California Partners in Flight Desert Bird Conservation Plan
Black-tailed Gnatcatcher (Polioptila melanura)
Photo by Ian Tait
Prepared by: Jason Tinant (firstname.lastname@example.org)
Science Education Coordinator
Oglala Lakota College
2212 Cerro Court
Rapid City, SD 57702
Tinant, J. 2006. Black-tailed Gnatcatcher (Polioptila melanura). In The Draft Desert Bird Conservation Plan: a strategy for reversing the decline of desert-associated birds in California. California Partners in Flight. http://www.prbo.org/calpif/htmldocs/desert.html
action plan summary
The Black-tailed Gnatcatcher was historically considered to be conspecific with the California Gnatcatcher (Atwood 1986). The coastal subspecies was identified as Polioptila melanura californica, while the desert subspecies was identified as P.m. lucida. Atwood (1988) identified vocal, morphological and ecological differences between the species that persist even in areas of sympatry. In 1989 the Ornithologists' Union split P. melunura into two species: California Gnatcatcher (Polioptila californica) and Black-tailed Gnatcatcher (Polioptila melanura) (A.O.U 1989).
There are three recognized subspecies of the Black-tailed Gnatcatcher: P.m.
melanura, P.m. curtata, and P.m. lucida. P.m. lucida is the subspecies
present in California.
No special status. Protected under the Migratory Bird Treaty Act.
HISTORICAL BREEDING DISTRIBUTION:
Grinnell and Miller (1944) described the Black-tailed Gnatcatcher in California as a common resident with a range extending from the Colorado Desert west to eastern San Diego County and northwest to Palm Springs, Riverside County. It also occurred along the Colorado River Valley from the Mexican boundary to the Nevada border and locally in the Mojave Desert.
CURRENT BREEDING DISTRIBUTION:
The Black-tailed Gnatcatcher is a resident species restricted to arid and semiarid zones of the Lower Sonoran Life Zone in the southwestern United States and central Mexico. P.m. lucida occurs throughout the Sonoran, Colorado, and Mojave deserts (Farquhar 2002). The northernmost breeding area is in the Panamint Mountains (Wauer 1964). In California, the main range extends south from extreme southern Inyo County (along the Amargosa R.) through eastern San Bernardino, Riverside, and Imperial counties to the Mexican border, and west through the Colorado and Mojave deserts to as far west as Barstow and Morongo Valley San Bernardino County, San Gorgonio Pass Riverside County, and Anza Borrego State Park (Small 1994). Southwestern Riverside County constitutes an area of sympatry with the California Gnatcatchers (Weaver 1998). Range extends through western and central Arizona, southwestern New Mexico (Farquhar 2002).
AVERAGE TERRITORY SIZE
Estimates for breeding territory size range from 0.8 to 2.7 ha (Hensley 1954, Laudenslayer 1981). Foraging territory expands over a much larger area (4.8 ha) in winter than during the nesting season (0.8 ha) (Smith 1967). Territorial boundaries are also subject to change in response to local population density fluctuations (Preston 1998), changes in vegetation composition, and habitat disturbances (Mock 1990).
TIME OF OCCURRENCE AND SEASONAL MOVEMENTS
Black-tailed Gnatcatchers are considered resident throughout their range, but are known to sometimes wander during the non-breeding season (Farquhar 2002). This wandering may be attributed to exploiting more seasonally sustainable foods (Rosenberg 1991), larger established territory size during the non-breeding season (Smith 1967), and/or a broadening of the breadth of foraging habitat (Rice 1980).
Gnatcatchers are primarily insectivorous and forage by gleaning (picking arthropods from a leaf, branch, or twig) and very occasionally use hawking or hovering to capture prey. Black-tailed Gnatcatchers spend most of the daylight hours searching for food, beginning 5-10 minutes before sunrise. In the late afternoon or on cloudy days, gnatcatchers select sunlit parts of plants for foraging. During hot afternoons, high winds, or precipitation events, gnatcatchers forage near the center or the most protected parts of the foliage (Thomas 1975).
