As I hiked along the reservoir's shore a fair-sized, long-tailed, slender-beaked bird with a rusty back and dark breast-streaks ascended to the top of a roadside tree and began very prettily, rather loudly singing an endless warble. If I'd been in eastern North America I'd have called him a Brown Thrasher without further thought, but I didn't think Brown Thrashers got this far south, and -- now that I looked and listened closely -- certain things weren't quite right.
For example, the song. Brown Thrashers tend to repeat their phrases twice: wheet-wheet, chew-chew, ow-ow... The song I was hearing halfheartedly repeated a phrase here and there, but not with the regularity of Brown Thrashers. Also, this bird sang louder and with more gusto than the average Brown Thrasher. It had the Brown Thrasher's yellow eye but its upper parts weren't quite as reddish, plus it seemed just a wee bit larger than the Brown.
Well, it was the very closely related Long-billed Thrasher, distributed in arid to semihumid brushy, scrubby and semi-open places from southern Texas to east-central Mexico -- not a big area. See the species at http://www.greglasley.net/longbillthras.html.
By no means am I less interested in bird species that are almost the same as I grew up with in the US. In fact, in some ways such species are more engaging. That's because I've always gotten a buzz from "variations on a theme."
For example, I love Bach's fugues, which consist mostly of a simple stated theme repeated again and again, each time in a different way, in a different context, with ever more daring artistry.
In this vein, the Brown Thrashers of my childhood in Kentucky constitute a simple theme, the thrasher theme, and now this Long-billed Thrasher is a slightly different variation on that pleasing theme.
Mexico is home to about eleven thrasher species, or thrasher variations. Most lack the Brown Thrasher's and the Long-billed Thrasher's rusty upper parts. The Ocellated Thrasher has dark, not yellow, eyes, and its chest is spotted like a thrush's, not short-streaked like a Brown Thrasher's. Maybe Mexico's most outlandish thrasher-theme is manifested by the Crissal Thrasher with his much curved beak, gray-brown overall color, and chestnut undertail coverts, like the catbird's.
In fact, when you look at all the members of the family to which thrashers belong, the Mimidae (and Mexico hosts 18 of them), the boundaries between thrashers, catbirds and mockingbirds aren't nearly as well defined as they seem farther north.