California Partners in Flight Riparian Bird Conservation Plan

Warbling Vireo (Vireo gilvus)

Photo by James Gallagher, Sea and Sage Audubon

Prepared by: Thomas Gardali (tgardali@prbo.org)

PRBO Conservation Science
4990 Shoreline Highway, Stinson Beach, CA 94970

RECOMMENDED CITATION

Gardali, T. 2003. Warbling Vireo (Vireo gilvus). 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.

SHORTCUTS

range map

references

SUBSPECIES STATUS:

Vireo gilvus swainsonii is the breeding sub-species in most of California and the rest of the Pacific slope west of the Cascade-Sierran divide south of southern British Columbia, reaching its southern breeding limit in San Diego County. "V. g. leucopolius" recognized y some researchers) breeding range is characterized as the Great Basin (Sibley 1940, Grinnell and Miller 1944).

MANAGEMENT STATUS: No federal or state special management status; on the 1978 Audubon Society Blue List.

DISTRIBUTION

HISTORICAL DISTRIBUTION 

Historically, the breeding distribution of the Warbling Vireo was described as the entire length of California west of southeastern deserts (in appropriate habitats).

CURRENT BREEDING DISTRIBUTION

Currently, the breeding range extends from the Canadian border south to the Santa Ana mountains (Orange County), San Bernadino mountains (San Bernadino County), Tehachapi mountains (Kern County), and east-central White and Inyo mountains (Inyo County), exclusive of the entire Central Valley.

ECOLOGY

AVERAGE TERRITORY SIZE

Territory size as follows: 2 pairs in Arizona were both 1.2 ha (Barlow 1977); 9 in riparian forest in coastal California in 1998 averaged 1.45 ha; 19 territories in eastern California averaged 1.2 ha in 1998 and 1999 (Gardali and Ballard 2000); 1 in Illinois was approximately 1.2 ha; 3 in southern Ontario near Toronto were approximately 1.2-1.5 ha; and 2 in Alberta were both 1.5 ha (J.C. Barlow pers. comm.).

TIME OF OCCURRENCE AND SEASONAL MOVEMENTS

Arrives to breeding grounds in Marin County from mid to late March with evidence to suggest that the first arriving birds in the area are the birds that will remain to breed (Ralph 1967, 1971). Earliest arrival date in the Sierra Nevada was reported by Gaines as April 7, 1931. Departs breeding grounds in Marin County from July and early August (Ralph 1971). Early spring arrival in California is 10 March and peaks from 15 April to 20 May (Weston 1948, Gardali et al. 2000, Flannery et al. in press). Hatch-year (young) pass through central coastal California in fall in peak numbers from mid-Aug to mid-Sep, with common late dates between 26 and 30 September (Stewart 1972, Gardali et al. 2000, Flannery et al. in press).

MIGRATION STOPOVER CHARACTERISTICS

Nocturnal migrant. Fall stopover duration in central coastal California 3.4 days ± 2.6 SD (Gardali and Ballard 2000). Spring stopover duration at the Salton Sea was 4 d ± 1.53 SD (Flannery et al. in press).

FOOD HABITS

FORAGING STRATEGY

Foliage gleening-perched and hovering. James (1976) reports that hovering was used to secure the major portion of food from leaves in the peripheral areas of the tree at an average of 7.7 meters in trees 10 meters in height. Airola (1979) reported that Warbling Vireos primarily perch-gleens twigs (76%). Petit et al. (1990) describe the foraging technique and prey location for Warbling Vireo as highly plastic, which is supported indirectly by James (1976) and Airola (1979).

DIET

Beal (1907) reported that 97% of diet was comprised of insects in California. The 3% veg matter was taken almost entirely in August and September suggesting that they feed almost exclusively on insects during the breeding season. Major food items include Lepidoptera, especially caterpillars but also moths and their pupae, were mostly taken at 43% and peaking at 82% of the diet in April. Hemiptera accounted for 21%, ladybird beetles for 19%, other beetles for 7%, and other insects for 4%; spiders made up another 2% of the diet. Of the little vegetable matter taken the majority was elderberries and poison oak seeds.

DRINKING

Drinks from wet vegetation and small streams.

