California Partners in Flight Coastal Scrub and Chaparral Bird Conservation Plan
Gray Vireo (Vireo vicinior)
Illustration by Adrienne Olmstead
Prepared by: Kirsten Winter (firstname.lastname@example.org)
Cleveland National Forest
10845 Rancho Bernardo Road
San Diego, CA 92127
Revised by: Lori Hargrove (Lori.Hargrove@email.ucr.edu)
University of California
Riverside, CA 92521
Winter, K and L. Hargrove. 2004. Gray Vireo (Vireo vicinior). In The Coastal Scrub and Chaparral Bird Conservation Plan: a strategy for protecting and managing coastal scrub and chaparral habitats and associated birds in California. California Partners in Flight. http://www.prbo.org/calpif/htmldocs/scrub.html
action plan summary
No subspecies are currently accepted, but studies of geographic variation are needed (Barlow et al. 1999). Stephens (1890) and Phillips (1991) suggested that western populations might be distinguishable as the subspecies Vireo vicinior californicus, by darker upperparts.
The Gray Vireo is protected under the Migratory Bird Treaty Act (USFWS 2003), and is on the national list of Birds of Conservation Concern 2002 (USFWS 2002), the Partners in Flight National Watch List (Rich et al. 2004), the first priority list of California Species of Special Concern (CDFG 2003), and the California-BLM Animal Sensitive Species List (BLM 2004). It has also been on watch lists or priority lists in other states, including Arizona, Colorado, Nevada, New Mexico, Texas, and Utah.
The Gray Vireo breeds in the southwestern United States and northwestern Mexico, and winters from southeastern California, southwestern Arizona, and southwestern Texas south into Sonora and southern Baja California, Mexico (AOU 1998, Barlow et al. 1999, Howell and Webb 1995, Unitt 1994, 2000).
HISTORICAL BREEDING DISTRIBUTION:
California: Grinnell and Miller (1944) described the Gray Vireo as fairly common to locally common in arid chaparral in the mountains and foothills of southern California from the Mexican boundary north to the upper Kern River Basin, and in the Mojave Desert from the Providence Mountains north to the Grapevine Mountains in Inyo County. Historic occurrences included Campo and Julian in San Diego County, the San Jacinto Mountains, Joshua Tree National Monument, 10 miles east of Riverside (Stephens 1890), the San Bernardino Mountains, Cajon Pass, Hesperia, San Gorgonio Pass, the San Gabriel Mountains, Mint Canyon near Saugus (Miller 1921), and Bodfish and Walker Pass in Kern County (Grinnell 1922). In 1908 in the San Jacinto Mountains, Grinnell and Swarth (1913) described the Gray Vireo as "common in many localities from 3000-6500 feet," and estimated that there were 960 individuals in the San Jacinto region. Johnson et al. (1948) described the Gray Vireo as locally "conspicuous, though not abundant" in the Providence Mountains in 1938, and collected 10 specimens.
Outside of California: The primary range included southern Nevada, Grand Canyon of Arizona, southeastern Colorado, and southwestern New Mexico, and Montoya New Mexico (Bailey 1928). In 1950, Taber described the range as including southern Nevada, southwestern Utah, Arizona, western Colorado, western Oklahoma, New Mexico, western Texas, and northern Lower California [Baja] (Taber 1950). In Arizona, described as a fairly common nesting species in northwestern and central Arizona, locally south to the Chiricahua Mountains (Phillips et al. 1964). In Nevada, reported from the Sheep and Spring Ranges (Johnson 1965). In Utah, populations apparently became established at Zion National Park during the 1960s (Wauer and Carter 1965).
CURRENT BREEDING DISTRIBUTION:
The Gray Vireo breeds in the mountains of southern California, northern Baja California, southern Nevada, southern Utah, Arizona, New Mexico, western and southern Colorado, western Texas, northwestern Coahuila, and possibly northwestern Oklahoma and southwestern Wyoming (AOU 1998, Unitt 1994).
California: The Gray Vireo is currently known as a summer resident in the mountains of the eastern Mojave Desert, on the northeastern slope of the San Bernardino Mountains, locally on the slopes of the San Jacinto and Santa Rosa Mountains, and in the mountains of San Diego County (Garrett and Dunn 1981, Small 1994). The breeding range is highly discontinuous, and increased fragmentation has probably occurred. Los Angeles County Breeding Bird Atlas data, collected 1995-1999, revealed a few scattered pairs in the San Gabriel Mountains (L. Allen, pers. comm.); few records are being reported from San Bernardino or Riverside Counties. The Gray Vireo has been extirpated from several locations in California, including Kern County, Joshua Tree National Park, the Kingston Range, Cajon Pass, and localities in Riverside and San Diego counties.
Inyo County: Single observations in the Grapevine Mountains near the Nevada border in 1991 and 2002 (ESAS 2002); known breeding location in the Panamint Mountains and possibly Kingston Mountains (Small 1994).
