On Apr 16, 2011, at 7:35 AM, Gary Bletsch wrote:

> Thus it appears that, some time in the mid-20th century, biologists sought to refine their terminology. Albinism was reserved for cases involving white body parts that lack melanin for whatever reason. Leucism was set upon as the term to use in cases of pallor of such body parts.
>
> However, it seems that nowadays there is a studied reluctance to use the "A-word" for any bird that does not have pink eyes and all-white plumage.

Perhaps for a good reason?

The underlying biology that controls pigmentation (melanocortin receptor and associated genes) is now reasonably well understood. So ornithologists interested in bird coloration use more precise terms that reflects the molecular biology. Then birders pick up on this when they read the papers and it's transmitted into the "wild" by word of mouth.

There is an important distinction between the two cases:

Albinism is a well understood genetic disease. It is a recessively linked trait i.e. the bird has two copies of the MC1R with mutations that render it defective. They cannot product any melanin anywhere in the body (hence the "red eyes": they have an uncolored iris). They can also pass one of their defective genes onto their offspring. If they mate with another albino bird (very very rare) they would have albino offspring. Otherwise the offspring will be carriers. That could be important in an isolated population.

Leucistic birds have some part of the pathway that generates melanin disrupted in some or all of the feather follicle (there are several ways it can break) so feathers don't get their melanin when they grow.

For birders does it matter? A little. True albinos are more rare than leucistic birds. There is a difference between the two cases so using the correct term aids communication e.g. a recent TWETTERS-ALERTS post mentioned an albino bird -- was it really albinistic (and really rare) or merely leucistic? How much effort you put into seeing it might depend on that difference.

As mentioned before Geoff Hills "Bird Coloration" is a great book for those interested in more details on this topic.
--
Kevin Purcell (Capitol Hill, Seattle, WA)
kevinpurcell at pobox.com
http://kevinpurcell.posterous.com
http://twitter.com/kevinpurcell