GNATCATCHERS   Polioptilidae
Gnatcatchers are a homogenous, easily recognized group of small New World passerines. Fourteen of the 17 species are in the genus Polioptila, and they are called gnatcatchers. The other three species are gnatwrens in two other genera [Microbates and Ramphocaenus]. Gnatcatchers are gray-and-white long-tailed and thin-billed woodland species, some of which also have black caps. The widespread species in North America is Blue-gray Gnatcatcher (left) which looks gray-and-white all year with a prominent white eyering except that spring males have a short black eyebrow. Their scolding notes is characteristic of brushy habitats in the West, but singing males have a delightful and melodic song.

North American species tend to be sexually dimorphic. A common characteristic of many species is that males have black caps but females do not. In the Neotropics, however, the sexes of many species look alike, and especially those in South America. Within the sexually dimorphic species, identification can be problematic with keys points being bill size, undertail pattern, and vocalizations.
 

This shot right) shows a female Black-tailed Gnatcatcher on a nest in a mesquite bush in Arizona. Nests are a deep cup woven in the fork of branches. It is held together by spider webs and lined with feathers. Both sexes help in nest building and often reuse materials from old nests. The finished product provides good camouflage. Male Black-tailed Gnatcatchers also share in incubating the eggs and feeding the young. In hot places like Arizona, another task is shading the young from the sun by perching at the nest edge with outstretched wings (Atwood & Lerman 2006).

The most recent genetic evidence is that gnatcatchers are most closely related to wrens, another (mostly) New World family (Barker et al. 2002, 2004, Alström et al. 2006). This finding resolves what has been a tortured taxonomic history. For many years they were thought to be related to Old World Warbler [Sylviidae], and for a while they were thought to be closely related to Kinglets [Regulidae]. In the more distant past, claims of relationships to Old World Flycatchers [Muscicapidae] or New World Warblers [Parulidae] were made. In the end, it sure makes some logical sense that gnatcatchers and gnatwrens are most closely related to wrens.
 

From North American experience, I always think of gnatcatchers as difficult to see, but not shy birds of thick undergrowth, or brush, or chaparral. This photo (below) of California Gnatcatcher captures it within that habitat.
California  Gnatcatcher was initially described as a new species in 1888, but was lumped with Black-tailed in 1926. It was not restored to full species level until 1993. The biochemical evidence is that it became isolated in Baja California during the Pleistocene but spread northward after the last Ice Age. Its DNA is divergent from Black-tailed Gnatcatcher by about 4% (Zink & Blackwell 1998). Today it is still common in Baja California, Mexico, but within the United States it is restricted to maritime chaparral in southern California. That habitat has undergone tremendous development pressure and is today very fragmented. Only small numbers of California Gnatcatcher remain in California, where it is listed as Threatened under Endangered Species laws (Atwood & Lerman 2006). 
 
Nearly all species in this family are resident. Only Blue-gray Gnatcatcher undergoes any regular migration. However, the edges of breeding ranges can expand and contract, and these developments have brought an occasional Black-capped Gnatcatcher (right) into Arizona. This otherwise Mexican species nests some years in southeast Arizona (as did this male in Chino Canyon in 1983) but in many years none are found within the U.S.

Our understanding of tropical gnatcatchers changed rapidly in recent years. What was once thought to be Guianan Gnatcatcher P. guianensis is now split into four species: Guianan, Rio Negros P. facilis, Para P. paraensis, and Iquitos P. clementsi Gnatcatchers across the Amazon basin (Whitney & Alvarez 2005). Tropical gnatcatchers and gnatwrens often forage with mixed feeding flocks, both in scrub and understory and (for some species) in the canopy.


Photos: The Blue-gray Gnatcatcher   Polioptila caerulea was in montane chaparral in Trinity County, California, on 19 June 2006. The nesting Black-tailed Gnatcatcher  P. melanura was in Chino Canyon, Arizona, on 2 July 1983. The California Gnatcatcher  P. californica was at San Elijo Lagoon, San Diego County, California, on 19 Apr 2006. The male Black-capped Gnatcatcher  P. nigriceps was in Chino Canyon, Arizona, on 2 July 1983. All photos © 2007 Don Roberson; all rights reserved.

Bibliographic notes

There is no "family book" covering the Gnatcatcher, but the summary by Atwood & Lerman (2006) is excellent, and contains many striking photos.

Literature cited:

Alström P., P.G.P. Ericson, U. Olsson, and P. Sundberg. 2006. Phylogeny and classification of the avian superfamily Sylvioidea. Molecular Phylogenetics & Evolution 38: 381-397.

Atwood, J.L., and S.B. Lerman. 2006. Family Polioptilidae (Gnatcatchers), pp. 350-377, in Handbook of the Birds of the World (del Hoyo, J., Elliott, A., and Christie, D.A., eds). Vol. 11. Lynx Edicions, Barcelona, Spain.

Barker, F.K., G.F. Barrowclough, and J.G. Groth. 2002. A phylogenetic hypothesis for passerine birds: taxonomic and biogeographic implications of an analysis of nuclear DNA sequence data. Proc. Roy. Soc. Lond. B. 269: 295-305

Barker, F.K., A. Cibois, P. Schikler, J. Feinstein, and J. Cracraft. 2004. Phylogeny and diversification of the largest avian radiation. Proc. Nat. Acad. Sci. 101: 11040-11045.

Whitney, B.M., and J.A. Alvarez. 2005. A new species of gnatcatcher from white-sand forests of northern Amazonian Peru with revision of the Polioptila guianensis complex. Wilson Bull. 117: 113-127.

Zink, R.M., and R. C. Blackwell. 1998. Molecular systematics and biogeogrpahy of aridland gnatcatchers (genus Polioptila) and evidence supporting species status of the California Gnatcatcher (Polioptila californica). Molec. Phyloen. Evol. 9: 26-32.

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