Kern County: No recent records
Santa Barbara and Ventura Counties: Single observation by Kevin Cooper (pers. comm.) near Big Pine Mountain, Los Padres National Forest in 2000. Additional potential habitat occurs in the Los Padres National Forest, where there is extensive pinyon-juniper woodland along Highway 33 as it approaches the Cuyama Valley. This area has not been extensively surveyed for Gray Vireos (P. Unitt, pers. comm.).
Los Angeles County: Los Angeles County Breeding Bird Atlas: data collected 1995-1999 (L. Allen, pers. comm.), revealed Gray Vireos at five locations. In the northern San Gabriel Mountains, there was possible or probable breeding at three locations, and confirmed breeding at one location (a nest with eggs, later with young). In addition, a pair was found in the Santa Susana Mountains just north of Chatsworth-a slightly westward extension of the previous known range. The estimated density of vireos was 1-9 individuals per 10 sq. mile block. Historic locations at Bob's Gap were surveyed but no Gray Vireos were found.
San Bernardino County: No focused surveys have been directed at the Gray Vireo in San Bernardino County (R. McKernan, pers. comm.), but intermittent records suggest that populations persist in the San Bernardino Mountains and the mountains of the Mojave National Preserve.
Formerly occurred at Blackrock Spring, Quail Spring, and Smithwater Canyon in Joshua Tree National Park (Miller and Stebbins 1964).
Eastern San Bernardino Mountains: A few records exist from BBS route #14066 at Big Bear in the San Bernardino Mountains, beginning with six individuals in 1969 and ending with one in 1983, but this route is no longer active (USGS 2004a). Although records are few and intermittent, Gray Vireos have bred consistently in the Round Valley/Rose Mine area (Small 1994, USDI BLM 2003).
Riverside County: Formerly occurred at Covington Flat in Joshua Tree National Park (Miller and Stebbins 1964). In the San Jacinto Mountains, where formerly described as common by Grinnell and Swarth (1913), it is not known how many birds persist but sightings are rare (CVMSHCP 2004). The few records are from Pinyon Flat (1977, 1997), and Santa Rosa Peak Road (1979, 1981) (CVMSHCP 2004).
San Diego County: The Gray Vireo is apparently most numerous in San Diego County, where the population was recently estimated to be in the low hundreds based on bird atlas data collected between 1997 and 2000 (Unitt 2004). However, the recent drought conditions and extensive fires in San Diego County that peaked in 2003 may have reduced this population size.
Outside of California: According to Breeding Bird Survey (BBS) data, the main population center is in Utah and Arizona on the Colorado Plateau, especially the Virgin River Area (3-10 relative abundance, measured as the average number of birds detected per 2.5 hours at roadside stops), but population densities appear very low throughout most of the range (0.05-1 relative abundance) (USGS 2004b). BBS data and distribution maps: http://www.mbr-pwrc.usgs.gov/bbs/bbs.html. In Mexico, breeding locations include northern Baja California, and the Sierra del Carmen, Coahuila (Barlow and Johnson 1969, Howell and Webb 1995). Other Partners in Flight Gray Vireo accounts (by state): Arizona (Latta et al. 1999), Colorado (Beidleman 2000), Nevada (Neel 1999), New Mexico (NMPIF 2003), Utah (Parrish et al. 2002).
Breeding Bird Atlases:
Arizona BBA - Gray Vireos were found patchily distributed in 315 of 1834 blocks surveyed (T. Corman, pers. comm.). Troy Corman, Coordinator for the Arizona BBA, provided the following information: "Some of the smaller and more local populations in southeastern Arizona may have been reduced or extirpated during the past 100 years, however, I would have to say that the distribution of the more northern populations have likely changed very little. The ABBA project did find this species in many areas that it had not been reported before, however, this is more likely due to increased survey efforts in remote areas where very little or no prior avian surveys had been conducted. I have no information on the increase or decrease of abundance for the species in Arizona."
Colorado BBA - Breeding evidence (possible, probable, or confirmed) reported in 45 (3%) of 1745 priority blocks, with population clusters in western and southeastern Colorado (Dexter 1998).
Nevada BBA - Ted Floyd, Coordinator of the Nevada BBA, provided the following comments: "Gray Vireos were found in 23 blocks, 18 of which were clustered in northern Lincoln and northeastern Nye Counties. The species does not seem to breed as far north in Nevada as it does at equivalent latitudes in neighboring Utah. Densities were estimated at 11-100 pairs per block in 12 blocks, 2-10 pairs per block in 6 blocks, and 1 pair per block in 5 blocks."
The species' winter range is in the lowlands of Sonora, Mexico as well as in southern Baja California, southwestern Arizona, southwestern Texas, and southeastern California (AOU 1998, Howell and Webb 1995, Unitt 1994, 2000). A single wintering bird was collected in northwest San Luis Potosi, Mexico (Fry et al. 1996). In California, the Gray Vireo is only known to occur in a large elephant tree stand in the Anza-Borrego Desert of San Diego County (Unitt 2000). Except in Texas, the wintering distribution of Gray Vireos overlaps with the distribution of elephant trees (Bursera microphylla) (Bates 1992b).
