California Partners In Flight Coniferous Bird Conservation Plan for the
Black-Throated Gray Warbler
Prepared by: Tina Mark, USDA Forest Service, Tahoe National Forest, 631 Coyote St., Nevada City, CA 95959, (530) 478-6240.
SPECIES: Black-Throated Gray Warbler (Dendroica nigrescens)
SUBSPECIES STATUS:
Two subspecies are recognized--D. n. nigrescens and D. n. halseii, but the races are very similar and not always distinguishable; the race halseii is not always considered as a valid race (Curson, et al. 1994, Guzy and Lowther 1997).
D. n. nigrescens breeds from the Pacific coast and in the coastal mountain ranges, from southern British Columbia to northern California. D. n. nigrescens winters in California and Arizona south to n. Mexico (Guzy and Lowther 1997).
D.n. halseii breeds over most of the species’ range, except for the Pacific northwest region where it is replaced by D. n. nigrescens (Guzy and Lowther 1997, Curson, et al. 1994). The two races are geographically separated by the Cascade Mountains and can also be distinguished by song (IBID). D. n. halseii winters only in Mexico (IBID).
MANAGEMENT STATUS:
No special status. Protected Under the Migratory Bird Treaty Act.
RANGE IN CALIFORNIA:
1. Historical References
According to Grinnell and Miller (1944), the historical breeding range for the species occurred throughout California, except for in the lower deserts of the southwest and the islands. Its occurrence was restricted primarily to the mountains, foothill or plateaus and to particular plant associations. Occurrence on the immediate coast vicinity was scarce or absent. The species was reported from 300 feet elevation up to 7500 feet elevation. The species was widespread during migration. Guzy and Lowther (1997) indicate that this species was not known to breed on Vancouver Island prior to around 1950, but has been breeding there since.
2. Breeding and Winter Distribution
The Black-throated Gray Warbler breeds all along the entire Sierra-Cascade axis, but are local and uncommon on the east slope of the central Sierra Nevada (Small 1994). In southern California, they breed in the Transverse Ranges, the San Jacinto Mtns, and possibly in the Santa Ana Mtns. In summer, they are rare and local in San Diego Co. in the Palomar Mtn., Doane Valley, Hot Springs Mtn., Mt. Laguna, and the Cuyamca Mtns. In the eastern desert ranges, they are common in summer in the White Mtns., Inyo Mtns., and Panamint Mtns. from 6000-8500 feet ranges.
Guzy and Lowther (1997) summarize breeding and winter range within California as follows:
Summer Range (Primarily in mountains): Winter Range:
Klamath Mtns. to Warner Mtns. Primarily winters in Baja
California Sur, and on Pacific slope and interior of Mexico from southern portions
of Sonora, Durango, Zactecas, and Coahuila south to central Oaxaca
n. Coast Ranges south to s. Sonoma and Napa Cos. Regularly
winters in small numbers within U.S., particularly along West coast and Gulf
Coast.
Santa Cruz Mtns. and Diablo Range of Santa Clara Co. (considered
an uncommon breeder), Oakland hills, n. Diablo Range south thru Santa Barbara
(uncommon and local) and Ventura Cos. Winters along California Coast primarily
in southern counties, becoming “very rare” north to Oregon border.
Cascade and Sierra Nevada ranges south through Piute and
Tehachapi Mtns.
Traverse Ranges, San Jacinto Mtns., Palomar Mtn, Mt. Laguna,
Cuyamaca Mtns., and possibly Santa ana Mtns. in extreme southwest.
White and Inyo Mtns., Panamint and Kingston ranges, and
Nw York Mtns. in southeast.
ECOLOGY
1. Average Territory Size: No information available.
2. Time of occurrence and seasonal movements.
a. Arrival Date on Breeding Grounds:
Small (1994) reports that this species is a fairly common spring migrant from late March to early May. Peak migration occurs from mid- to late April in southern California and from mid-April to mid-May in the north. It is a fairly common summer visitor. Low elevation breeding birds establish territories by mid-April, while northern birds are on territories by the first week in May.
