Naturalist Newsletter of November 19, 2007
issued from Yerba Buena Clinic just outside
Pueblo Nuevo Solistahuacan, Chiapas, MÉXICO
about 1740 meters in elevation, ± LAT. 17° 11' 27"N, LONG. -92° 53' 35"W
I heard the beautiful, familiar, before-the-rain song of the Northern Cardinal, si-weet si-weet si-weet and though I've not yet seen North America's "Red Bird" here I wasn't surprised. We're at the southernmost limit of the Cardinal's distribution and the species makes its home up to 1800 meters in elevation (5900 ft), so it should be here.
But then the song started breaking up, notes were introduced I've never heard a Cardinal articulate, and finally the song flipped into something gurgly no Northern Cardinal has the savoir faire to whistle.
The bird flew across my path and landed in shadows enabling me to see its thrasher-like silhouette: Slender body, long tail and a slender, slightly down-curved bill. A puff of wind for half a second opened the forest's canopy and a shaft of light illuminated a slaty-blue top. So, this was no kind of thrasher.
When the bird flew again I saw its slaty-blue top, white underparts, and black mask. It was exactly like the Blue Mockingbird along the lake back in Jalpan in Querétaro except for the white underparts. In fact, it was the very closely related (same genus) Blue-and-white Mockingbird, MELANOTIS HYPOLEUCUS, shown here.
Blue Mockingbirds live as far south as Oaxaca but, being partial to arid habitats and uplands, so far they haven't crossed the moist, lowland Isthmus of Tehuantepec to this side. Blue-and-white Mockingbirds, being highland birds, aren't found on the other side of the Isthmus, and are in fact endemic from here through the Guatemalan mountains to Honduras.
It's easy to imagine that at one time the Blue and Blue-and-white Mockingbirds were the same species, but then as the Tehuantepec lowlands grew hot and humid, maybe as an ice age ended, the population fragmented into two groups. West of the Isthmus Blue Mockingbirds evolved adaptations for arid habitats while to the east Blue-and-white Mockingbirds adapted to moister uplands. Or maybe the similarities result from some other process entirely. What's certain is that the Blue and Blue-and-white Mockingbirds are very closely related species, what's called "sister species" nowadays, meaning the two species arose from a common ancestor.
What a beautiful bird this Blue-and-white Mockingbird is, and how good to hear his song beginning with such a familiar melody, then gradually jazzing up the tune as if he were playing with me.