TIME OF OCCURRENCE AND SEASONAL MOVEMENTS
Arrival date on breeding grounds:
In San Diego County, birds arrive regularly in late March, with the earliest arrival date in the southern Laguna Mountains 14 March 2001 (Unitt 2004). Gray Vireos may not arrive until early May further north (Garrett and Dunn 1981).
Departure date from breeding grounds:
In California, this species remains on breeding grounds until late August or early September. Five Gray Vireos were detected on 19 August 2001 in the southern Laguna Mountains, including three singing males (Unitt 2004), and have been reported in the San Jacinto Mountains as late as 27 August (Grinnell and Swarth 1913), and in Joshua Tree National Park as late as 10 September (Miller and Stebbins 1964).
Spring migration period:
In California, spring migration extends from March to May, but records of spring migrants are rare. In Texas, migration season may begin as early as February, and in Colorado it may extend as late as May (Barlow et al. 1999). Gray Vireos remain on wintering grounds in Sonora through the end of April (Bates 1992a).
Fall migration period:
In California, fall migration extends from August through mid-September, but records of fall migrants are rare. Birds begin to arrive on wintering grounds in Arizona by 25 August (Phillips et al. 1964), Texas by 5 October (Barlow et al. 1999), and Sonora, Mexico by 9 September, with most arriving late September (Bates 1992a, Russell and Monson 1998).
Extent of wintering in CA:
The San Diego County Bird Atlas resulted in the detection of Gray Vireos overwintering in the Sonoran desert of Anza-Borrego Desert State Park (Unitt 2000, 2004). This is the first known record of Gray Vireos overwintering in California. A total of 5 individuals were detected in association with a large grove of elephant trees (Bursera microphylla) in December 1999. However, follow-up surveys have revealed only a single individual at most per survey of the area from 1999 through 2003 (Hargrove and Rotenberry 2003). Elephant tree habitat also occurs in Imperial County, but it is unknown whether any focused winter surveys have been conducted there. Gray Vireos are also known to overwinter in elephant tree habitat in southern Arizona and northern Mexico (Bates 1992b).
MIGRATION STOPOVER CHARACTERISTICS:
Little information, but may be more likely to occur in riparian habitat at lower elevations during spring migration (Brown et al. 1986, Parrish et al. 2002). In the Chisos Mountains of Texas, transient birds stop at sites where the vegetation is similar to that in breeding and wintering ranges (Barlow et al. 1999).
Little information. In California, Gray Vireos are reported as casual to the
Channel Islands (AOU 1998), and have been reported sporadically at various non-breeding
inland locations. In spring, records include 2 May at Whitewater Canyon (Garrett
and Dunn 1981) and 1 May at Bonita in San Diego County (Unitt 2004). In fall,
records include 3 September at Harper Dry Lake (USDI BLM 2003) and 19 September
at Palomar Mountain (Unitt 2004). Outside of California, reported to migrate
through Baja California and Durango (AOU 1998). Populations breeding from Utah
south to Arizona migrate to southwest Arizona and Sonora, Mexico (Barlow et
al. 1999). Barlow et al. (1999) suggest that populations that breed in Colorado
may winter in Texas, while populations that breed in Texas may winter as far
southeast as San Luis Potosi, Mexico.
Forages within a shrub or tree, catching prey primarily through gleaning, stalking, and hawk-capture; less commonly used foraging techniques include hovering, flycatching, and pouncing (Barlow et al. 1999). Grinnell and Swarth (1913) described Gray Vireos foraging in redshank (Adenostoma sparsifolium), chamise (A. fasciculatum), scrub oak (Quercus spp.), manzanita (Arctostaphylos spp.), ceanothus (Ceanothus spp.), four-leaved pinyon (Pinus quadrifolia), and sagebrush (Artemisia tridentata), at 0.3-1.5 m (1-5 ft.) above the ground. In winter, the birds search slowly through B. microphylla, often mandibulating encapsulated fruit, and only removing fruit with shed capsules; aril-covered seeds are swallowed whole and seeds are probably regurgitated (Bates 1992b).
Insectivorous and frugivorous. The relatively large bill and gape of Gray Vireos, compared to other small insectivorous passerines, appear to aid in handling large-bodied insects (Orenstein and Barlow 1981) and small fruits (Bates 1992b). Summer: Arthropods, especially geometrids, large caterpillars, and grasshoppers (Barlow et al. 1999). Winter: Arthropods and fruits. In the Sonoran Desert including southeastern California, the diet may be composed primarily of the fruit of elephant trees (Bates 1992b). One species of elephant tree, Bursera microphylla, produces fruits that are primarily eaten by the Gray Vireo and Ash-throated Flycatcher (Myiarchus cinearascens) (Bates 1992b). A possible mutualistic relationship between Gray Vireos and B. microphylla may explain the high degree of overlap between the winter range of the Gray Vireo and the geographic distribution of B. microphylla (Bates 1992b).
