It’s shaped like a robin, sings like a robin, and acts like a robin. But if it's a robin, where is its cheery red breast? For those of you lucky enough to spot a pure-white American Robin, you might think you've seen a ghost—or an albino. But make sure to check its eyes, too. If it's albino, its eyes will be red or pink. On the other hand, pale-feathered birds with normally colored eyes have a condition known as leucism. Unlike albino birds, which completely lack the natural pigment known as melanin, leucistic birds produce melanin but can't deposit it into their feathers. Some may even have partial leucism, resulting in a “pied” appearance. Melanin is responsible for any black or brown color on birds, dogs, snakes, and humans. (Some brightly hued birds have so much melanin that their plumage turns completely black—a variation known as melanism.) The pigment is widespread partly because these colors are useful for camouflage. Meanwhile, albino...