Bent (1963) reported a spring arrival date of March 24 for the Black-throated Gray Warbler in the Sierra Nevada Mountains in Grass Valley, CA.
b. Initiation of Nesting
Nesting occurs from May to July (Curson, et al. 1994). No incubation and fledging periods are recorded.
c. Spring Migration Period
In California, the Black-throated Gray Warbler migrates from early-March into late-May depending on geographic region. Peak spring migration in southern California occurs in April; in northern California peak spring migration is April to mid-May. Most lower elevation migration peaks in mid-April (Small 1994, Curson, et al. 1994, Guzy and Lowther 1997). Reports from San Diego and Santa Barbara counties document that spring migration generally is initiated in early March (Guzy and Lowther 1997).
d. Fall Migration Period
In California, fall migration begins in mid-August, but may extend into October, with a few stragglers into December (Small 1994).
e. Extent of wintering in California
The species was reported to use oaks and riparian willows and cottonwoods for wintering habitat in California (Paige, C., et al. 1999) but few regular winter visitors occur in west-central California, where they are found mostly along the coast. In southern California, it is a rare but regular winter coast visitor. The subspecies migrates to different parts of winter range (Guzy and Lowther 1997).
3. Foraging Strategy
The Black-throated Gray Warbler is highly insectivorous and predominantly forages by gleaning from foliage. Ryser (1985) reports that the species gleans insects in the dense terminal foliage of pinyon and juniper trees in the Great Basin. They can exploit a variety of vegetation types throughout western North America (Keane and Morrison 1999).
Keane and Morrison (1999) found within season differences in the foraging behavior and habitat use patterns of the Black-throated Gray Warbler in pinyon-juniper woodlands of the White and Inyo Mountains of eastern California from April to August. Use of shrubs was greater in the early summer of each year; which correlated with greater relative abundances of arthropod numbers sampled on bitterbrush and sagebrush. The greater number of arthropods on bitterbrush was associated with early season flowering phenology. Greater use of pinyon pine in the late summer of each year corresponded with increases in numbers of arthropods sampled on pinyon pine and decreases in arthropod numbers on bitterbrush and sagebrush.
4. Displays
Pre- and post-copulatory displays are not known. No information on courtship displays (Guzy and Lowther 1997).
5. Mating System
No data available, thought to be monogamous as in other Dendroica warblers (Ehrlich et al. 1988, Guzy and Lowther 1997).
6. Clutch Size
Lays 3 to 5 white to creamy eggs, with brown marks or speckling (Bent 1953, Ehrlich et al. 1988).
7. Incubating Sex
The female apparently incubates the eggs (Bent 1953).
8. Incubation Period
No information.
9. Nestling Period: No information
10. Development at hatching
Young are altricial (nearly naked and helpless) and nidiculous (confined to nest) (Ehrlich et al. 1988, Guzy and Lowther 1997).
11. Number of Broods
Generally thought to have only 1 brood per year, but observations of nests with eggs as late as July and observations of adults with nest material into June suggests double broods (Guzy and Lowther 1997).
12. Who tends the young:
Both male and female feed the young (Bent 1953).
HABITAT
Overview of Breeding Habitat
The Black-throated Gray Warbler generally uses habitats characterized by brushy understory and trees of dry oak woodlands, oak and pine mixed forests, and pinyon-juniper woodlands (USDA 1994). In the northern portion of its range, it inhabits conifer forests that are open and interspersed with shrubs or forest edges. Farther south, seems to prefer shrubby stands of oaks, pinyon-juniper, and manzanita. Throughout its range, prefers and is perhaps limited to dry slopes (Ibid).
Small (1994) describes breeding habitat as follows: west of the eastern desert ranges and the east slope of the Sierra Nevada mountains, the Black-throated Gray Warbler breeds in a variety habitats from sea level to 7000 feet in the Upper Sonoran, Transition and lower Canadian life zones. At lower coastal elevations, they occur in mixed forests dominated by Douglas fir in the north, and to the interior and south, they breed in oak forests mixed with chaparral in the lower elevations, and mixed oak and Ponderosa pine in the higher mountains. In the arid eastern portion of their range they prefer pinyon-juniper woodlands interspersed with shrubs in the understory, but will also nest in pure pinyon pine stands.