BREEDING HABITAT

Shows a strong association with mature mixed deciduous woodlands especially along riparian corridors throughout range. May also prefer young deciduous stands that emerge after a clear-cut (Ward and Smith 2000). Found at edges or openings (both natural and human-made) as well as forest interiors. In general, overall habitat structure consists of large trees with a semi-open canopy; apparently indifferent to density of undergrowth (James 1971, MacKenzie et al. 1982, Marzluff and Lyon 1983). Elevations of breeding habitats range from sea level to 3,200 m (10,500 ft). A predictive model of Warbling Vireo abundance showed that areas with high canopy (25+m) and upper (8-25m) stratum cover, and low tree basal area explained variation in abundance (Marzluff and Lyon 1983). Other habitats include urban parks and gardens; orchards; farm fencerows; campgrounds; deciduous patches in pine forests; mixed hardwood forests; and, rarely, pure coniferous forests.

NEST SITE

Nest generally placed in the periphery of tree or shrub (James 1976a), essentially always supported by two lateral limbs (forked) for attachment of nest material. Distances of 27 nests from tree trunks in southern Ontario ranged from 0.3 to 6 m, with 14 nests averaging 1.2 to 3 m (Peck and James 1987).

NEST SUBSTRATE

In coastal California, 14 nests were built in willow (Salix sp.), 6 in red alder (Alnus rubra), 5 in oregon ash (Fraxinus latifolia), 3 in California bay (Umbellularra californica) and 1 in Bigleaf Maple (Acer macrophyllum) (PRBO). In the Owens Valley foothills of e. California, 14 nests were built in Black Oak (Quercus kelloggii), 5 in Black Cottonwood (Populus trichocarpa), 4 in Water Birch (Betula occidentalis), and 1 in Arroyo Willow (Salix lasiolepis) (PRBO). In the Sierra Nevada Mountains of California, 30 nests were built in quaking aspen (Populus tremuloides), 30 in lodgepole pine (Pinus contorta), 7 in Fremont cottonwood (Populus fremontii), and 7 in willows (Reynolds and Smith pers. comm.). Nests have also been found in apple (in orchard), pear, and pecan trees (Bent 1950).

HEIGHT OF NEST

In Montana, height of nests were on average 65.5% ± 17.8 SD (n = 161) of the of the height of the nest tree (Tewksbury pers. comm.).
Recorded nest heights range from 1.1 m to 37 m (Gardali and Ballard 2000). Nests in e. California averaged 8.3 m (range 1.1 - 30 m, n = 24; PRBO). Nests in the Sierra Nevada mountains of California averaged 7 m ± 4 SD (range 1.2 - 19 m, n = 69; Reynolds and Smith pers. comm.).

NEST TYPE

Open-cup nest placed on horizontal fork and bound at the rim. Often made of hair, grasses, bark strips, plant down, and lichen bound with spider webs and lined with finely shredded weed stems (Gardali and Ballard 2000).

BREEDING BIOLOGY

MATING SYSTEM

Monogamous

DISPLAYS

Display involves constant vocalizing described as "squeets," tail spreading, moving body from side to side, with head oriented toward mate (potential mate) and advancing toward mate. When close (~10cm) tail is closed and contour plumage is fluffed with wings quivering with mouth open wide (Dunham 1964).

CLUTCH SIZE

Mode = 4 (range 1-5).

INCUBATION

Both sexes incubate and brood.

INCUBATION PERIOD

12 days (range 10.5-16).

DEVELOPMENT AT HATCHING

Altricial.

NESTLING PERIOD

15-16 days

PARENTAL CARE

Both sexes tend the young.

NUMBER OF BROODS

Two

BROOD PARASITISM

Frequent victim of Brown-headed Cowbird (Molothrus ater) (Friedmann 1963, Friedmann et al. 1977, Friedmann and Kiff 1985). Parasitism in California reported as early as 1928 (Sherwood 1929). See Gardali and Ballard (2000) for review of site-specific parasitism proportions.
Typically if a cowbird egg is accepted, the entire natal brood is lost (reviewed in Gardali and Ballard 2000). In a study in riparian forests of w. Montana however, 2 of 27 (11.8%) parasitized nests in unfragmented forested landscapes were successful (i.e., fledged at least one natal young) and 4 of 36 (28.6%) parasitized nests in fragmented agricultural landscapes were successful, with an average of 2.0 (n = 3) natal young produced per parasitized nest (Tewksbury et al. 1998, Tewksbury pers. comm.). However, of the nests in both the fragmented and unfragmented landscapes in which a cowbird egg hatched only 1 of 39 (2.6%) was successful (Tewksbury pers. comm.).

