Last weekend I hitched a ride on the recycling truck up to El Madroño, to where the main highway crosses the Eastern Sierra Madre before plummeting into the Gulf lowlands. This time instead of climbing into the cloudforest I headed to a lush little cove in a valley where trees had been cut from a house-size spot and grasses and wildflowers had grown back, forming a little meadow. In the afternoon with the sky intensely blue, the sun stinging my skin but the air cool, I couldn't resist lying in the grass and just watching clouds.
The first thing to draw my attention, though, wasn't a cloud, but a Black Vulture circling me. Or, was it a Black Vulture at all? It certainly looked like one, but somehow it flew differently. The binoculars, moreover, didn't show white "window patches" on the wings, which Black Vultures have. And the bird's tail feathers backlit by sunlight glowed with a bright, rusty hue. The bird was a black Red-tailed Hawk, and it was circling with another hawk of the usual whitish/light-gray color.
I've seen black Red-tailed Hawks before, but not often. The last one I remember observing was soaring around the crater of the volcano called Nevado Toluca in central Mexico, at 15,036-ft (4583 m) in elevation. I'm getting the impression that this "dark morph" most frequently appears at high elevations.
Many pictures of dark Red-tailed Hawks can be found on the Internet, but I can't find any showing exactly what I saw last weekend. Both last weekend's bird and the volcano one looked perfectly black to me, except for the reddish tail. The ones on the Internet show pale spots and blotches. Maybe my birds, seen so starkly against the brilliant, high-elevation sky, showed up as sheer silhouettes even though they had some white feathers.
The Red-tail's "dark morph" has its own name, the Harlan's Red-tailed Hawk.
Seeing a normal pale Red-tail circling with the black morph, you can't help wondering what's to keep the much rarer black form from disappearing as its genetic material mingles with and disperses into the much more abundant pale-bird gene pool. I don't know what the deal is with Red-tails, but it's understood that the appearance of blue-morph Snow Geese (the Blue Goose recognized as a distinct species in older books) is controlled by a single gene, with dark being partially dominant over white. If a dark morph mates with a normal white goose, the offspring will all be dark. If two white geese mate, they have only white offspring. If two dark geese mate, they have mostly dark offspring, but some white ones.