Source: http://explorer.natureserve.org/servlet/NatureServe?searchName=Vireo+bellii+
Timestamp: 2019-04-19 15:03:20+00:00

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Taxonomic Comments: See Johnson et al. (1988) and Murray et al. (1994) for analyses of the phylogenetic relationships among vireos. Four subspecies recognized: V. b. bellii breeds from Colorado to Illinois south through e. Texas to Tamaulipas, Mexico; V. b. medius breeds from sw. Texas to c. Mexico; V. b. arizonae breeds from se. California to sw. New Mexico south to Sonora and Chihuahua, Mexico; and V. b. pusillus breeds in c. and sw. California south to Baja California, Mexico (AOU 1957, Brown 1993).
Comments on USESA: Subspecies pusillus of California and Mexico is listed by USFWS as Endangered. Considered Extremely High Priority on 1998 Watch List (Carter et al. 1998). Considered of High Priority on 1996 WatchList (Carter et al. 1996). Subspecies pusillus designated as endangered by the State of California in 1986. Subspecies arizonae designated endangered by California in 1988 (Thelander and Crabtree 1994). On WatchLists of Partners in Flight in 24 central and western states. State WatchLists are based on the PIF/Colorado Bird Observatory database, updated February 1999 (Carter et al., in press; 1996, Muehter 1998). Contact: Mike Carter, Colorado Bird Observatory, 13401 Piccadilly Rd., Brighton, CO 80601. Ph. 303-659-4348. email: "cbobirdobs@aol.com".
Range Extent Comments: BREEDING: southern California, southern Nevada, southwestern Utah, Arizona, southern New Mexico, northeastern Colorado, Nebraska, South Dakota, western North Dakota, southeastern Minnesota, southern Wisconsin, northeastern Illinois, northwestern Indiana, and southwestern Michigan south to northern Baja California, southern Sonora, southern Durango, Zacatecas, southern Tamaulipas, southern Texas, north-central Louisiana, Arkansas, and southwestern Tennessee, southwestern Kentucky, southern Indiana, and western Ohio (Brown 1993, AOU 1998). NON-BREEDING: southern Baja California and southern Sonora south to Honduras, primarily on Pacific slope; casually north to California, Arizona, Texas, Louisiana, and southern Florida, and south to Nicaragua (Brown 1993, AOU 1998).
Number of Occurrences Comments: Several populations have been reduced or extirpated (Brown 1993). Arizona Bell's Vireo (V. B. ARIZONAE) has expanded range and increased in abundance along the Colorado River through Grand Canyon, Arizona as a result of increased breeding habitat created by flood control at Glen Canyon Dam; a breeding range expansion of at least 219 km was observed over 11 years (Brown et al. 1983).
Population Size Comments: North American Breeding Bird Survey (BBS) data (1966-1995) indicate a survey-wide relative abundance of 1.08 (n = 238 routes) (Sauer et al. 1996). Least Bell's Vireo (V. B. PUSILLUS) in 1994 numbered 400-500 breeding pairs north of Mexico (Thelander and Crabtree 1994, USGS 1999). One California population experienced an increase from 19 to 122 breeding vireos over eight years (Robinson et al. 1995). Another management area increased vireos from 15 territories in 1980 to 259 in 1991 (USGS 1999). The number of breeding pairs declined by approximately 20% in 1999 in the two largest populations; the reasons for this decline are unknown (Hays, pers. comm.). Although populations of Arizona Bell's vireo (V. B. ARIZONAE) in Arizona and northern Mexico are fairly stable (Corman, pers. comm.; Sauer et al. 1996; Thelander and Crabtree 1994), this subspecies has nearly disappeared from California; thirty-five singing males were noted in 1981 but only four were noted by 1986 (Thelander and Crabtree 1994). In Grand Canyon territorial males increased from 67 to 136 over six years (Brown et al. 1983).