Parasitism may damage natal eggs and at this time the nest may be abandoned in response to the reduced clutch size (see Rothstein 1982).
Abundance of Warbling Vireos in the Sierra Nevada of California was negatively correlated with the abundance of cowbirds (Verner and Ritter 1983). The authors concluded that such a relationship reflects the significant impact cowbirds have on Warbling Vireos. Rothstein et al. (1980) suggest that cowbird parasitism may be severe enough to depress numbers of this species in the Sierra Nevada. Gaines (1977) noted that Warbling Vireos began to decrease in number in Yosemite in the 1930's, about the time the cowbird first appeared. In the Oakanagan Valley of British Columbia, population models predict that the current levels of parasitism are sufficient to drive small populations to extinction in the absence of emigration (Ward and Smith 2000).

LANDSCAPE FACTORS

ELEVATION

Breeding records for swainsonii have been reported as high as 8500 feet as in the San Bernadino mountains and as low as 300 feet near Red Bluff, Tehama county (Grinnell and Miller 1944). Overall CA = sea level to 8500 ft.

FRAGMENTATION

In terms of presence as a breeder the Warbling Vireo does not seem to be effected by habitat fragmentation. However, when habitats are fragmented by land use types that likely benefit nest predators and cowbirds (e.g., campgrounds and picnic areas) nest success of Warbling Vireos likely is reduced (but see Tewksbury et al. 1998).

DISTURBANCE

Densities apparently not affected by campground activities in riparian areas (Blakesley and Reese 1988).

PESTICIDE USE

Reductions in populations attributed to spraying of pesticides in shade trees (Bent 1950). In s. central British Columbia Warbling Vireos nearly disappeared from herbicide thinned deciduous forests while abundance increased in control and manually thinned plots (Easton and Martin 1998).

PREDATORS

Predators of adult Warbling Vireos not documented. Little information on types of nest predators. Steller's Jays and Western Scrub-jays (Aphelocoma coerulescens) are ferociously mobbed when near nest, implicating them as likely nest predators (Gardali and Ballard 2000). Significance of mammals as nest predators unknown; red (Tamiasciurus hudsonicus) and western gray (Sciurus griseus) squirrels are suspected nest predators in the West (Tewksbury et al. 1998, PRBO).

DEMOGRAPHY AND POPULATION TRENDS

DEMOGRAPHICS

Survivorship. Maximum recorded longevity is a 13 yr old male originally banded as an adult in coastal California (Klimkiewicz et al. 1983).
Three estimates of annual adult survival (the probability an adult will survive from one year to the next, whether or not it is recaptured) exist, all from w. North America. These estimates range from 50-83% for adults. Time-constant estimates of annual adult survival probability from modified Cormack-Jolly-Seber mark-recapture analyses from 5 yr of data were 0.565 ± 0.045 SE for the Northwest region (63 sites) and 0.827 ± 0.146 SE for the Southwest region (2 sites; DeSante et al. 1998). Using 19 yr of capture-recapture data annual adult survival for a single breeding population in coastal California yielded an estimate of 0.504 ± 0.052 SE (Gardali et al. 2000).

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 probability that a nest fledged at least 1 natal young using the Mayfield method was 61.6% (n = 41) in Arizona (Martin and Li 1992), 21% (n = 41) in central coastal California (Gardali et al. 2000) and 52% (n = 66) in Montana (Breeding Biology Research and Monitoring Database). Percent of nests located that fledged ³ 1 natal young was 43.5% (n = 23) in British Columbia (Campbell et al. 1997), 15% (n = 20) in the Sierra Nevada Mountains of California (Reynolds and Smith pers. comm.), and 10% (n = 20) in e. California (PRBO).