Black-tailed Gnatcatchers appear to preferentially forage on thorn trees (Parker
1986). Other plant species used for foraging include honey mesquite, honey-screwbean
mesquite, and mesquite-salt cedar. Canopy height appears to be important for
gnatcatcher foraging. Parker (1986) found gnatcatchers spent at least 75% of
their time foraging on substrates less than 3m high. Laudenslayer (1981) found
that gnatcatchers spent the majority of their time foraging in the lower portion
of the canopy, which corresponded to the maximum foliage volume. Gnatcatchers
respond to seasonal changes in food supplies by altering foraging height (Laudenslayer
Black-tailed Gnatcatchers are primarily insectivorous, but will occasionally eat vegetable matter. Euriciform larvae were the predominant prey type along the lower Colorado River (Laudenslayer 1981). Other prey items include coleopterans, hemipterans, larval lepidopterons, wasps (Sphecidae), ants (Formicidae), flies (Diptera), moths (Lepidoptera), small grasshoppers (Orthoptera), insect eggs, and spiders (Arachnida) (Farquhar 2002).
In Mojave, Great Basin, Colorado and Sonoran desert communities, prefers nesting and foraging in densely lined arroyos and washes dominated by creosote bush (Larrea tridentata, L. divaricata) and salt bush (Atriplex sp.) with scattered bursage (Franseria sp.), burroweed (Ambrosia dumosa), ocotillo (Fouquieria splendens), saguaro (Carnegia gigantea), barrel cactus (Ferrocactus sp.), nipple cactus (Mammaleria sp.), and prickly pear and cholla (Opuntia spp.). The community typically contains <10% honey mesquite (Prosopis glandulosa), screwbean mesquite (P. pubescens), palo verde (Cercidium sp.), ironwood (Olneya tesota), catclaw acacia (Acacia greggii, A. constricta), wolfberry (Lycium exsertum), spiny hackberry (Celtis pallida), and/or smoke tree (Psorothamnus spinosus) (Smith 1967, Thomas 1975, Parker 1986, Small 1994, Farquhar 2002).
Gnatcatchers tend to avoid scrub composed of the introduced salt cedar and
agricultural areas (Small 1988). Gnatcatchers have become very scarce in the
well-irrigated agricultural portions of the Coachella Valley RIV, the Imperial
Valley, and the lower Colorado River Valley.
Nests are often located on the plant periphery, but with plenty of overhanging vegetation (Smith 1967, Thomas 1975).
Nests built in species of trees that afford the greatest shade and camouflage (Thomas 1975). Nests can be found in spiny hackberry, mesquite species, palo verde, mistletoe (Phoradendron sp.), smoke tree, and rarely creosote bush (Smith 1967, Thomas 1975, Parker 1986, Atwood 1988, Farquhar 2002).
HEIGHT OF NEST:
Nest height appears to be related to the availability of tall trees and shrubs (Smith 1967, Parker 1986). Black-tailed Gnatcatchers usually nest 1 to 4 feet above the ground in the fork of a small shrub such as mesquite, creosote bush or other desert scrub (Ehrlich et al. 1988). Of 14 nests monitored near Tucson, Arizona, nest height ranged from 2 to 14 feet (Smith 1967). Of six nests found in palo verde, nest height ranged from 3 to 6 feet (Parker 1988).
HEIGHT OF PLANT:
In one study, plant height ranged from 6 to 15 feet (n = 6 nests) (Parker 1988).
Black-tailed Gnatcatchers position nests so that the inner lining of the nest is not exposed to direct sunlight. Nests are built in a fork of two or more branches, and are situated under a canopy of small leaf-bearing twigs or under a large branch (Smith 1967).
Black-tailed Gnatcatchers construct open cup nests 1.25 to 2.0 inches in height. The inside diameter of the nest is 1.0 to 1.5 inches, the outside diameter is 2.0 to 2.25 inches, and the depth ranges from 1.0 to 1.5 inches (Smith 1967). Nests are constructed of fine plant fibers, grasses, hair, feathers and spider webs and lined with fine materials (Ehrlich et al. 1988). Gnatcatchers will reuse materials from abandoned nests when constructing new nests. Both sexes participate in the nest construction process; however the female spends more time incorporating material into the nest (Smith 1967).
BREEDING TERRITORY SIZE AND DENSITY:
Laudenslayer (1981) recorded breeding densities of one pair per 7.0 - 7.8 acres in honey and screw-bean mesquite dominated woodlands (n = 9). Parker (1986) reported breeding densities of one pair per 6.1 acres in palo verde woodlands. Raitt and Maze (1968) reported a much lower density estimate of one pair per 50 acres in a creosote-dominated community where gnatcatchers were nesting in sumac (Rhus micorphilla) and graythorn (Condalia lycioides).
