B. Cord) at Old Mission Dam and October 5, 1884 in Poway (F. E. Blaisdell in Belding 1890).

Extent of wintering in CA: Vireos occasionally occur in California during the winter. Some records include: one individual on the San Diego River (San Diego County) on 1 January 1963 (McCaskie and Banks 1964), one at Bonita (San Diego County) between 18 December 1969 and 17 January 1970 (McCaskie 1970), two on the Otay River (San Diego County): one on 27 December 1970 (McCaskie 1971) and the other on 6 January 1979 (McCaskie 1979), and one at Coronado (San Diego County) on 15 December 1980 (McCaskie 1980).


Little information on habitat use; species as a whole described as using coastal scrub, riparian, and other woodland habitats during migration (Brown 1993).



Least Bell's Vireos obtain prey primarily by foliage gleaning (picking prey from leaf or bark substrates), and hovering (removing prey from vegetation surfaces while fluttering in the air). Salata (1983) noted foliage gleaning during 93 percent of his observations of foraging vireos (N=131), and hovering during 30 percent. Miner (1989), in a study of vireo foraging ecology at the Sweetwater River (San Diego County), observed that 50.4 percent of prey attacks (N=413) consisted of foliage gleaning, and 38.7 percent hovers. Both Salata (1983) and Miner (1989) observed vireos occasionally capturing prey by hawking (pursuit and capture of flying prey), and Miner (1989) noted a behavior she called "clinging," which she described as hovering, but with the feet in contact with the vegetation.

Foraging occurs at all levels of the canopy, but appears to be concentrated in the lower to mid-strata, particularly when pairs have active nests (Grinnell and Miller, 1944; Goldwasser 1981; Gray and Greaves 1984; Salata (1983), Miner (1989). Salata (1983) found that 69 percent of 131 foraging observations were within 4 meters (12 feet) of the ground. Miner (1989) found a similar peak in foraging activity in vegetation between 3-6 meters (9-18 feet) in height. Moreover, she determined that the distribution of vireo foraging time across all heights was not simply a function of the availability of vegetation at those heights, but rather represented an actual preference for the 3-6 meter zone.


Least Bell's Vireos are insectivores, preying on a wide variety of insect types including bugs, beetles, grasshoppers, moths, and particularly caterpillars (Chapin 1925; Bent 1950).


Vireos probably do not require water for drinking.



Data collected for color-banded birds indicate that site fidelity is high among adults, with many birds not only returning to the same territory, but placing nests in the same shrub used the previous year (Salata 1983b, Kus unpubl. data). Return rates of first-year breeders to their natal drainages ranged from 15-18% over the course of nine years of study on the Santa Ynez River in Santa Barbara County (Greaves 1987; Greaves and Gray 1991). Kus (unpubl. data), drawing from 10 years of study at several San Diego County sites, found that on average, 20% of first-time breeders dispersed away from their natal drainages, with a higher proportion of males (22%) than females (13%) dispersing.


Least Bell's Vireos place their nests in a variety of plants that provide concealment in the form of dense foliage. The most frequently used species include willows (Salix sp.), mulefat (Baccharis glutinosa), California wild rose (Rosa californica),poison oak (Toxicodendron diversilobum), mugwort (Artemisia douglasiana), and cottonwood (Populus fremontii) (Olson & Gray 1989, RECON 1989).


Nests are typically placed within one meter of the ground. Average height of 25 nests at the Sweetwater River (San Diego County) was 0.9 ± 0.4 meters; for 24 nests at the San Diego River (San Diego County); 1.3 ± 0.6 meters, and for 16 nests at the San Luis Rey River; 1.1 ± 0.3 meters (RECON 1989). Nest height of 226 nests at the Santa Margarita River (San Diego County) ranged from 0.3 - 2.4 meters, averaging 1.0 meters (Salata 1984). The average height for 32 nests at Santa Ynez River (Santa Barbara County) was 0.7 ± 0.35 meters, with a range of 0.4 - 1.2 meters (Olson & Gray 1989).


Average host heights range from 2.8-5 meters; some examples are: 3.2 ± 1.8 meters for 29 nests at the Sweetwater River (San Diego County); 4.1 ± 2.4 meters for 23 nests at the San Luis Rey River (San Diego County); 5.0 ± 2.5 meters for 21 nests at the San Diego River (San Diego County) (RECON 1989), 2.8 ± 0.4 for 32 nests on Santa Ynez River (Santa Barbara County) (Olson & Gray 1989).


