Adapted from Jim Conrad's online book A Birding Trip through Mexico, this excerpt from "Nevado de Toluca Volcano" in México state
Hawks are thick up here. Tunnels and earthen mounds among the boulder fields and along talus-slope edges show that many mammals live here. It's surprising how many lizards and grasshoppers loll on sunlit rocks. There are no trees or bushes to hide under, so all these creatures must make attractive targets for sky-high predators.
One hawk dives at something but misses, and this hawk is unlike any portrayed in the field guides. With broad, rounded wings and a stocky body, it's obviously one of that group of hawks birders call buteos, but it's completely black.
Cold, wet wind hisses through the boulders, thundering in the ears, burning cheeks, numbing the hands and stiffening the binoculars' focusing wheel. The sky is evenly dark blue and the high-elevation sun fairly screams its bright glare. Wind lashing all around causes the black bird's binocular image to jiggle and then suddenly the black-hawk mystery is solved when the bird draws near the sun: With sunlight filtering through it, the black silhouette's tail glows a rusty red.
Of course, it's just an old friend in a different guise, a black form of the Red-tailed Hawk.
Nature is doing something interesting here. It's playing with the facts that black objects and black organisms absorb more sunlight energy than lighter colored ones, and that organisms at high elevations need all the help they can get staying warm. It's "melanism" -- an instance of an animal being black or nearly black, when most of the members of its species are otherwise. Except for their rusty tails, most Red-tailed Hawks are brown and white. This high-elevation "melanistic" form, however, was born black, and its offspring will be black.