The Gray Vireo is thought to not require water for drinking (Miller and Stebbins 1964, Weathers 1983). However, many records are from near water sources (e.g., Miller and Stebbins 1964), and nests were often near water sources in Arizona and Texas (Barlow et al. 1999).
California: Gray Vireos typically breed in mature, arid chaparral, or open pinyon-juniper woodland mixed with chaparral, desert scrub, or sagebrush. Grinnell and Miller (1944) described the habitat as "dry chaparral, which forms a continuous zone of twig growth from one to five feet above the ground in which the birds forage, sing, and nest." The chaparral habitat used in the Transverse and Peninsular Ranges is most often dominated by chamise (A. fasciculatum), and sometimes also redshank (A. sparsiflora) (Grinnell and Swarth 1913) or cupleaf ceanothus (Ceanothus greggii) (Unitt 1984). Other common species may include scrub oak (Quercus spp.), manzanita (Arctostaphylos spp.), and big sagebrush (A. tridentata). In the mountains of the Mojave Desert, the typical habitat used is open pinyon-juniper woodland (Miller and Stebbins 1964), which may be mixed with sagebrush (Johnson et al. 1948).
Outside of California: Mostly pinyon-juniper woodland, with chaparral, oak scrub, sagebrush, or thorn scrub associations (Barlow et al. 1999). In the Colorado National Monument where Gray Vireos overlap in elevation with Plumbeous Vireos (Vireo plumbeus), Giroir (2001b) found that Gray Vireos preferred relatively lower elevation habitat with shorter and less dense junipers and pinyons, and greater shrub density. In Nevada, Johnson (1972) noted that Gray Vireos occur at the lower edges of pinyon-juniper habitat where it mixes with shrubland. In Texas, occur in xeric deciduous woodland association where Gregg ash is dominant, rather than in pinyon-juniper woodland (Wauer 1971). In Coahuila, Mexico, locations were dominated by gray oak (Quercus grisea), lechuguilla (Agave lechuguilla), mesquite (Prosopis juliflora), and Texas persimmon (Diospyros texana) (Barlow and Johnson 1969).
Nests are situated in the upper parts of a shrub or small tree typical of the habitat, supported mostly at the rim by a small fork or by multiple twigs (Taber 1950). In California, nests are more often in shrubs that tend to be shorter than in other parts of the range, but plant height data are lacking. In the Providence Mountains, a nest was reported in a clump of sagebrush 1.2 m (4 ft.) high (Johnson et al. 1948). In Colorado, mean nest tree height (n = 27) was 3.04 m (10.0 ft.), with a range of 1.8-4.8 m (5.9-15.7 ft.), and a similar width (Hutchings and Leukering unpubl. cited in Barlow et al. 1999). In Texas, nests are usually in trees or shrubs that are 1.5-5 m tall and 1.8-5 m wide (Barlow et al. 1999).
Nests are in shrubs or trees typical of the habitat. In California, nests have been reported in chamise, big sagebrush, mountain mahogany, cupleaf ceanothus, scrub oak, and pinyon pine (Grinnell and Swarth 1913, Johnson et al. 1948, Miller and Stebbins 1964, Taber 1950, Unitt 2004). In Arizona and Colorado, most nests were in juniper, and less often in pinyon pine (T. Corman pers. comm., Hutchings and Leukering unpubl. cited in Barlow et al. 1999). In Texas, most nests were in Gregg ash (Fraxinus greggi) (Barlow et al. 1999).
HEIGHT OF NEST:
In California, nest heights range from 0.7 m (2.4 ft.) to 1.5 m (5 ft.) (Grinnell and Swarth 1913, Johnson et al. 1948, Miller and Stebbins 1964, Unitt 2004). Of 26 nests measured in Colorado, mean nest height was 2.04 m (6 ft.), with a range of 1.3-3.4 m (4.3-11.2 ft.) (Hutchings and Leukering unpubl. cited in Barlow et al. 1999). Of 33 nests measured in Arizona, mean nest height was 2.3 m (7.5 ft.) and the range was 1.0-5.2 m (3.3-17.1 ft.) (T. Corman, pers. comm.).
Nests are usually screened by surrounding foliage (Barlow et al. 1999).
VEGETATION SURROUNDING THE NEST:
Not described in detail. See overview of breeding habitat.
Semi-pensile, cup-shaped nest mostly supported by twigs at the rim but also often supported by twigs at the base and sides (Taber 1950, Baicich and Harrison 1997). Nests are mostly constructed of dry grasses, plant fibers, soft bark, spiders' webs and cocoons, and may be lined with hair-like fibers and decorated by leaves, particularly sagebrush (Taber 1950, Baicich and Harrison 1997). Nests are not reused, but some nest materials may be recycled to construct new nests (Barlow et al. 1999).
Little studied in California. Most data are based on two unpublished small-scale studies summarized by Barlow et al. (1999), one in Colorado and one in Texas.
BREEDING TERRITORY SIZE AND DENSITY:
Nest in loose "colonies" with extensive, apparently suitable habitat left unoccupied.