In the White Mountains of eastern California,
Morrison et al. (1993) found that the relative abundance of the Black-throated
Gray Warbler was greater in the pinyon-pine zone as compared to its relative
abundance in the bristle-cone-limber pine zone. In fact, the Black-throated
Gray Warbler was the most abundant bird species of 58 breeding bird species
encountered in the pinyon-juniper zone in 2 of 3 years. The Black-throated
Gray Warbler declined in abundance as elevation increased, but was not
replaced by any other warbler species.
1. Nest Site
a. Nest type
The Black-throated Gray Warbler builds a deep, cup-shaped nest from plant fibers, grass and weed stalks, lined with feathers and hair (Bent 1953, Curson et al. 1994, Ehrlich, et al. 1988). Bent (1953) describes a Black-throated Gray Warbler nest from Washington as “plant fibers, dry grasses and a few very small weed-stalks were all neatly woven together to form the walls of the nest. The lining was a few feathers—two being those of the Ruffed Grouse, with others from sparrows, the quill of each being worked into the walls of the nest; next to this lining were soft and very fine plant fibers, with a few horsehairs.” Nearly all nest descriptions have included mention of “feathers” as part of the lining, including feathers of Steller’s Jay, Greater Roadrunner, Mountain Quail, Varied Thrush, Ruffed Grouse, and Mexican Jay (Guzy and Lowther 1997). In southeast Arizona, Harrison (1994) described all of his nest observations as “a profusion of small feathers in the lining, and I have never heard of a nest of this species that did not have feathers. One beautiful nest, destroyed by some predator, had 289 feathers by actual count. About 75 percent were the soft blue feathers of the [Mexican] Jay”.The nest dimensions measure 7.7 cm in diameter with a height of 7.7 cm for the outside of the nest; inside diameter is 4.5 cm and height is also 4.5 cm (Guzy and Lowther 1997).
b. Nest Substrate
The most common breeding habitat documented in California includes ponderosa pine, valley foothill hardwood-conifer, montane hardwood, and pinyon-juniper areas. Builds a deep, cup-nest usually out on horizontal branch or fork, near trunk of shrub or tree (Guzy and Lowther 1997).c. Height of Nest
The Black-throated Gray Warbler generally builds nests from 2 feet to 15 feet above the ground, but nests have been observed at 50 feet above the ground (Bent 1953, Zeiner et al, 1990). In the Santa Rita and Chiricahua Mountains of southeastern Arizona, Harrison (1994) found nests from 12 to 40 above the ground; the average was 24.5 feet within white or Emory oak and one in juniper.d. Height of Nest Plant
Height of nest plant depends upon type of plant association and successional stage of the habitat used for nesting. Oak woodlands can vary from less than 20 feet up to over 80 feet. Pinyon-juniper woodlands can vary anywhere from approximately 20 feet to over 50 feet. Whereas, ponderosa pine forests can vary from 20 feet in young forests to over 100 feet in mature forests.e. Plant Species Concealing Nest
See “Nest Substrate”, above.f. Percent Nest Cover
Very little information available on cover requirements. Cover requirements for this species may vary depending on the site-specific habitat it uses. The species tends to inhabit a variety of relatively dry, open woodlands and open forests with brushy understories of foothills and mountains (Guzy and Lowther 1997).