Behavior termed "recognition display" performed by members of a pair when together in a dense bush after separation. Display consisted of facing one another while perched about 50 cm apart, and directing vigorous "tsh" call toward each other. Display continued for 5-30 seconds, followed by foraging together or singly (Thomas 1975).
Black-tailed Gnatcatchers are considered to be monogamous (Ehrlich et al. 1988).
3-5 eggs (Ehrlich et al. 1998).
Males and females share incubation responsibilities (Woods 1928, Ehrlich et al. 1988). Males and females alternate foraging trips and incubation periods. After a period of 15-40 minutes, the incubating bird will call to its mate. The foraging bird will then return to the nest and trade places with the incubating bird (Smith 1967).
14 days (Ehrlich et al. 1988).
DEVELOPMENT AT HATCHING:
Altricial (Ehrlich et al.1988).
9-15 days (Ehrlich et al. 1988).
Both male and female gnatcatchers tend young (Ehrlich et al. 1988). Nestling feeding trips are influenced by rising environmental temperatures. The number of nestling feeding trips was markedly lower during the heat of the day than during the cool, early morning hours (Smith 1967).
POST BREEDING SOCIAL BEHAVIOR:
Male and female gnatcatchers continue territory defense into the non-breeding season. The male is most active in territorial defense, but the female also plays an active, if less intensive, role (e.g., scolding but not chasing). The female Black-tailed Gnatcatcher's role in territory defense is unusual among passerine species. Female participation may be required to maintain a large territory successfully and thereby ensure acquisition of resources necessary for survival and successful reproduction (Preston 1998).
Thomas (1975) found interspecific aggression directed towards three species:
Black-throated Sparrow (Amphispiza bilineata), Cactus Wren (Campylorhynchus
bunneicapillus), and Brown-headed Cowbird (Molothrus ater). The Black-throated
Sparrow and Cactus Wren were only attacked when in close proximity to male gnatcatchers.
Brown-headed Cowbirds were attacked whenever they were found in a gnatcatcher's
territory (Thomas 1975).
NUMBER OF BROODS:
Black-tailed Gnatcatchers have multiple broods. Smith (1967) reported that a pair of gnatcatchers abandoned their fourth nest in mid-July. In Chemehuevi wash along the lower Colorado River, gnatcatchers were among the latest breeders, abandoning parasitized nests or fledging Brown-headed Cowbirds in the last week of May (n = 3). Typically the gnatcatchers had already successfully reared young earlier in the breeding season (pers obs.).
Black-tailed Gnatcatchers are susceptible to cowbird parasitism. Historical records for Brown-headed Cowbird parasitism exist from 1874, 1882, 1905, and 1915 (Friedmann 1963). At a site near Las Cruces, NM, each of 14 nests found were parasitized by Brown-headed Cowbirds (Thomas 1975).
Friedmann (1963) suggested that the earlier breeding season of gnatcatchers
may be motivated by cowbird parasitism. Along the washes of the lower Colorado
River Valley, cowbirds arrive after the first nesting attempt of Black-tailed
Gnatcatchers. Thus, the first Black-tailed Gnatcatcher broods were able to fledge
without competition from cowbirds. Second and later broods of gnatcatchers were
all parasitized by cowbirds at Chemehuevi Wash (PRBO unpubl. data).
Black-tailed Gnatcatchers range limited in elevation to 75 - 900m, with breeding common below 300m (Grinnell and Miller 1944, Atwood 1988, Small 1994).
Destruction of the mesquite brushland in the Coachella, Imperial and Colorado River valleys is the main factor causing the decline of the Black-tailed Gnatcatcher. Destruction of habitat in desert washes by off-road vehicles may also be a factor (Remsen 1978).
ADJACENT LAND USE:
Black-tailed Gnatcatchers are a good indicator species for undisturbed areas (Farquhar 2002). Gnatcatchers cannot adapt to exotic vegetation or high density of buildings; one of most sensitive to vegetation changes and urbanization (Tweit and Tweit 1986). Does not recolonize areas overtaken by urbanization (Emlen 1974).
No direct data exists, however Black-tailed Gnatcatchers have almost disappeared from intensive agricultural areas in the Coachella and Imperial valleys of California (Remsen 1978).
Not well documented.
DEMOGRAPHY AND POPULATION TRENDS:
Rosenberg et al. (1991) suggest that gnatcatchers may regulate their population size through territorial defense. By forcing juveniles to disperse to suboptimal habitats, mature gnatcatchers holding territories would suffer very low mortality through the winter months.