California: Territory size has only been roughly estimated in the southern Laguna Mountains at 3-8 ha (8-20 acres) per pair based on spot-mapping of unmarked singing males (USDA Forest Service 2002). The density at this location was estimated at 14 pairs per square mile (259 ha) of suitable habitat, or 4.3 birds per 40 ha (USDA Forest Service 2002). In the San Jacinto Mountains, Grinnell and Swarth (1913) estimated density at 16 pairs per 259 ha (1 sq. mi.) of suitable habitat, or 4.9 birds per 40 ha. At Deep Canyon in the Santa Rosa Mountains, Weathers (1983) estimated density at 1.6 birds detected per 40 ha. In the Providence Mountains, density was estimated at four pairs per 259 ha (1 sq. mi.) of favorable terrain (Johnson et al. 1948).
Outside of California: In Texas, territory size was dependent in part on population density, with adjacent territories averaging 2-4 ha (5-10 acres) and isolated territories 4-10 ha (10-25 acres) (Barlow et al. 1999). At the Colorado National Monument, density was estimated by point transect and distance sampling as 0.055 birds per ha (95% CI: 0.038-0.082) (Giroir 2001a). In another study using point transects and distance sampling on the Colorado Plateau, density was estimated at 0.064 birds per ha (95% CI: 0.046-0.089) (Schlossberg unpubl. manuscript).
Twenty of 22 birds banded in Texas returned to the same site the following year, and one returned four consecutive years (Barlow et al. 1999).
Males sing throughout the breeding season, and also during migration and on wintering grounds (Barlow et al. 1999). Males begin singing shortly after sunrise, and sing across the day throughout all stages of the nesting cycle (Barlow et al. 1999). In California between April and May, males sang with equal frequency in the mornings and afternoons, and often even while foraging, flying, preening, incubating, and carrying insects (USDA Forest Service 2002). Although singing is also heard during the summer (Unitt 2004), it may be reduced. In Texas, singing breaks in the mid-afternoon and resumes later at a slower pace (Barlow et al. 1999). Females occasionally utter a brief, buzzy version of the male Primary Song (Barlow et al. 1999).
Solicitation displays and "Aggressive Threat Posture" for territorial defense (Barlow et al. 1999).
Typically monogamous (Barlow et al. 1999).
By both sexes; 5-6 d to complete nest (Barlow et al. 1999).
Egg dates range from 20 April to 2 August, but most are from late May (Barlow et al. 1999).
Typically 3-4 eggs, with a range of 2-4 eggs (Barlow et al. 1999).
Both sexes sit on eggs, but only female incubates (Barlow et al. 1999).
12-14 days (Barlow et al. 1999).
DEVELOPMENT AT HATCHING:
Altricial (Barlow et al. 1999).
13-14 days (Barlow et al. 1999).
Young found in care of parents within 15-20 m of nest for 5-10 d postfledging (Barlow et al. 1999). Family groups with immatures are seen late in the breeding season (Barlow et al. 1999).
Both parents brood young, shade the nest, feed young and remove fecal sacs (Barlow et al. 1999).
POST FLEDGING BIOLOGY OF OFFSPRING:
May remain with family group until migration, and thought to be capable of breeding by first spring after hatching (Barlow et al. 1999). Juveniles may depart breeding grounds earlier than adults (Phillips et al. 1964).
POST BREEDING SOCIAL BEHAVIOR:
Small flocks occasionally observed prior to migration, and vireos may join mixed flocks on wintering grounds (Barlow et al. 1999).
NUMBER OF BROODS:
Two broods per year are probable in Texas and Arizona (Barlow et al. 1999).
Remsen (1978) suggested that brood parasitism by Brown-headed Cowbirds (Molothrus ater) might be a reason for the apparent decline of Gray Vireo populations in California, but records are few. In California, brood parasitism on Gray Vireos has only been reported from three locations near Cajon Pass (Hanna 1944, Friedmann 1963), and Gray Vireos have been extirpated from this area. No incidences of cowbird parasitism on Gray Vireos were observed during the San Diego or Los Angeles bird atlas surveys. During the Arizona bird atlas surveys, only one incident was documented. In Colorado, one out of 27 nests was parasitized (Hutchings and Leukering unpubl. cited in Barlow et al. 1999), and there are only two records for the state (Dexter 1998). In Texas and Arizona 1967-1986, 10 out of 50 nests were parasitized (Barlow et al. 1999). Parasitized nests are often abandoned (Barlow et al. 1999).
In California, Grinnell and Miller (1944) described the Gray Vireo occurring at elevations of 600-2000 m (2000-6500 ft). In the San Jacinto Mountains, Gray Vireos occurred in the zone of 900-1800 m (3000-6000 ft) (Grinnell and Swarth 1913), and may occur up to 2400 m (7870 ft) in the San Bernardino Mountains (USDI BLM 2003). In San Diego County, Gray Vireos occur mainly at 900-1500 m (3000-5000 ft), but have been found as low as 200 m (640 ft) and as high as 1600 m (5400 ft) (Unitt 2004). Outside of California, the breeding range is 914-2380 m (3000-7800 ft) (Barlow et al. 1999). In the Colorado National Monument, 60% of Gray Vireo territories were in canyon bottoms and 40% were on plateau tops (Giroir 2001b), and Schlossberg (unpubl. manuscript) found an inverse relationship between Gray Vireo density and elevation on the Colorado Plateau.
