Sooner or later everyone who watches birds will come across a real oddball. Some of the more frequently seen abnormal birds are the partial albinos. They can be all white, have white patches (a white head, perhaps), or appear pale grayish tan all over. Many terms are used for such birds, but I prefer simply to call them partial albinos. (Leucistic means the same thing.)
A partial albino may be almost completely white, or it may have a few white feathers. On some birds, such as the junco above, the white feathers are clustered along normal feather tracts. On other birds, they can be scattered all over. This form of partial albinism, where normally colored feathers exist alongside white ones, I call pied.
Another common version of partial albinism, in which the feathers still have a little pigment, is called dilute plumage. The bird looks pale tan or gray and shows a ghost of its normal feather pattern.
Birds have two kinds of melanin pigment. One is a reddish brown to buff color and is responsible for the red breast of American Robin and the pale buff flanks of Black-capped Chickadee. The other melanin ranges from the black of crows to the soft gray of juncos. Most birds have both pigments; they work together to produce all black-brown-buff colors and, with the addition of carotenoid pigments (red to yellow), produce almost all of the varied colors of birds.
Each kind of melanin can be lost independently. An American Robin that is mostly white but has a more or less normal reddish breast is a bird that lacks the black-gray kind of melanin but retains the red-brown kind.
A loss of melanin can have many causes and may be permanent or temporary. True albinism is a genetic condition causing a complete absence of melanin, including pink eyes. Partial albinism is more common and can occur in healthy birds that are otherwise normal.
If you see a partially albino bird, ask yourself: Is it pied or dilute? Is it missing both kinds of melanin or just one? Much about its appearance can be explained by these relatively simple factors. – David Sibley, Contributing Editor
About David Sibley
David Sibley’s column “ID Toolkit” appears in every issue of BirdWatching magazine. Subscribe. This article appeared in the February 2014 issue. David is the author of The Sibley Guide to Birds (2000), Sibley’s Birding Basics (2002), field guides to the birds of eastern and western North America (2003), and The Sibley Guide to Trees (2009). He is also the illustrator and a co-author of The Sibley Guide to Bird Life and Behavior (2001). He writes frequently about birds on his blog Sibley Guides.