Excerpts from Jim Conrad's
from the December 18, 2006 Newsletter issued from Sierra Gorda Biosphere Reserve, QUERÉTARO, MÉXICO
When North Americans think of warblers they visualize nervous little yellow-and-black, migrant birds with slender, insect-eating bills. The quintessential North American warbler is a member of the genus Dendroica, such as the Yellow-rumped Warbler, Black-throated Green Warbler, Hermit Warbler and the Pine Warbler.
At San Juan de los Durán a common warbler flitting among the weeds and along forest edges is neither a migrant nor a member of the genus Dendroica or any other genus most North American birders are familiar with. It's the Rufous-capped Warbler of the genus BASILEUTERUS. See one at http://www.siti.com.mx/musave.dir/htm.dir/1333.htm.
Though the Wood-Warbler Subfamily, the Parulinae, is assumed to have arisen in North America, Basileuterus warblers are thought to have evolved in South America. In continental-drift terms, it's conjectured that before the Central America land bridge connected North and South America Basileuterus's ancestors passed from the warbler homeland in North America into South America and there evolved a bit before re-invading back northward through Central America. Basileuterus warblers are a bit more robust and have stouter bills than North American genera. Most Basileuterus species have olive or gray upperparts and yellow underparts, striking "eyebrows" (superciliums) and colored crowns or crown stripes. When you've spent a lot of time learning North America's many migrant woodwarbler species it's a treat to see these southern "variations on the warbler theme."
The Rufous-capped Warblers at San Juan de los Durán were very curious about me as I sat in the sun near a spring. They came tsik-tsik-tsiking and hanging sideways on weed stems looking at me the way a chickadee might.
Rufous-capped Warblers sometimes show up accidentally in the US along the border with Mexico, in Texas and Arizona. Otherwise they're normally found throughout most of Mexico, except in the hot lowlands, into South America. They specialize in disturbed areas such as clearings and woods edges and usually are found in lower mountains and foothills -- in other words, places exactly like San Juan de los Durán.
Probably the most interesting species in that list are the Military Macaw and the Bearded Wood-partridge.