Little information. In southern California, Gray Vireos occur in unbroken expanses of chaparral and do not favor edges (Unitt 1994). Some loss of habitat has occurred in California, and pinyon-juniper woodland has been extensively cleared over much of the range (Barlow et al. 1999). Gray Vireos seem to prefer remote habitat-which could protect the species, but could also make the species more sensitive to fragmentation when it does occur. However, Neel (1999) describes Gray Vireo habitat as naturally fragmented, and populations seem to be naturally disjunct. It is possible that fragmentation could make areas more accessible to Brown-headed Cowbirds which may adversely affect Gray Vireos through brood parasitism (Unitt 1994).
Minimum patch size has been estimated at 0.8-1.2 ha (2-3 ac) (Neel 1999), but this is probably an underestimate since it is smaller than a typical territory size. Barlow (1977) suggests that large tracts of undisturbed habitat are required, given the large territory sizes.
DISTURBANCE (natural or managed):
Little information. Potential disturbance threats include fire, grazing, habitat conversion, off-road activity, and recreational shooting. Too-frequent fire may adversely affect Gray Vireos, which seem to favor mature stands of chaparral. Extensive clearing of pinyon-juniper woodland for firewood or to improve forage availability for cattle operations in Arizona, New Mexico, Utah, and Colorado has resulted in loss and degradation of habitat (Barlow et al. 1999).
ADJACENT LAND USE:
In California, seem to prefer habitat with high aridity, away from coastal influence. Lack of shrubs may preclude use of desert habitat. Temperature tolerances are not known. Potential response to climate change is not known. In this region, temperatures are increasing by 0.12° C per decade (Lane et al. 1994) and precipitation has decreased by 20% over the last century (Shriner et al. 1998).
Little information. Probable predators include Loggerhead Shrike and Cooper's Hawk; predators that may eat eggs or nestlings include Western Scrub Jay, Mexican Jay, Northern Mockingbird and Scott's and Hooded Orioles (Barlow et al. 1999). Other nest predators may include rats, chipmunks and reptiles (Hanna 1944), and coyotes and gray foxes (Barlow et al. 1999). At Colorado National Monument, jays, rock squirrels, and chipmunks destroyed one-half of the nests found in 1995 (Dexter 1998).
WINTERING HABITAT AND BIOLOGY
California and most other areas except Texas: Occur in elephant tree stands in rocky canyons, arroyos, and bajadas with Sonoran desert scrub. In San Diego County, elephant trees are mixed with desert lavender (Hyptis emoryi), catclaw acacia (Acacia greggii), and creosote bush (Larrea tridentata) (Unitt 2000). In Sonora, vegetation is dominated by "stem succulents," including elephant trees (Bursera microphylla and B. hindsiana) and columnar cacti (Pachycereus pringlei and Lophocereus schotti) (Bates 1992a).
In Big Bend National Park, Texas: an isolated population of Gray Vireos was discovered in Chihuahuan desert scrub with Texas persimmon (Diospyros texana), honey mesquite (Prosopis juliflora), whitethorn acacia (Acacia constricta), catclaw mimosa (Mimosa biuncifera), desertwillow (Chilopsis linearis), and Gregg's acacia (Acacia greggii) (Barlow and Wauer 1971).
Bates (1992a) reported mean territory size in Sonoran elephant tree habitat in winter as 0.9 ha (2.2 ac) for 9 nests, with range 0.3-1.4 ha (0.7-3.5 ac). Males and females sing or trill to defend territories, and territorial interactions occur frequently (Bates 1992a). Up to ten of 14 (71%) banded Gray Vireos returned to the same territories the following winter, and some returned to the same territory up to five consecutive winters (Bates 1992a). Gray Vireos may also join mixed flocks on wintering grounds (Barlow et al. 1999, Unitt 2000).
Little information. Winter in rocky canyons, arroyos, bajadas, and islands where elephant trees occur. In Baja California, Gray Vireos winter at lower elevations (Wilbur 1987), which corresponds to elephant tree distribution (Bates 1992b).
DEMOGRAPHY AND POPULATION TRENDS:
AGE AND SEX RATIOS:
Little information. In California, Hanna (1944) reported one Gray Vireo nest parasitized by cowbirds, and noted that many Gray Vireo nests failed due to predation. In Colorado, a study of 27 nests found that nesting success, estimated with the Mayfield method (Mayfield 1961), was 0.33 (Hutchings and Leukering, unpubl. cited in Barlow et al. 1999). In Texas, of three observed nests, two nests fledged two young each and one fledged three young (Barlow et al. 1999).
Longevity of males is at least 4-5 years (Barlow et al 1999). Winter return rates of birds banded the previous winter in Sonora, Mexico (10 of 14 birds) showed that survivorship between years was at least 71% (Bates 1992a).
