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.
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