Leucism, or leukism, is an abnormal plumage condition caused by a genetic mutation that prevents pigment, particularly melanin, from being properly deposited on a bird’s feathers. As a result, the birds do not have the normal, classic plumage colors listed in field guides, and instead the plumage have several color changes, including: white patches where the bird should not have any; paler overall plumage that looks faint, diluted or bleached; or overall white plumage with little or no color discernible.
Leucism is relatively unusual in birds, and albinism is rare. From 2000-2006, Project FeederWatch participants reported fewer than 1000 leucistic birds. Given that participants report about 5.5 million birds each season, the percentage of leucistic birds being reported is very small.
Leucistic Red-Tailed Hawk photos by J Centavo on Flickr