Early to mid-successional riparian habitat is typically used for nesting by the Least Bell's Vireo because it supports the dense shrub cover required for nest concealment as well as a structurally diverse canopy for foraging. Vegetation characteristics of riparian stands between five to ten years of age are most suitable for nesting Least Bell's Vireo (Goldwasser 1981, Kus 1998, RECON 1989, Fish & Wildlife Service 1998). Restored riparian in the coastal lowlands of southern California has the habitat structure to support breeding vireos within 3-5 years particularly if they are adjacent to established riparian areas (Kus 1998).


Least Bell's Vireo nests are normally found in areas with dense understory (RECON 1989, Salata 1981 and 1983, Goldwasser 1981). At the Santa Ynez River (Santa Barbara County), below 1.0m, mugwort (Artimisia douglasiana) and summer mustard (Brasica nigra) contribute most to foliage density (Olson & Gray 1989).


Open space within a one meter radius surrounding a nest was calculated at three river drainages in San Diego County: Sweetwater River, San Diego River and San Luis Rey River (RECON 1989). Open space represents sections in which there was less than 50 percent vegetation coverage in six sections including above and below the nest. Results represent the average number of openings within one meter of the nest: Sweetwater River = 0.7 ± 0.8 (N=29), San Diego River = 2.3 ± 1.7 (N=24), and San Luis Rey River = 1.6 ± 1.5 (N=23).


The canopy of riparian habitat is mainly dominated by willows. On the Santa Margarita River, at Camp Pendleton, 97% of the canopy around the nest is willow spp. (N=38) and the average percent canopy cover within 0.4ha of a nest = 25% (Salata 1983).


Santa Ynez River (Santa Barbara County): average = 8.3 m, range = 1.8-18.3m (Olson & Gray 1989). Santa Margarita River (Camp Pendleton, San Diego County) average = 7 m, range 3-15m (Salata 1983).


On Santa Ynez River in Santa Barbara mainly Fremont cottonwood (Populus fremontii), arroyo willow (Salix lasiolepis) and red willow (Salix laevigata) (Olson and Gray 1989). San Diego County most dominant trees are black willow (Salix goodingii) and arroyo willow (Salix lasiolepes) (RECON 1989).


Vireos tend to occupy areas which support dense shrub cover (Salata 1981, Salata 1983, Goldwasser 1981). The proportion of tress with shrub understory was significantly higher at sites occupied by vireos than at those in which vireo did not occupy (RECON 1989 p 27).


On Santa Ynez River in Santa Barbara County mainly mugwort (Artimisia douglasiana), mulefat (Baccharis salicifolia), and willow shrubs (Salix spp.) (Olson and Gray 1989). On Sweetwater River, San Diego River, and San Luis Rey River in San Diego County the most common shrub species (92 % of territories) is mulefat (Baccharis glutinosa) (RECON 1989).


On Sweetwater River, San Diego River, and San Luis Rey River (San Diego County), mulefat (Baccharis glutinosa) is the most dominant followed in high number by the willow shrubs (Salix spp.), and tamarisk (Tamarix sp.) (RECON 1989).


The greatest foliage density around nests occurs between 0.2 and 1.0m and consists mostly of mugwort (Artemisia douglasiana) and summer mustard (Brassica nigra) (Olson & Gray 1989, RECON 1989).


Curly dock (Rumex crispus) and western ragweed (Ambrosia psilostachya) (RECON 1989).


The proportion of nests, at several sites in San Diego County, that were concealed by ground cover are: Sweetwater River 62 percent (18/29), San Luis Rey River 65 percent (15/23) and San Diego River 29 percent (6/21) (RECON 1989).


Least Bell's Vireo prefer to nest in areas with low aquatic and herbaceous cover (RECON 1989).


In a study along the Santa Ynez River (Santa Barbara County), trees at successful nest sites were significantly greater in mean DBH than unsuccessful nests (Olson & Gray 1989). The average DBH for trees surrounding nests was 15.5cm with a range of 1.8-50.0cm) (Olson & Gray 1989).


There is no data on nests and distance to water, however, nests are found within the active floodplain of a waterway and are, therefore, within about 300m of surface water.


Open-cup nest placed in the horizontal fork of a tree or shrub branch and bound at the rim. Nests are typically constructed of soft plant strips and shreds, leaf fragments, small pieces of bark, spider webs, and other materials, and are usually lined with soft substances such as plant down or hair (Bent 1950).


The density of breeding Least Bell's Vireo is difficult to calculate due to the clumped nature of their territories. In many areas along the river a group of territories will be found interspersed with areas containing no territories. In 1994, along the San Diego River (San Diego County), Least Bell's Vireo territories were averaged at 0.41 territories per ha (Kus 1994). In 1988, along the San Luis Rey River (San Diego County) territories were averaged at 0.22 territories per ha (Kus 1988). This same river, however, had areas where densities were as high as 2.8 territories per ha (Kus 1988).