Overall Threat Impact Comments: HABITAT: Declines may be related to loss of riparian habitat (USFWS 1988), particularly in western portions of range. Urban development, water diversion, flood control projects, grazing, and the spread of agriculture have destroyed much western nesting habitat. The Central Valley of California, where the species was once a common breeding bird but is now extirpated, has lost 95 percent of its riparian vegetation in this century to agriculture and other anthropogenic factors (Smith 1977). The species also once bred in the Owens River Valley, where today water diversion to supply urban areas has killed most of the once lush riparian vegetation (Robinson et al. 1995). Overgrazing suppresses shrub growth and reduces available nest sites and vireo density (by 50 percent in Oklahoma; Overmire 1963). BROOD PARASITISM: Observed rates of brood parasitism by Brown-headed Cowbirds (MOLOTHRUS ATER) vary geographically, ranging from 6 percent in Grand Canyon, Arizona, to 69 percent in Kansas. Based on a model developed for Least Bell's Vireos (V. B. PUSILLUS) in California, cowbird parasitism rates exceeding 30 percent lead to unstable populations that could be extirpated by stochastic events, while rates of 48 percent and 69 percent lead to population extinction in 18 and 8 years respectively (Brown 1993, Laymon 1987). In one California study, parasitism rates were reduced from 47 percent to 10 percent, increasing fledglings per pair from 2.08 to 2.86; the latter productivity rate, but not the former, should allow the population to persist and experience moderate growth (Robinson et al. 1995). Elsewhere, nonparasitized nests successfully fledge more young than parasitized nests (Barlow 1962, Brown 1993). Birds forced into fragmented habitat or marginal nesting areas are more vulnerable to parasitism. There is reason to believe, however, that large vireo populations in suitable habitat can maintain themselves in the face of cowbird parasitism (Robinson et al. 1995). In addition, different subspecies may naturally have different vulnerability to cowbird parasitism due to the timing of migration and nest initiation. In Arizona, V. B. ARIZONAE initiates nesting in early April before cowbirds become common, whereas V. B. PUSILLUS initiates nesting during a later period that coincides with the seasonal arrival of cowbirds; this reduces the relative rate of brood parasitism experienced by the earlier nesting subspecies (Corman, pers. comm.). PREDATION: Because often nests near the ground, brooding adults and young are commonly depredated by various mammals and reptiles, including domestic cat (FELIS DOMESTICUS), raccoon (PROCYON LOTOR), 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; Brown 1993, Bent 1950), and various snakes (Cink 1977, Nolan 1960). Suspected or confirmed avian predators include greater roadrunner (GEOCOCCYX CALIFORNIANUS), American crow (CORVUS BRACHYRHYNCHOS), and scrub jay (APHELOCOMA CALIFORNICA; Collins et al. 1989). POPULATION ISOLATION: One isolated population with relatively low rates of brood parasitism and intact habitat has declined; parent-offspring matings were documented, so inbreeding depression may be responsible (Robinson et al. 1995).
Short-term Trend Comments: North American Breeding Bird Survey (BBS) data (1966-1995) indicate significant survey wide declines averaging 3.2 percent per year (n = 238 survey routes), with steepest regional declines in the BBS Central region (-4.8 percent average per year; n = 173). Steepest declines by state evident with V. B. BELLII in Oklahoma (-8.3 percent average per year; n = 35), and more recently in Nebraska (-7.7 percent per year; n = 12; 1980-1995; Sauer et al. 1996). Too rare in North Dakota to be regularly encountered on regional BBS routes (Dobkin 1994). For status in northcentral U.S. see Hands et al. (1989). Several populations have been reduced or extirpated (Brown 1993). The Least Bell's Vireo, V. B. PUSILLUS, was designated "endangered" by the U.S. Fish and Wildlife Service and the State of California in 1986 when about 300 pairs were identified (USGS 1999, Brown 1993). By 1994 the subspecies numbered 400-500 breeding pairs north of Mexico (Thelander and Crabtree 1994, USGS 1999). The number of breeding pairs declined by approximately 20 percent in 1999 in the two largest populations; the reasons for this decline are unknown (Hays, pers. comm.). The Arizona Bell's vireo, V. B. ARIZONAE, was designated endangered by California in 1988 (Thelander and Crabtree 1994). Although populations in Arizona and northern Mexico are fairly stable (Sauer et al. 1996, Thelander and Crabtree 1994), the Arizona Bell's vireo has nearly disappeared from California; thirty-five singing males were noted in 1981 but only four were noted by 1986 (Thelander and Crabtree 1994). Has expanded range and increased in some areas. In one California population, brown-headed cowbird (MOLOTHRUS ATER) removal is credited with an increase from 19 to 122 breeding vireos over eight years (Robinson et al. 1995). A management program on Camp Pendleton, which included cowbird control, increased vireos from 15 territories in 1980 to 259 in 1991 (USGS 1999). Also increased along the Colorado River through Grand Canyon, Arizona as a result of increased breeding habitat created by flood control at Glen Canyon Dam. The breeding range expansion of at least 219 kilometers was observed over 11 years and territorial males increased from 67 to 136 over six years (Brown et al. 1983). No additional trend information available for Arizona Bell's Vireo in Arizona, except from the BBS; population is considered stable overall (Corman, pers. comm.).