Breeding bird survey data show Black-tailed Gnatcatcher populations to be stable
or declining slightly throughout much of its range. In areas where gnatcatcher
habitat has been lost to agricultural or urban use, populations have experienced
significant declines or even extirpation.
EXOTIC SPECIES INVASION/ENCROACHMENT:
Black-tailed Gnatcatchers avoid agricultural areas and scrub composed of the introduced salt cedar (Small 1994).
Retaining or replanting native vegetation is an important consideration in the conservation of the Black-tailed Gnatcatcher. Germaine et al. (1998) suggested 4 management actions for Sonoran Desert range:
1. Retain or replant native vegetation and limit/exclude housing developments
dominated by exotic vegetation.
2. Leave riparian and other native vegetation corridors intact and undisturbed, especially within highly developed areas.
3. Retain 1-ha patches of native Sonoran vegetation throughout developed areas, leaving a network of native-habitat patches interspersed throughout an urban matrix. This should be at an interpatch distance that readily affords juveniles the ability to disperse among them.
4. Reduce dominance of non-natives by managing for sensitive native species (e.g., Black-tailed Gnatcatcher).
ACTION PLAN SUMMARY
SPECIES: Black-tailed Gnatcatcher (Polioptila melanura)
STATUS: The Black-tailed Gnatcatcher is protected under the Migratory Bird Treaty Act.
The Black-tailed Gnatcatcher prefers nesting and foraging in densely lined arroyos and washes dominated by creosote bush and salt bush. Black-tailed Gnatcatchers are very sensitive to human disturbances, with populations quickly declining in areas where urbanization, irrigated agriculture, or intensive off-highway vehicle use occur.
Destruction of the mesquite brushland in the Coachella, Imperial and Colorado River valleys is the main factor causing the decline of the Black-tailed Gnatcatcher in California. Off-road vehicle use in desert washes may also contribute to population declines.
Housing developments dominated by exotic vegetation will not support this species. The invasion of salt cedar along ephemeral drainages reduces habitat quality for gnatcatchers.
Locate, monitor and protect the remaining key breeding locations for Black-tailed Gnatcatcher in the Mojave Desert, the Coachella, Imperial and Colorado River valleys.
Continue and expand the study of bird populations in xeric riparian woodlands along the lower Colorado River.
Coordinate and maintain data sharing among appropriate agencies and organizations including the Bureau of Land Management, National Park Service, PRBO Conservation Science, and Department of Defense.
Mesquite bosque, riparian areas and washes must be left vegetatively intact and undisturbed by excluding off-road vehicle use and limiting feral burro populations.
Protect dense catclaw acacia-smoke tree washes in the Colorado and Mojave deserts.
Protect mesquite brushlands in the Coachilla, Imperial, and Colorado River valleys.
In areas where urban or agricultural development is imminent, working with
developers to retain native Sonoran vegetation patches of greater than 1-ha,
especially along washes and arroyos, is critical. These patches must be interspersed
throughout the urban or agricultural matrix at a distance of less than 0.5 kilometers.
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Atwood, Jonathan Lee. 1986. The gnatcatchers: Polioptila californica, P. melanura and P. nigriceps: species limits, vocalizations, and variation. Unpublished Thesis (Ph. D.)--University of California, Los Angeles, 1986. 297pp.
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Friedmann, H. 1963. Host relations of the parasitic cowbirds. U. S. Natl. Mus. Bull. no. 233. Washington, D.C.
Germaine, S.S., S.S. Rosenstock, R.E. Schweinsberg, and W.S. Richardson. 1998. Relationships among breeding birds, habitat, and residential development in Greater Tuscon, AZ. Ecol. Applic. 8:680-691.
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Thomas, Kenneth Gary. 1975. Foraging and breeding behavior of the black-tailed gnatcatcher (Polioptila melanura) in southern New Mexico. Unpublished Thesis (M.S.)--New Mexico State University. 36 pp.
Tweit, R.C. and J.C. Tweit. 1986. Urban development effects on the abundance of some common resident birds of the Tuscon area of Arizona. American Birds 40:431-436.
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Weaver Kenneth L. 1998. A new site of sympatry of the California and Black-tailed Gnatcatchers in the United States. Western Birds 29(4):476-479.
Woods, R. S. 1928. Nesting of the black-tailed gnatcatcher. Condor 30:39-143.
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