erythrism

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Unusual red pigmentation, as of hair or plumage.

Century Dictionary and Cyclopedia

1. n. In ornithology, a condition of dichromatism characterized by excess of red pigment in the plumage of birds which are normally brown, gray, etc. It is constantly exhibited by sundry owls, as species of Scops and Glaucidium, the common screech-owl of the United States (Scops asio), for example, occurring indifferently in the red or the gray plumage. Compare albinism and melanism.
2. n. In anthropology, redness of hair, generally combined with light complexion.
3. n. In zoology, the assumption of red in animals, in place of their ordinary coloration.

Wiktionary

1. n. Abnormal red colouration.

GNU Webster's 1913

1. n. A condition of excessive redness. See erythrochroism.