Monogamous. Birds may switch mates between successive nesting attempts within the same season and between years (serial monogamy). Spiegelberg (1997), using microsatellite techniques to study the genetic structure of family groups, failed to detect any evidence of extra-pair copulatory activity in the three vireo populations he studied.


Nest-building can begin soon after arrival of the pair, typically in late March, although prolonged inclement weather can delay nest-building for several weeks (pers. obs.). Nest initiations peak during April, but can continue through the first week of July.


Males use high, often exposed, perches in the canopy as singing perches during territorial defense and advertisement. Courtship includes displays in which birds flick their wings and alternately fan and depress their tails, often accompanied by rapid calls (Bent 1950, Brown 1993).


Typically 3-4, occasionally 2, rarely 5 (Bent 1950).


Both male and female share in incubation, although females incubate more than males during the day (Hensley 1950, Nolan 1960). Nighttime incubation appears to done exclusively by females (Barlow 1962).


Incubation in this subspecies typically commences with the penultimate egg (Kus, unpubl. data), although reports for other subspecies indicate that it can begin as early as laying of the first egg (Pitelka and Koestner 1942). Incubation lasts about 14 days (Bent 1950).




Nestlings fledge 10-12 days after hatching.



Both sexes feed and brood nestlings. Fledged young may be cared for by both parents, or, if the pair re-nests, primarily by the male.


Fledglings are cared for by their parents for at least two weeks after fledging, during which time territorial boundaries are relaxed as family groups range over larger areas. Studies of banded birds reveal that fledglings generally remain in the territory or its vicinity for most of the season; however, the behavior of older fledglings produced early in the year has not been well studied (Kus, unpubl. data).


Little information. Territorial boundaries are relaxed at the end of the breeding season, and male singing frequency declines substantially during the post-breeding molt, making it difficult to detect and locate birds.


Birds typically begin breeding as first-year adults.


Least Bell's Vireos can initiate as many as five nests during a season, but typically do not raise more than two broods, with most pairs raising no more than one brood per season.


Least Bell's Vireos are extremely vulnerable to cowbird parasitism, which, in concert with habitat loss and degradation, is considered a primary factor responsible for the species decline (Linton 1908; Dawson 1923; Hanna 1928; Rowley 1930; Bent 1950; Grinnell 1950). In heavily parasitized areas, up to four cowbird eggs may be found in vireo nests (Salata 1983; B. Jones, unpubl. data), particularly during the second half of the nesting season when fewer hosts are available. Reports prior to the implementation of cowbird management programs indicate that cowbirds parasitized 33-100% of vireo nests (Goldwasser et al. 1980; L. Salata, unpubl. data; B. Jones, unpubl. data; Gray and Greaves 1984; L. Hays, unpubl. data). Even with cowbird management, in some areas, up to 43% of nests are parasitized, of which, on average, 29% are abandoned (Kus 1999).



Grinnell and Miller (1944) reported elevational extremes of -54 m (-175 ft.) in Death Valley to 1,260 m (4,100 ft.) at Bishop, Inyo County.


Much of the riparian habitat throughout the range of the Least Bell's Vireo has been destroyed leaving fragmented remnants. The riparian system in southern California has decreased by about 90% of what was present in 1850 (Smith 1977). In San Diego County the loss is reported at 61% (Oberbauer 1990).


Vireos occur in disproportionately high frequencies in the wider sections (greater than 250m) of the riparian relative to site availability (RECON 1989).


The riparian system is adapted to periodic flooding. The dynamic aspect of the riparian vegetation allows for fast recovery to disturbance as long as the natural water flow and sedimentation regimes are intact (Fish & Wildlife 1998). Flooding is currently restricted in almost all habitat occupied by the Least Bell's Vireo due to upstream dams. Potential disturbance to riparian habitat and nesting Least Bell's Vireo are associated with urbanization and agriculture and include: runoff from both agricultural fields and roadways, traffic noise, feral pets, recreational use of habitat, and increased foraging habitat for brown-headed cowbird (Molthrus ater).

Least Bell's Vireo often nest near open spaces or trails. Nest failure and abandonment can be caused by human disturbance such as trampling of nests or nest sites or clearing of vegetation (Fish & Wildlife 1998). Brood parasitism and habitat fragmentation are the primary factors causing the species decline and are both results of human-induced disturbance.


Due to increased urbanization and agriculture in southern California, much of the riparian habitat is now surrounded by agricultural areas such as farming, cattle grazing and horse ranching as well as urban development such as roads, golf courses, residential development, and commercial development. Vireo territories (n=35) bordering on agricultural and urban areas were significantly less successful in producing young than territories bordering on coastal sage scrub, grassland and chaparral (RECON 1989).


No specific data but it is possible that pesticides could be incorporated into the riparian system due to runoff by neighboring agricultural fields.