2. Vegetation Surrounding the Nest – See “Nest Substrate”, above.
3. Landscape Factors
a. Elevation – In California, breeds from 3,000 to 7,000 feet in the Sierra Nevada range, up to 9,500 feet in the White Mountains in eastern California (Paige, et al. 1999).
b. Fragmentation – it is not well understood how fragmentation affects this species in the long-term, although younger successional forests may be beneficial to this species in the short-term as they regenerate, especially in western Douglas fir or other dense, mature forest types (Paige, et al. 1999). The results of one study within Engelmann spruce-subalpine fir forest which had interconnected forest remants surrounding clearcuts four years after logging, found that densities of Black-throated Gray Warblers were comparable to unlogged continuous forests (8.3 singing males per 100 hectare of logged forest vs. 9.96 males per hectare of unlogged forest). However, impacts of woodland habitat fragmentation on productivity and survivorship has not been studied. More information is needed to understand the relationship of this species and fragmentation at the landscape level.
c. Patch Size - no information available.
d. Climate - no information available.
4. Special Factors
Brood ParasitismIt is thought to be a rare Brown-headed Cowbird host (Ehrlich et al. 1988), but this has been little studied. A total of 20 brood parasitism records are known; parasitism rate in very limited samples in California, Arizona, and New Mexico were estimated to be 10 percent to 20 percent (Lowther and Guzy 1997).
Diet
The Black-throated Gray Warbler feeds primarily or entirely on insects during the breeding season (Ehrlich et al. 1988). Guzy and Lowther (1997) report that the only prey species which have been specifically mentioned are identified as “small green caterpillars” (infesting oaks in s. Oregon and in California).
5. Sensitivity to Human-Induced Disturbance
The Black-Throated Gray Warbler tends to be relatively tame and can often be observed from short distances as it forages among foliage. There is little information about the effects of human-induced disturbance to this species. Guzy and Lowther (1997) suggest that human influence may have little effect to the overall population due to its widespread distribution and its occurrence in a variety of habitats that are often of little economic value. However, intensive harvest of pinyon-juniper woodlands may affect patterns of habitat use. One study reported that Douglas fir stands which had been thinned, tended to support fewer birds than unthinned stands, but differences were neither statistically significant nor consistent (Ibid). Other studies found that mean bird densities after clear-cutting of Douglas fir forests were highest in shrub/sapling stage plots compared to pole timber and medium sawtimber plots. These data suggest that the Black-throated Gray Warbler can occupy sites that have been severely altered, but may not address how such habitat changes may affect habitat use.
6. Predators
Information on predation is scanty. Bent (1953) reported that this species is subject to predation by accipiters, small mammals, snakes, and jays. Guzy and Lowther (1997) report that the Sharp-shinned Hawk (Accipiter striatus) and the Cooper’s Hawk, (A. cooperii) are predators of adult Black-throated Gray Warblers. Jays and crows are suspected to be nest predators.
Both male and female of the species have been described as sometimes giving distraction displays when predators are a potential threat. One observer noted that the female “set up a great fuss, chattering and fluttering her wings” (Guzy and Lowther 1997). While others have expressed that the Black-throated Gray Warbler merely stays near the nest and acts unconcerned towards its observer.
7. Exotic Species Invasion/Encroachment - No information found
8. Population Trend
BBS trend data indicate population trend for this species is stable or increasing range-wide (Sauer, et al. 2000). North America BBS data shows significant population increase survey-wide. BBS data from 1966-1996 (2.2 percent average annual change; P<0.05; n = 202 survey routes). Other trend estimates for 1966-1996 and 1980-1996 did not show statistically significant trend increase for population estimates.
9. Demographics
Very little information is available on demographics for this species, particularly no information is reported for age at first breeding, breeding intervals, life span and survivorship, and home range.
Guzy and Lowther (1997) cite some information on population densities of nesting birds for various locations: 24 singing males/40 ha in southeast Arizona oak-juniper woodland, 2 territories on 16 ha (oak woodland in Sonoma Co.; 9 territories on 40 ha (pinyon-juniper) in Grand Co., UT; and 21 territories per 40 has (pinyon-juniper) in San Juan Co, UT.
10. Management Issues and Options
Impacts from timber harvest activities and grazing
impacts are mostly unknown.
No information is available for how the species
responds to habitat alterations, including productivity and survivorship;
changes in population density, rates of brood parasitism and behavioral
response to parasitism.