Global Range: BREEDING: southern California, southern Nevada, southwestern Utah, Arizona, southern New Mexico, northeastern Colorado, Nebraska, South Dakota, western North Dakota, southeastern Minnesota, southern Wisconsin, northeastern Illinois, northwestern Indiana, and southwestern Michigan south to northern Baja California, southern Sonora, southern Durango, Zacatecas, southern Tamaulipas, southern Texas, north-central Louisiana, Arkansas, and southwestern Tennessee, southwestern Kentucky, southern Indiana, and western Ohio (Brown 1993, AOU 1998). NON-BREEDING: southern Baja California and southern Sonora south to Honduras, primarily on Pacific slope; casually north to California, Arizona, Texas, Louisiana, and southern Florida, and south to Nicaragua (Brown 1993, AOU 1998).
General Description: A rather nondescript vireo with one or two faint pale wing bars on otherwise dark wings; dark tail; indistinct spectacles; overall color ranges from olive in the central U.S. to grayish in Southwest (NGS 1983, Peterson 1990).
Diagnostic Characteristics: Differs from the warbling vireo (VIREO GILVUS) by having wing bars and a pale eye ring.
Reproduction Comments: Clutch size is three to five (commonly four). Incubation lasts 14 days. Young are tended by both parents, leave nest at 10-12 days, remain with adults for 25-30 days more. Breeding season begins early April in south to late May in north of range. Both parents incubate eggs and tend young (Harrison 1978). In Kansas, two broods per season possible, but most pairs rear only one (Barlow 1962). In California, two to four renesting attempts per season possible, two broods per season normal. Most pairs double-brooded in the Lower Colorado River Valley and Grand Canyon, Arizona (Brown 1993, Franzreb 1989). Nests constructed with grasses, bark, and other plant parts, along with spider webs and hair, to construct a nest loosely suspended from thin, forked branches; they line it with fine grasses and hairs. Most nests located 0.5 to 1.5 meters above ground, ranging from 0.2 to 8.0 meters (Thelander and Crabtree 1994).
Usually returns to same nesting territory in successive years (Franzreb 1989). More than 60 percent of male and 30 percent of female returning birds utilize the previous year's territories (Greaves 1989). Most fledglings that survive to breed, return to their natal area (Greaves and Gray 1991). Nest site occasionally found in same shrub as in previous years (Greaves 1987).
Ecology Comments: In California, territory size was 0.2-1.7 hectares (Franzreb 1989). Observed nesting densities range from 0.5 breeding pairs per 40 hectares of mesquite bosque habitat in the lower Colorado River Valley where nearly extirpated, to 26 pairs per 40 hectares in willow-tamarisk habitat elsewhere in Arizona (Meents et al. 1984, Szaro and Jakle 1985). Extrapolated densities of up to 200 pairs per 40 hectares have been calculated from small, extremely productive riparian study areas (Brown 1987).
