For the Birds Radio Program: Albinism and Leucism
Today Laura Erickson talks about oddly-colored birds. This program was inspired by KAXE programmer Lauren Childs. 4:15 (date confirmed)
A couple of weeks ago, I got an interesting phone call from Lauren Childs, a programmer at KAXE, who found a unique junco among the enormous flocks at his feeder. It wasn’t an albino—at least, not quite. It wasn’t pure white, or even white in patches as a partial albino would be, but was a creamy color where normal juncoes are slate gray or brown.
This genetic condition is very rare, and has a special name—leucism. It’s caused when the body produces some, but very little, normal pigment. Albinism is a more common genetic occurrence caused when the body lacks an enzyme, tyrosinase, which produces melanin in the first place. Brown and gray pigments are more likely to be replaced with albinistic white than yellow or red pigments are. There are many more albino blackbirds, sparrows, crows, hawks, and robins than albino goldfinches or cardinals.
When specific areas of the body lack tyrosinase, while other areas have it, we have a partial albino–a bird with normal colors in parts of the body while other parts are pure white. Since a true albino cannot produce melanin at all, the eyes are pink, and albinos are very sensitive to light. Some birds have white feathers but normal eyes—these partial albinos have a greater survival rate than birds with normally colored feathers but albino eyes. Human albinos have a far better prognosis for survival than bird albinos, since standing out visually to predators is hardly a factor with people, and since humans can stay indoors or wear sunglasses to protect them from excessive sunlight. Perhaps avian albinos would have a better survival rate if they could grow little ear flaps to hold their sunglasses up.
A related condition, schizochroism, occurs in species whose natural color comes from more than one pigment. If an individual lacks the enzyme necessary to produce one of these pigments, it will have a washed out appearance. Schizochroism is known to occur in murres and dovekies—both oceanic birds—and also California Quail and Mourning Doves. Another condition, xanthochroism, happens in species whose color comes from both melanin and carotenoids. If the bird, like an albino, doesn’t produce melanin, but still produces carotenoids, its feathers will be yellow. This happens most frequently in inbred captive parrots.
Birds that produce too much melanin–sort of the opposite of albinos–are called melanistic. This is a regular and common occurrence in some hawks, like Rough-legs and the Short-tailed Hawk, while it’s extremely rare but still happens in other species, like the Broad-wing. This year at Hawk Ridge, Frank Nicoletti counted over 50,000 Broad-wings, but found only one dark one among them. Squirrels may not be birds, but since they come to bird feeders we might as well pay some attention to them. In some areas melanistic squirrels become rather common.
Melanism and albinism are both genetic conditions, and when anyone with either condition successfully reproduces, the young are likely to inherit either the condition or at least the gene. Dark squirrels may be more likely to be taken by predators, but to offset that they probably stay a bit warmer on frigid days. We don’t have any black squirrels on Peabody Street, but one visited our feeder for a day or two a couple of years ago. The rainbow of birds and other creatures that brighten the Northland broadens its spectrum with these oddball little jewels, and seeing a leucistic junco at our feeder is a vividly remembered experience for a lifetime.