Excerpts from Jim Conrad's
from the November 19, 2007 Newsletter 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
CHIAPAS'S HAIRY WOODPECKERS
The first time I saw a Hairy Woodpecker here in Chiapas my heart skipped a beat, for it didn't look like any woodpecker I'd ever seen. It was just like the Hairy Woodpecker that's fairly common coast-to-coast in North America, including most of Canada, except that its throat and underparts were a definite tan color, not white as in every Hairy I'd ever seen. Also maybe it was a little smaller and shorter-billed than North America's Hairy. At that time I only had the Peterson Field Guide to Mexican Birds and that didn't illustrate the woodpecker I was seeing, or say anything about a tan-breasted Hairy look-alike.
Eventually Howell's masterpiece on the birds of Mexico and Northern Central America came out and finally I understood. As with many plant and animal species with extensive distributions (Hairys occupy upland pine-oak and oak forests all the way to Panama), the species has fragmented into subspecies. Our Chiapan subspecies is PICOIDES VILLOSUS ssp. SANCTORUM, and it's brownish below.
Actually, even Howell leaves me scratching my head a little, for his illustration of the sanctorum subspecies shows a bird with much darker underparts than what I see here, and a whiter back. Our birds have equally tan backs and underparts, but a pure white eye-stripe.
Howell states that our southern birds are a little smaller than the northern ones. In fact, there's an ecological rule, Bergmann's Rule, which asserts that "geographic races of a species possessing smaller body size are found in the warmer parts of the range, and races of larger body size in cooler parts." This makes sense. Species in cold climates have problems losing heat as it radiates from their body surfaces. It's a physical fact that a large sphere has less surface area per unit volume than a smaller one. Therefore, a large sphere would radiate less heat from its surface area than a small one, and the same would go for animals.
One can imagine that if Hairy evolution should continue in its current direction eventually there'll be a larger, white-breasted northern species, and a smaller, brown-breasted southern species.