Mobility and Migration Comments: Breeding populations in U.S. are long-distance migrants. Most individuals of subspecies PUSILLA migrate from breeding area late July-late September; some may overwinter in U.S. Migrates regularly through northern Mexico; winters primarily in central and southern Mexico along the Baja peninsula, the Pacific slope and in the interior of middle America to Honduras (AOU 1998, Hays pers. comm.). Spring migrants return to U.S. early to mid-March, reach northern limits of breeding range in May. In Kansas and Indiana, males arrive on breeding areas several days before females (Barlow 1962, Nolan 1960, Terres 1980). Leave northernmost limits of breeding range in August and September; most have left the U.S. by early Oct., although sizable numbers remain in Arizona until late November (Barlow 1962, Rosenberg et al. 1991). Arrives in Oaxaca Mexico and Honduras by late September (Monroe 1968, Binford 1989). Chiefly a nocturnal migrant (Brown 1993). Males sing full song during migration (Dickey and van Rossem 1938). No information on control and physiology of migration.
Habitat Comments: BREEDING: The types of habitat used vary widely among the four subspecies (Ehrlich et al. 1992). Dense brush, willow thickets, mesquite, streamside thickets, and scrub oak, in arid regions often near water, also adjoining uplands (AOU 1998, Kus and Miner 1989). Nests in shrub or low tree, usually averaging about one meter above ground, usually in horizontal or downsloping twig fork, typically near edge of thicket.
Nesting vegetation in California averages three to five meters in height (Franzreb 1989). Over 60 species of nest substrate plants used including willow (SALIX spp.), mesquite (PROSOPIS SPP.), rose (ROSA spp.), oak (QUERCUS spp.), and cottonwood (POPULUS spp.; Brown 1993). Wild rose and coast live oak (QUERCUS AGRIFOLIA) used more frequently than expected on the basis of their availability in the riparian zone (Gray and Greaves 1984). In the Grand Canyon, Arizona 64 percent of nests were in the introduced shrub tamarisk (TAMARIX RAMOSISSIMA) and 24 percent were in honey mesquite (P. GLANDULOSA; Brown 1993).
May nest in any successional stage with dense understory vegetation; most critical structural component of habitat in California is a dense shrub layer 0.6-3.0 meters above ground (Goldwasser 1981, Franzreb 1989). Most Least Bell's Vireos (V. B. PUSILLUS) nest sites are in willow stands 5-10 years of age (RECON 1989).
Nesting success depends on an optimum microclimate, and adequate shade may be critical for successful nesting at low elevations. Tree canopies provide cooler environments for static temperature of the eggs while adults are foraging (Thelander and Crabtree 1994). For additional analysis of V. B. PUSILLUS nest site characteristics see Hendricks and Rieger 1989, and Olson and Gray 1989.
NON-BREEDING: Quantitative information needed (Brown 1993). In migration and winter, primarily in dense scrub (AOU 1998). West coast of Mexico and Honduras in thornscrub adjacent to watercourses, riparian gallery forests, tropical deciduous forest, and arid tropical scrub (Hutto 1989); rarely in interior subtropical scrub and tropical evergreen forest (Binford 1989).
Food Comments: Eats primarily insects and small spiders (99.3 percent), rarely fruits (0.7 percent) (Chapin 1925). Forages in dense brush, occasionally in treetops (Terres 1980, NGS 1983). In California, 69 percent of foraging observations within 4 meters of ground, but rarely on ground. Gleans prey from leaf and bark substrates, also obtains some prey by hovering and occasionally by hawking (Salata 1983). No observations of drinking; may obtain adequate water from diet (Brown 1993).
Stewardship Overview: Has seriously declined in several portions of range, particularly in arid southwest where endangered. Vulnerable to loss and fragmentation of riparian and dense scrub habitats, and to brood parasitism by brown-headed cowbirds (MOLOTHRUS ATER). These factors continue to threaten remaining breeding populations. Breeding habitat restoration and cowbird control has led to population recovery in limited areas.
Restoration Potential: Species responds, at least locally, to brown-headed cowbird (MOLOTHRUS ATER) control and restoration of riparian habitat (Brown 1993). A procedure for developing a specific vegetation restoration model for the Least Bell's Vireo (V. B. PUSILLUS) habitat is available which addresses mean percent cover, density, abundance, species composition, and expected plant mortality rates (Baird and Rieger 1989).
