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