11. Habitat and Population Objectives
This species has been little studied despite being widespread and relatively common in certain habitats. Since this species uses a wide variety of woodlands and appears to respond positively to early successional stages of forest regeneration within certain forest types, restoration potential would likely be high in landscapes that achieved sustainable forests conditions either though management or natural disturbance regimes (Paige, et al. 1999).
12. Associated Bird Species
Little information is available. On Vancouver Island, British Columbia, the Black-throated Gray Warbler was among 4 species (Orange-crowned Warbler, Black-capped Chickadee, Wilson’s Warbler, and Song Sparrow) that were observed to “dew bathe” or flutter against dew-covered leaves (Guzy and Lowther 1997). It has been observed to join mixed-species flocks in the winter and during migration; often found with Townsend’s and Hermit Warblers in the spring (Ibid).
13. Related Species
The Black-throated Gray Warbler is one of five
species in the genus Dendroica (Harrison 1994). This species is placed
within or near the Dendroica virens superspecies comprised of the Black-throated
Green (D. viren), Townsend’s, Hermit, and Golden-cheeked (D.chrysoparia)
Warblers (Guzy and Lowther 1997). Phylogenetic history of this warbler
group indicates that the Black-throated Gray Warbler may have branched
from the other warblers prior to Pleistocene glaciation events. Black-throated
Gray Warblers have been documented to hybridize with Townsend’s and Hermit
warblers. The five warbler species are not only similar in plumage
but have similar songs.
Bent, A. C. 1953. Life histories of North American wood warblers. U. S. Natl. Mus. Bull. 203.
Dunn, J. L., and K. L. Garrett. 1997. A field guide to warblers of North America. Houghton Mifflin Co., Boston.
Curson, J., D. Quinn, and D. Beadle. 1994. Warblers of the Americas: an identification guide. Houghton Mifflin Company, New York, New York.
Ehrlich, P. R., D. S. Dobkin, and D. Wheye. 1988. The birder’s handbook: a field guide to the natural history of North American birds. Simon and Shuster, Inc, New York. 785 pp.
Grinnell, J. and A. H. Miller. 1944. The distributions of birds of California. Pacific Coast Avifauna Number 27, 608 pp.
Guzy, M. J. and P. E. Lowther. 1997. Black-throated Gray Warbler (Dendroica nigrescens) In The Birds of North America, No. 319 (A. Poole and F. Gill, eds.).
Harrison, H. H. 1984. Wood warblers’ world. Simon and Shuster, Inc., New York, New York. 335 pp.
Keane, J. J. and M. L. Morrison. 1999. Temporal variation in resource use by black-throated gray warblers.
Morrison, M. L. 1982. The structure of western warbler assemblages: Ecomorphological anlysis of the Black-throated Gray and Hermit Warblers. Auk 99: 503-513.
Morrison, M. L., L. S. Hall, J. J. Keane, A. J. Kuenzi, and J. Verner. 1993. Great Basin Naturalist 53(3), pp. 246-258.
Paige, C.; revisions by M. Koenen and D.W. Mehlman. 1999. Wings Info Resources/Species Information and Management Abstracts: Black-throated gray warbler (Dendroica nigrescens). The Nature Conservancy. Edition date: 1999-10-20.
Ryser, F. A. 1985. Birds of the Great Basin: a natural history. University of Nevada Press, Reno.
Sauer, J. R., J. E. Hines, I. Thomas, J. Fallon, and G. Gough. 2000. The North American Breeding Bird survey, Results and analysis 1966 – 1999, version 98.1, USGS Patuxent Wildlife research Center, Laurel, MD.
Small, A. 1994. California birds: their status and distribution. Ibis Publishing Company, Vista, CA. 342 pp.
USDA Forest Service. 1994. Neotropical migratory bird reference book: volume 1. USDA Forest Service, Pacific Southwest Region, California. 832 pp.
Zeiner, D. C., W. Laudenslayer Jr., K. Mayer, and M. White, eds. 1990. California’s wildlife, Vol. 2, Birds. Calif. Dept. Fish and Game, Sacramento. 732 pp.