The wrens are an American family. With the notable exception of the winter or northern wren, they are confined entirely to the Nearctic and Neotropical zoogeographic regions. Within this vast area, ranging from 62° north in Alaska to 55° south in Tierra del Fuego, the abundance of wren species varies greatly, with the maximum diversity in southern Central and South America. Canada, with an area of 3.85 million sq mi (10 million km2), has eight species; the United States (3.8 million sq mi, 9.8 million km2) has ten; Mexico (760,000 square miles, 1.98 million km2) has 35, with 11 species that are endemic; Panama (30,000 sq mi, 77,700 km2) has 21; and Colombia (440,000 sq mi, 1.14 million km2) has 30. Species abundance remains high in the Andean chain, but drops off sharply in the lowlands of the Amazonian basin.
The high diversity in the mountainous regions of Central and South America is at least partially the result of the varied terrain; frequently several species are found in close proximity in the different habitats created by different altitudes and the varying precipitation levels caused by mountains and rain shadows. Conversely, the Amazonian basin is of almost uniform altitude. Furthermore, in lowland areas the families of the antbirds and ovenbirds reach their maximum abundance, doubtless competing for food resources with wrens. In South America south of Bolivia, the number of species diminishes rapidly, with essentially only two species south of the tropic of Capricorn.
Curiously, wrens are almost absent from the Caribbean subregion; the southern house wren extends to some of the Windward Islands and the peculiar and unique Zapata wren (Ferminia cerverai) occupies a few square miles of swampland in Cuba, but much apparently excellent habitat on the large islands is inexplicably wrenless.
One species, known in North America as the winter wren (Troglodytes trogodytes) but in Britain simply as the wren, crossed the Bering Strait into the Old World. Lacking competition, it has expanded across three further continents, from Kamchatka and Taiwan in the east to Morocco and Iceland in the west, occupying a diverse range of habitats, from remote sea-girt islands to Himalayan scrubland and suburban gardens.
The different genera of wrens have different centers of abundance. By far the most widespread genus is Troglodytes. It is both the northernmost and the southernmost genus in the Americas, and the only one in the Old World. It includes a modest number of species, some of which have very restricted distributions in the mountains of Costa Rica, Panama, and Colombia. The genus Thryothorus, at 21 species, is by far the largest, and has an almost exclusively tropical distribution. The genera Salpinctes, Catherpes and Hylorchilus, comprising rock, canyon, and alllied wrens, occur from western Canada to Costa Rica and specialize in living in rocky habitats and cliff-faces. From Arizona to Ecuador and Brazil are found the largest wrens, the rambunctious and boisterous members of the genus
Campylorhynchus, which includes the familiar cactus wren (Campylorhynchus brunneicapittus) of the arid American southwest as well as several highly restricted species found in limited areas of Mexico. In the thick tropical forests from Mexico to Brazil and Bolivia is found the genus Henicorhina, which includes two widely distributed species, one highland and one lowland, as well as one that was only recently discovered in a few mountains in Peru and Ecuador. The genus Microcerculus is a specialist of dense tropical forest, usually at low elevations, and occurs in these habitats from Mexico to Brazil and Ecuador. In Andean South America the genus Cinnycerthia is found exclusively in wet mountain forest, often at considerable elevations, while the genus Cyphorhinus is found at lower elevations from Honduras to Bolivia and the Guianas. The distribution of the four species of the marsh-specializing genus Cistothorus is peculiar. Two are found in restricted areas of Venezuela and Colombia. One occurs across the whole of North America, from California to Florida and New Brunswick. The fourth, the sedge wren (Cistothorus platensis), is found discontinuously from the Canadian prairies through Central and South America to Cape Horn and the Falkland Islands. The other wren genera are almost all confined to the New World tropics, with the exception of Bewick's wren (Thryomanes bewickii), which occurs across western and central North America.
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