All of five restoration sites monitored in one study supported nesting V. B. PUSILLUS within 3-5 years, providing the first evidence that it is possible to create suitable nesting habitat for this species in coastal California lowlands. Nests at restoration sites have successfully fledged young, and have been no less productive than nests in natural habitats. Key components of site restoration success and use were water availability, structure of planted vegetation, and the site's proximity to natural habitat (Kus 1998). Translocation and captive breeding for release into areas within their historical range has been analyzed but not undertaken (Franzreb 1990).
Preserve Selection & Design Considerations: Several factors should be considered. Riparian habitats are critical in arid and semi-arid regions. Riparian functions must support establishment and maintenance of mid-successional shoreline vegetation. Minimizing habitat patchiness may reduce rates of cowbird parasitism and preserves should be located in areas free of brown-headed cowbirds (MOLOTHRUS ATER). Even isolated habitat islands occupied by apparently unpaired males should be protected and enhanced because the observed tendency of females to wander between mates may make these males important to tying together isolated sub-populations of the species (Greaves 1989).
Management Requirements: No special management attention needed for apparently stable populations in eastern half of breeding range (Brown 1993), but those in central and western range would likely benefit from conservation of breeding habitat. Abundance is strongly influenced by land use patterns in breeding habitat, particularly in arid western regions where it is more limited by the quality and availability of riparian habitat than elsewhere. Removal of brown-headed cowbirds (MOLOTHRUS ATER) from breeding habitat during spring may be essential in some areas (Thelander and Crabtree 1994). Selective shooting and trapping of cowbirds, relocation of livestock facilities away from riparian areas, and reduction of grazing in riparian areas to maintain a dense understory are recommended, as well as revegetation of riparian areas to increase extent of nesting habitat and to deter cowbirds (Laymon 1987). Relocation or elimination of dairies, livestock feed lots, waste grain, bird feeders, and other cowbird attractants may reduce local parasitism rates. Periodic disturbance of riparian areas may be required to maintain the 5-10 year age structure of vegetation preferred for breeding. Scouring by flooding and river meandering rejuvenates gallery vegetation, an important factor in maintaining habitat (Franzreb 1990).
Monitoring Requirements: Often located by song. Adult males may sing year round, except for fall migration, from shortly before sunrise throughout the day, but most frequently before 10:00 am. Rarely sings during afternoon when temperatures exceed 30 C (Brown 1993). During breeding, males sing principally on and near the nest, rarely in flight (Barlow 1962). Sing most frequently early in breeding season and during territorial disputes; frequency may decline sharply after pair formation (Nolan 1960). Winter habitat selection has been documented using point count census and other basic field observation methods (Hutto 1989).
Management Research Needs: Continued research and monitoring is needed of the factors influencing vegetation development and use of restored riparian habitat; modification of restoration practices may be appropriate (Kus 1998). Management objectives for V. B. PUSILLUS populations should assume a high degree of isolation from each other until there is evidence that birds from one drainage become breeders in other drainages (Greaves 1989). Reasons for population declines in mesquite habitat need further understanding (Ehrlich et al. 1992). Little data on density, ecology, and habitat use during migration or from its winter range (Brown 1993). No information on fidelity to winter range.
Biological Research Needs: It is not known what constitutes a viable genetic pool among Least Bell's Vireos (V. B. PUSILLUS). Additional research is needed to understand what role genetic factors, such as inbreeding depression, may affect the conservation of isolated populations (Greaves 1989, Robinson et al. 1995).
Management Information Acknowledgments: The author thanks Troy Corman, Neotropical Migratory Birds Coordinator, Arizona Department of Fish and Game, for providing information on the status and ecology in Arizona, and Loren Hays, USFWS, for providing an update on the trends and management of the endangered subspecies in California. Funding for the preparation of this abstract was made possible by the U.S. Fish and Wildlife Service, Division of Endangered Species.
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