This topic is confused and confusing, with no clear consensus of terminology. The Birding article was good, but I do not completely agree with its conclusions. One problem is that we're dealing with two completely different problems here: the description of what a bird looks like, and the genetic and physiological mechanisms that might be producing that appearance.
Leucism does have a technical definition involving the development and migration of melanocytes during embryonic development. Who cares about that? It results in a bird with scattered pure white feathers That appearance can also be achieved by an older bird experiencing the death of melanocytes in places (just like the hairs in my beard). Again, who cares? The birds look piebald, and might be described as "pied," meaning patched dark and white. The beautiful Hermit Thrush that started the conversation, however, is a nice example of "diluted" coloration. Again, the physiological mechanisms that can result in this physical example or "phenotyte" are varied. If you're describing such a bird for other birders, do you want to indicate what it looks like, or pretend that you have some knowledge about the physiology of the origin of its appearance? If the latter, I'd suggest you settle down and wait for a lot more research to be conducted. If the former, I would recommend using terms like "pied," "dilute plumage," and "pigment-challenged." Best, Kevin Kevin J. McGowan, Ithaca, NY 14850 k...@cornell.edu 607-254-2452 ________________________________ From: bounce-120416032-3714...@list.cornell.edu <bounce-120416032-3714...@list.cornell.edu> on behalf of Anders Peltomaa <anders.pelto...@gmail.com> Sent: Sunday, April 24, 2016 7:23 PM To: Peter Post Cc: NYSBIRDS-L Subject: Re: [nysbirds-l] Central Park, NYC: Leucistic Hermit Thrush Thanks Peter, I have since learnt from others on Facebook better. Michael Retter replied on Facebook, where I shared it to ABA's page. He said that he like to call birds like this Hermit Thrush, "hypomelanistic" and then followed up with another comment. --- quote --- There is a great article in Birding (39.5: 38) about pigment abnormalities in birds. It's from that article that I first learned of the terms "hypomelanistic", "amelanastic", and "hypermelanistic". Two key passages: "Melanins are the most prevalent pigments in birds. They produce almost all of the black, brown, gray, rufous, and buff shades and patterns, and they come in two general types--eumelanins and phaeomelanins... Eumelanins are more common and predominate in black and brown feathers, whereas phaeomelanins are less common and predominate in chestnut and rufous feathers...The relative proportion of these two pigment types, therefore, determines most colors...the ruddy throat of a Barn Swallow owes its color mainly to phaeomelanins." "The history of the term leucism is ... convoluted and controversial. It was first applied exclusively to the particular condition that results in all-white plumage or pelage and normally pigmented eyes and skin (Rensch 1925). Several decades later it was used for the abnormal condition that results in the dilution of plumage pigments (Harrison 1964). And more recently it has been applied to everything from an entirely white plumage to a single white feather (Buckley 1982). Harrison's usage appears to be the most widely recognized today [2007], but many still favor Buckley's definition. No matter which definition you prefer, however, the term leucism falls short in its usefulness for classifying color abnormalities in birds because it does not account for loss of pigment in unfeathered areas [such as in this Hermit Thrush's bill]." ---unquote--- Very interesting and certainly the best bird I have seen for a while. One of those birds that sticks with you and becomes real learning moments. Anders Peltomaa On Sun, Apr 24, 2016 at 5:52 PM, <pwp...@nyc.rr.com<mailto:pwp...@nyc.rr.com>> wrote: A very interesting and unusual looking bird. However, I would not call this leucistic as there are no areas that show a complete lack of pigment. This is clearly what is technically known as a dilution. It is caused by one or a number of genes that deposit melanin in a lower than normal concentration. Peter Post Sent from my iPhone On Apr 24, 2016, at 3:22 PM, Anders Peltomaa <anders.pelto...@gmail.com<mailto:anders.pelto...@gmail.com>> wrote: Hi all, Yesterday, I and many other birders saw this exceptional Hermit thrush in Central Park. None in my group had ever seen anything like it, and some in the group have 50+ years of birding experience so calling it exceptional is not an exaggeration. Photo by visiting French birder Hubert Pottiau. <https://www.facebook.com/profile.php?id=100010342935903> https://flic.kr/p/GsFaeD good birding, Anders Peltomaa Mannahatta -- NYSbirds-L List Info: Welcome and Basics<http://www.northeastbirding.com/NYSbirdsWELCOME> Rules and Information<http://www.northeastbirding.com/NYSbirdsRULES> Subscribe, Configuration and Leave<http://www.northeastbirding.com/NYSbirdsSubscribeConfigurationLeave.htm> Archives: The Mail Archive<http://www.mail-archive.com/nysbirds-l@cornell.edu/maillist.html> Surfbirds<http://www.surfbirds.com/birdingmail/Group/NYSBirds-L> BirdingOnThe.Net<http://birdingonthe.net/mailinglists/NYSB.html> Please submit your observations to eBird<http://ebird.org/content/ebird/>! -- -- NYSbirds-L List Info: Welcome and Basics<http://www.northeastbirding.com/NYSbirdsWELCOME> Rules and Information<http://www.northeastbirding.com/NYSbirdsRULES> Subscribe, Configuration and Leave<http://www.northeastbirding.com/NYSbirdsSubscribeConfigurationLeave.htm> Archives: The Mail Archive<http://www.mail-archive.com/nysbirds-l@cornell.edu/maillist.html> Surfbirds<http://www.surfbirds.com/birdingmail/Group/NYSBirds-L> BirdingOnThe.Net<http://birdingonthe.net/mailinglists/NYSB.html> Please submit your observations to eBird<http://ebird.org/content/ebird/>! -- -- NYSbirds-L List Info: http://www.NortheastBirding.com/NYSbirdsWELCOME http://www.NortheastBirding.com/NYSbirdsRULES http://www.NortheastBirding.com/NYSbirdsSubscribeConfigurationLeave.htm ARCHIVES: 1) http://www.mail-archive.com/nysbirds-l@cornell.edu/maillist.html 2) http://www.surfbirds.com/birdingmail/Group/NYSBirds-L 3) http://birdingonthe.net/mailinglists/NYSB.html Please submit your observations to eBird: http://ebird.org/content/ebird/ --
