Today, here at the very peak of the eastern rim of the Mexican highlands, we see our first mainly (not entirely) eastern North American species, the Black-and-white Warbler.

All this makes sense because Mexico's highlands are basically extensions of North America's western uplands, the Rockies. Mexico's hot, steamy Gulf lowland -- which lie at the foot of the slope beginning just a few kilometers east of here -- is an extension of eastern Texas's Coastal Plain, the same Coastal Plain that passes through Louisiana and extends northward via the Mississippi Embayment into southern Illinois and western Kentucky, and follows the US's Atlantic coast to New Jersey..

Thinking like this we can visualize two vast bird-zones fusing along the slope just the other side of this ridge on which I'm camped. If the teeming biology of Mexico's Gulf lowlands is an ocean, then the Black-and-white Warbler seen here is a smidgen of that ocean's spume splashed very high and a tiny bit onto the very shore of the highland realm of a geographic domain populated by bird species from western North American.