Fledglings remain within 10-20 meters of the nest for the first four or five days. Full-sized juvenile birds may remain in natal territory for several weeks (Barlow et al. 1999). Banding studies in Texas and Sonora have revealed that Gray Vireos show a high degree of fidelity to breeding sites and wintering sites (Barlow et al. 1999, Bates 1992a).
BBS trend data (Sauer et al. 2004): http://www.mbr-pwrc.usgs.gov/bbs/bbs.html.
BBS data are inadequate for any trend assessments in California. Data from all regions and even survey-wide are considered deficient. Survey-wide, BBS data 1966-2003 showed an overall increase of 1.9% (not significant). The 1966-2003 trends for selected regions range from -5.8% in New Mexico to +4.5% in Texas (neither significant). There is a tendency for declines in northern and easternmost populations and increases in western populations. An increase in surveyor ability to detect Gray Vireos may also be influencing results (Barlow et al 1999).
Breeding Bird Atlas: Recent breeding bird atlas efforts in Arizona, Colorado,
Nevada, and southern California have found Gray Vireo to be slightly more common
and widespread than previously reported (L. Allen pers. comm., T. Corman pers.
comm., Dexter 1998, T. Floyd pers. comm., Unitt 2004). However, many of the
new records have resulted from surveys of areas that have never been previously
visited, so no trend is indicated. In California, increases in Gray Vireo records
locally due to atlas surveys still fall short of abundance and range indicated
by historical records (Unitt 2004). DeSante and George (1994) assert that the
species has declined in Nevada by more than 50% in this century.
EXOTIC SPECIES INVASION/ENCROACHMENT:
No information. Non-native grasses that may be introduced after clearing of pinyon-juniper woodland or frequent fires could contribute to type conversion of the habitat to grassland.
Extensive loss of habitat due to clearing of pinyon-juniper woodlands has occurred (Barlow et al. 1999). Amount of loss of suitable chaparral habitat in California is unknown. Development of private inholdings could fragment habitat and introduce non-native predators (dogs, cats) as well as encourage expansion of cowbird populations. Amount of loss of elephant tree habitat for wintering is unknown.
Little information. Fire return intervals may affect habitat suitability. In San Diego County, Gray Vireos were found in both "young-age" (average six years post-fire) and "old-age" (average 31 years post-fire) chaparral (USDA Forest Service 1997). Historical records indicate that relatively mature habitat is probably preferred. Type conversion to grassland would render habitat unsuitable for the Gray Vireo.
Cowbird brood parasitism may be a concern in some areas. Predation appears to be a major cause of nest failure in some areas. The effects of disturbances such as grazing and off-road activities are unknown.
Possible associated bird species in California include: California Quail, Mountain Quail, Gray Flycatcher, Ash-throated Flycatcher, Western Scrub-Jay, Wrentit, Oak Titmouse, Juniper Titmouse, Bushtit, Bewick's Wren, Blue-gray Gnatcatcher, California Thrasher, Phainopepla, Orange-crowned Warbler, California Towhee, Spotted Towhee, Rufous-crowned Sparrow, Black-chinned Sparrow, Sage Sparrow, Lazuli Bunting, Bullock's Oriole, Scott's Oriole, and Lawrence's Goldfinch
MONITORING METHODS AND RESEARCH NEEDS:
1. Use GIS models to compare the distribution of Gray Vireos to basic habitat and landscape features such as fire history, vegetation type, precipitation, etc.
2. Survey historic sites in California to determine whether Gray Vireos are present.
3. Complete additional surveys and bird atlases to acquire more detailed information on the current distribution, abundance, and habitat associations of this species, especially in San Bernardino and Riverside Counties.
4. Establish a monitoring program in California. Gray Vireos are readily sampled by point counts or other rapid survey techniques due to their loud and persistent singing (USDA Forest Service 2002). A combination of extensive rapid surveys for presence-absence and local intensive surveys for detailed demographic information ("double sampling") would be ideal (Bart and Earnst 2002, USDA Forest Service 2002). The addition of BBS routes and/or PIF monitoring stations in suitable habitat should be considered.
5. Investigate additional elephant tree stands in southern California for possible wintering populations.
6. Band and/or collect genetic or stable isotope information from California populations and Mexican populations to determine where California birds overwinter.
7. Investigate populations of Gray Vireos to determine if they are genetically distinct, possibly meriting subspecies status.
8. Collect basic biological data that are lacking in California, including breeding phenology, behavior, and habitat and nest site characteristics.
9. Investigate productivity issues by monitoring nest success and determining the factors that may be limiting reproduction, especially predation and brood-parasitism.
10. Study the effects of grazing, land-use, and fire on Gray Vireos.
ACTION PLAN SUMMARY
SPECIES: Gray Vireo, Vireo vicinior
STATUS: California Species of Special Concern, First Priority.
The species is very poorly studied, especially in California. It has undergone range contraction and population declines in California, and possibly elsewhere, due in part to habitat loss and fragmentation. However, extensive areas of apparently suitable habitat are unoccupied. The total population size in California is probably in the low hundreds, with the majority occurring in San Diego County.