POPULATION TREND

The 1966 to 2001 Breeding Bird Survey (BBS) trend data for California show a statistically significant -0.1 percent decline (P = 0.03, n = 166 routes, 95% CI = -1.9 to -0.1). Declines from BBS are more dramatic in recent period, 1980 to 2002 (-2.4%, P < 0.001, n = 106 routes).

Mist-netting data (1968-1992) from South East Farallon Island show a significant decline in captures of hatching year Warbling Vireos but neither a decline or increase in Spring or Fall capture rates (Pyle et al. 1993). Mist-net capture data from coastal Marin County reported significant long-term declines in abundance of both breeding and migratory Warbling Vireos (Gardali et al. 2000). In addition, Gardali et al. (2000) found a significant long-term decline of hatching-year birds captured during the breeding season. The mist-net captures of fall migrants (primarily hatching-year birds; 86.8%) in Santa Clara County declined significantly from 1987-1998 (b = -0.13, SE = 0.046, P = 0.02; Gardali and Jaramillo 2001).
 

MANAGEMENT ISSUES AND OPTIONS

EXOTIC SPECIES INVASION/ENCROACHMENT

HABITAT LOSS

Habitat loss, especially the loss of deciduous trees continues to be a threat to Warbling Vireos (reviewed in Gardali and Ballard 2000).

MANAGEMENT

Perhaps the top concern to Warbling Vireo population viability in California is poor reproductive success (see Demography; Reproductive Success above). For example, the dual affects of nest depredation and parasitism in the Bitterroot Valley of western Montana appear to be limiting Warbling Vireo populations in both unfragmented forested landscapes and fragmented agricultural landscapes (Tewksbury et al. 1998). Importantly, the inference that breeding season processes contribute to population dynamics is strengthened because productivity in a given year influences recruitment the following year (Gardali et al. 2000).

Management at the landscape scale would be the most inclusive strategy. I suggest limiting land use patterns that are likely to positively influence the abundance of nest predators and Brown-headed Cowbirds.

Maintain all existing suitable riparian vegetation. Restore riparian vegetation with particular attention given to deciduous tree species (see Breeding above).

ASSOCIATED BIRD SPECIES

Canopy nesting species will mostly likely benefit from management activities aimed at the Warbling Vireo (e.g., Hutton's Vireo, Bullock's Oriole, Western-wood Pewee, Black-headed Grosbeak, and Yellow Warbler).

HABITAT AND POPULATION OBJECTIVES

1. Initiate studies on nest predators. Determine which species depredate Warbling Vireo nests, at what frequencies, and their relative densities. Examine the relationship (if any) to the landscape composition (e.g., human habitation versus regenerating clear-cut).

2. Assess the importance of migratory stop-over sites and associated habitat characteristics.

3. Information on the wintering ecology of this species would be particularly useful. For example, studies of its fitness and survival in a variety of landscapes (e.g., pine-oak woodlands vs. coffee plantations) would help determine its vulnerability to human activities in the tropics.

4. Establish intensive monitoring sites in all bioregions. Monitor abundance, reproductive success, and survival.

5. Monitor site-specific response to habitat restoration. 

SCIENTIFIC REFERENCES

The following list contains additional references than those cited in the body of the account. It is a bibliography to be used by all those interested in further understanding the Warbling Vireo. If there are missing citations please contact the author and he will be happy to send them to you.

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Avise, J. C., C. F. Aquadro, and J. C. Patton. 1982. Evolutionary genetics of birds. V. genetic distances within the Mimidae (mimic thrushes) and Vireonidae (vireos). Biochemical Genetics 20: 95-104.

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Barlow, J. C. 1977. Effects of habitat attrition on vireo distribution and population density in northern Chihuahuan Desert. Pp. 591-596 in Trans. symp. on the biological resources of the Chihuahuan Desert region, United States and Mexico (R. H. Wauer and D. H. Riskind, eds.). U. S. Dep. Inter., Natl. Park Serv. Trans. Proc. Ser. 3.

Barlow, J. C. 1980. Patterns of ecological interactions among migrant and resident vireos on the wintering grounds. Pp.79-107 in Migrant birds in the Neotropics: ecology, behavior, distribution, and conservation (A. Keast and E. S. Morton, eds.). Smithso. Inst. Press. Washington, D. C.

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