Re: Why are there partially leucistic birds ?

Dear Frederic--

Unfortunately, my short answer is I don't know. I've often wondered the same thing, and haven't found a definitive answer. Here's what I do know:

Everything I've found on leucism indicates that the condition (in which pigment cells for color are not present in the skin or feathers) is purely genetic, and not nutritional in origin. For those who are unfamiliar with leucism, this condition differs from albinism, in which the pigment cells that create color do not exist at all.

Leucistic reptiles are common in the pet trade (as evidenced by numerous web sites advertising rare color morphs for snakes, geckos, and lizards), and more than one site documents leucism in birds. One of the sites echoes your statement that leucism is more common in male blackbirds than it is in females.

As for the expression of leucism, (why is only part of the body lacking in color in some leucistic birds?) my guess is that the genetic mutation responsible for leucism is a case of incomplete dominance. In other words, this does not appear to be an 'all or nothing' genetic mutation.

I don't know enough about leucism to provide you any definitive statements about why the birds you see look the way they do. However, here are some additional resources which may help.

Here are a few web sites with images and descriptions of leucism in birds:

• Leucistic Adelie penguin
• Leucistic yellow-rumped warbler (poor visibility in this image, but perhaps interesting), and a further descriptions of the same bird.

Craves, J.A.  1997.  Extreme leucism in a Yellow-rumped Warbler (Dendroica 
coronata coronata). Michigan Birds and Natural History 4:199-200. 

Owen, M.; Shimmings, P. 1992. The occurrence and performance of leucistic 
barnacle geese Branta leucopsis. Ibis 134:22-26.

Weiloch, M.; Czapulak, A. 1991. Cygnus olor immutabilis in Poland. Wildfowl 
Suppl. 1:304-309.

best,

Ruth

12-18-2004B: Added by Melissa Radcliff of the West Virginia Raptor Rehabilitation Center

We have a red-tail hawk that is leucisistic. Our site has wonderful
pictures of him showing this genetic condition.
His name is Snow.