BBS data are inadequate for trend assessment in California. No monitoring programs
or research projects are currently in place.
Breeding territory size estimates range from 2-10 ha (5-25 ac) per pair, a very large territory size for a small passerine.
The primary breeding range includes the mountains of southern California, southern Nevada, much of Arizona and New Mexico, western and southern Colorado, southwestern Texas, and the northern portion of the Baja California Peninsula in Mexico. Breeding range is typically occupied beginning in March to May, and ending in August or September.
Gray Vireos are migratory, overwintering primarily in the Sonoran Desert of Mexico or in the extreme southwestern U.S. (Texas, Arizona and California).
In the Peninsular and Transverse Ranges of southern California, Gray Vireos are found in arid chaparral mostly dominated by Adenostoma sp. Where found in chaparral, the vegetation is often dense and mature (i.e. not recently burned in a wildfire). In this habitat type the vireos appear to prefer large, unbroken, remote stands of chaparral, while avoiding edges.
In the mountains of the Mojave Desert and throughout most of the range, Gray Vireos are found mostly in pinyon-juniper woodland. Isolated populations in southern Texas and Mexico occur in ash and oak trees mixed with thorn scrub.
The winter range in California, Arizona, and Mexico consists of sonoran desert scrub dominated by elephant trees (Bursera microphylla). The Gray Vireo is thought to have a mutualistic relationship with B. microphylla in winter. An isolated wintering population in Texas occurs in thorn scrub dominated by Texas persimmon.
During the breeding season, Gray Vireos glean large insects from shrubs or low trees. During the winter, the diet is supplemented by the fruit of elephant trees (B. microphylla).
Open-cup, semi-pensile nests are built in shrubs or small trees at heights typically 1-2.5 m (3-8 ft.) above the ground. Average clutch size is 3-4 eggs, and double brooding is probable.
Nests are often predated, and occasionally parasitized by cowbirds.
Most of the areas occupied by the Gray Vireo in California are under state or federal management and protection. The breeding range, in the Peninsular and Transverse Ranges and in the eastern mountains of California, is managed by the Cleveland National Forest, San Bernardino National Forest, Joshua Tree National Park, Death Valley National Park, and the Desert District of the Bureau of Land Management. The single known wintering location of the Gray Vireo in California is within the Anza-Borrego Desert State Park. Thus, most of the occupied habitat in California is relatively well protected.
Possible concerns include:
Population declines and range retraction may be due to reasons other than habitat loss and degradation, so protection of habitat alone may not be sufficient to conserve the species.
Small, disjunct populations may be at increased extinction risk.
Breeding populations in California may be limited by conditions and extent of the wintering habitat in Mexico. The status of the Gray Vireo and B. microphylla may be intertwined, but it is not known where populations that breed in California overwinter.
Development of private inholdings could fragment habitat and introduce non-native predators (dogs, cats) as well as encourage expansion of cowbird populations.
Cowbird brood parasitism may be a concern in some areas, though records are few.
Predation appears to be a major cause of nest failure in some areas.
Disturbances such as grazing and off-road activities may have a negative impact on populations.
Loss of habitat due to clearing of pinyon-juniper woodlands has occurred through much of the range; amount of loss of suitable chaparral habitat in California is unknown.
Reduced fire return intervals or large-scale fires may adversely affect habitat
suitability. Type conversion of chaparral to grassland would render habitat
unsuitable for the Gray Vireo.
Improve knowledge of the species' distribution, status, and habitat associations throughout its range. In California, Riverside and San Bernardino Counties should be prioritized.
Determine the impact of brood parasitism and predation on nest success.
Determine the effect of disturbances such as grazing and off-road activities.
Determine appropriate fire return intervals for Gray Vireo habitat.
Locate wintering grounds for birds that breed in California.
1. Develop habitat models. Use GIS models to compare the distribution of Gray Vireos to basic habitat and landscape features such as fire history, vegetation type, land use, precipitation, etc.
2. Conduct targeted surveys. Support and encourage focused surveys of historic sites in California to determine whether Gray Vireos are present, and additional surveys and bird atlases to acquire more detailed information on the current distribution, abundance, and habitat associations of this species, especially in San Bernardino and Riverside Counties. Investigate additional elephant tree stands in southern California for possible wintering populations.
3. Establish a monitoring program in California. Monitoring of the Gray Vireo can be combined with other species of arid chaparral, including the Mountain Quail, Sage Sparrow, and Black-chinned Sparrow.
4. Band and/or collect genetic or stable isotope information from California populations and Mexican populations to determine where California birds overwinter.
5. Investigate populations of Gray Vireos to determine if they are genetically distinct, or exhibit sufficient morphological variation to possibly merit subspecies status.
6. Collect basic biological data that are lacking in California, including breeding phenology, behavior, and habitat and nest site characteristics.
7. Investigate productivity and nest success, and factors that may be limiting
reproduction, especially predation, brood-parasitism, and disturbances.
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