Predators may include Western Scrub-jays (Aphelocoma californica), Cooper's Hawk (Accipiter cooperii), raccoon (Procyon lotor), Virginia opossum (Didelphis virginiana), coyote (Canis latrans), long-tailed weasel (Mustela frenata), dusky-footed woodrat (Neotoma fuscipes), deer mouse (Peromyscus maniculatus), house mouse (Mus musculus), rat (Rattus rattus), domestic cat (Felis domesticus), gopher snakes (Pituophis melanoleucus) (Franzreb 1989).



Survivorship. It is estimated that 5 to 29 percent of Least Bell's Vireos survive to their first breeding season. This is based on studies of color-banded birds returning to their natal breeding grounds and may not include birds that have dispersed to unstudied areas (Fish & Wildlife 1998).

No sex specific differences in survival have been reported. No estimates of survival are available for period from fledging to sexual maturity.

Reproductive success. The annual percentage of fledglings per nest range from 33 percent to 89 percent with long term averages ranging between 41 percent and 74 percent. Annual average numbers of fledglings per nest range between 0.7 and 3.3 with averages falling between 1.1 and 2.4. The Least Bell's Vireo can attempt as many as 5 nests per season, therefore, it is appropriate to relate the number of fledglings per pair to emphasis an individual's reproductive success. The annual average number of fledglings per pair ranges from 0.9 and 4.5, with long term averages ranging between 1.8 and 3.2. (Fish & Wildlife 1998).


Historical accounts of Least Bell's Vireo described them as common to abundant in the late 1800's and early 1900's (Cooper 1861, 1874, Anthony 1893 and 1895, Baird et al. 1874, Belding 1878, Fisher 1893, Grinnell and Swarth 1913, Grinnell and Storer 1924, Grinnel et al. 1930, Grinnell and Miller 1944). By 1986, the population had declined to an estimated 300 pairs, with the majority occurring in San Diego County. Restoration efforts and Brown-headed Cowbird control have allowed populations to increase in recent years. In 1998, the population size was estimated at 2,000 (L. Hays, USFWS, pers. comm.). A population viability analysis, using computer simulations, indicates that the Least Bell's Vireo populations currently exceed minimum viable population size (Fish & Wildlife 1998). This was based on eight populations in San Diego County, Riverside County and Santa Barbara County. This means that the population has less than a five percent probability of going extinct in the next 100 years (Soule 1987) as long as habitat size and quality remains the same or increases and brown-headed cowbird control continues.



The invasion of exotic plant species into the riparian system increases habitat fragmentation and can decrease suitable nesting habitat in some cases. Invasive non-natives found in current Least Bell's Vireo habitat include castor bean (Ricinus communis), cocklebur (Xanthium strumarium), tamarisk (Tamarix sp.) and giant reed (Arundo donax) (Fish & Wildlife 1998). Arundo donax is of prime concern due to it's ability to disperse throughout the drainage and it's rapid growth that allows it to outcompete and restrict growth of native riparian habitat.


1. Preserve and enhance existing riparian habitat within the vireo historic range.
2. Control exotic vegetation.
3. Continue cowbird removal and/or develop alternative means of controlling cowbird parasitism.
4. Management on a community level in order to reduce predation levels.


Southwestern Willow Flycatcher (Empidonax trailii extimus), Yellow Warbler (Dedroica petechis brewsteri), Yellow-breasted Chat (Icteria virens), Song Sparrow, Common Yellowthroat, Black-headed Grosbeak, Tree Swallows, Downy Woodpeckers, House Wren, Pacific-slope Flycatcher, Ashthroated Flycatcher, Spotted Towhee, Orange-crowned Warbler, Hutton's Vireo, Nuttall's Woodpecker, Black Pheobe, Bushtit, Swainson's Thrush, American Goldfinch, Lesser Goldfinch, Wrentit, Bewick's Wren, Cooper's Hawk, Red-shouldered Hawk, White-tailed Kite, Ruby-crowned Kinglet, Sharp-shinned Hawk, Hermit Thrush



1. Conduct regular monitoring of vireo populations.
2. Conduct thorough rangewide surveys.
3. Conduct a statewide inventory of riparian habitat.
4. Color banding to collect data for demographic and dispersal analysis.


1. Determine whether any reproductive parameters are density-dependent.
2. Determine whether dispersal is density-dependent.
3. Examine the effect of different cowbird control regimes on vireo parasitism rates and reproductive success.
4. Evaluate the use of restored habitat by vireos.
5. Investigate the status of wintering habitat and identify current or potential threats.
6. Identify predators and establish means of control.
7. Identify additional and potential Least Bell's Vireo breeding habitat within its historical range.


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