I occasionally hear from readers who report a partially white bird — a bluebird, a house sparrow or some other type - in their yards.
While my wife and I were conducting a recent birding workshop at Block Creek Natural Area, between Fredericksburg and Comfort, we saw a chipping sparrow with a white head and neck, as though it had a cowl over its otherwise brown head. The other day, I was visiting a friend in Houston who had a blotchy white house sparrow coming to his bird feeder.
White patches in bird plumage is called leucism, a condition in which some feathers have scant deposits of pigment. It's caused by defective genes that restrict the body's melanin from entering certain feathers or feather groups. Unlike albino birds, leucistic birds have normally colored eyes, beaks, legs and feet.
The white patches on leucistic birds have resulted in such monikers such as "pied" or "piebald" birds, terms that seem to have originated among poultry farmers. Some people refer to leucistic birds as having partial albinism, which is misleading because leucistic birds are not albinos.
Albino birds have a genetic mutation that causes an absence of an enzyme called tyrosinase, which synthesizes melanin. As a result, albino birds generally lack melanin in their bodies and plumage. Their eyes, skin, beak, legs and feet typically have no color. However, carotenoids, organic pigments that come from plants in the birds' diets, may enter some feathers, giving albino birds slight coloration.