Wednesday, April 8, 2009

thoughts on albinism in birds; particularly redtail hawks


The photo attached to this posting shows a white gyrfalcon on the left and a white redtail hawk on the right.
Note the talons of the hawk. The photo was sent to me by Henry Kendall of Chesterfield, Missouri who was caring for the birds at the time the photo was taken, in the 1960's if I recall correctly. I believe the falcon was the mascot at the time for the U.S. Air Force Academy and the hawk (named "Whitey") was part of Henry Kendall's breeding project in which he hoped to produce white redtail hawks for falconers. Kendall is the first falconer to breed a prairie falcon in captivity in the early 1960's, which was before the first successful breeding of peregrine falcons in captivity in the U.S. and helped demonstrate that medium to large falcons could be produced in a captive breeding program.
I know of at least one person who would look at the photo of "Whitey", the redtail hawk on the right and say Whitey was not an albino hawk or albinistic, because the bird had normal, dark eyes and thus was not missing the normal pigment in the eyes. Thus, such a bird was just a hawk with some extra white in it, but not albino nor albinistic, any more than a woman can be partially pregnant.
Of course, such a thought process depends on a particular, simplistic definition of albinism that limits albinism to birds with no pigment at all, including feathers, skin or eyes and does not account for birds with lesser degrees of albinism. I consider such self-limiting definitions to be scientifically flawed and unnecessarily confusing because they do not describe the fullness of relevant reality, but impose artificial limitations not present in reality.
A better definition of albinism is used by Paul Johnsgard in "Hawks, Eagles and Falcons of North America". His succinct definition is as follows: "Albinism: Absence (locally or overall) of melanins or other pigments in individuals that normally are more highly pigmented."
Terres, in "The Audubon Society's Encyclopedia of North American Birds" describes albinism as:
"an albino has white feathers instead of the usual colors of its species and the white feathers may cover the bird wholly or in part, as there are various degrees of albinism. Some birds such as swans, geese, herons, egrets, gulls and others that are normally white are not albinos." (I would add that the same is true of horses and other animals with all or partial white in their normal coloration).
Terres goes on to describe the four degrees of albinism as follows:
"Total Albinism: the rarest form, in which the bird has a complete absence of melanin from the eyes, skin and feathers.
Incomplete Albinism: Pigment is completely absent from either eyes, skin or feathers, but not all three.
Imperfect Albinism: Pigment formation is partially inhibited (reduced) in eyes, skin or feathers, but pigment is not totally inhibited in any
Partial Albinism: The commonest form: ... within local parts of the body may involve certain feathers only; it is often symmetrical and each side of the bird may show white feathers in the same pattern.
Clearly, "Whitey" was an incomplete albino redtail hawk. That is the best available precise scientific terminology for that bird. The eyes were normal but the feathers and the talons were white. I have photos of similar redtails white normal talons and normal eyes, but all white feathers. I have photos of partially white redtail hawks with normal talons and eyes, and these would be scientifically called "partial albino" redtail hawks, not "hawks with some white feathers".
Interestinly, I happened to look in Ron Austing's very important book "The World of the Redtailed Hawk and found photos of a hawk he and his friends captured in Ohio in 1960 which is shown on page 27 of that book. The photo is black and white. It shows an adult redtail white white feathers and white talons and a white eye with yellowish tinge. The text describes the color of the feathers as white, though the photo appears to me to show some degree of pigmentation. In the absence of better information, I would tend to describe that bird as an "imperfect albino" hawk, or with dilute plumage. The eyes were not pink, but whitish and the bird was described as having excellent vision, which should not be typical of a total or pure albino redtail hawk.
I personally trapped and banded an imperfect albino (or leucistic) redtail hawk in my study area and its eyes were paler than normal, but not pink and the bird showed diluted pigmentation throughout its plumage.
In summary, to limit the definition of albinism to its purest and rarest form is to limit understanding of what albinism is and how it is expressed in birds. This information on the four degrees of albinism has been published in ornithological textbooks for five or more decades and has always been readily available. Why some authors continue to provide only a superficial and abbreviated definition of albinism, making it synonymous only with pure or total albinism is beyond me. Clearly, partial albinism is the most common form of albinism in birds and the other forms are well-documented. I have seen photos and descriptions of pure or total albino owls in the literature, but hardly so of hawks. Ron Austing mentions an albinistic red-shouldered hawk discovered by Dr. Heinz Meng in New York that had pale blue irises and which lived less than three years in captivity. Such a bird may not have survived to adulthood in the wild. Several sources reveal that albinism in hawks is not expressed in the plumage until adulthood.
Henry Kendall told me and others that he had documented over 550 case histories of albinism in redtail hawks. I am unaware that he ever documented any case of pure or total albinism, including pink eyes. Clearly, those hundreds of birds were either incomplete, imperfect, or partial albinos, according to the most precise definitions available. The most common form of albinism in birds, including redtail hawks is partial albinism, which is a term that has been used in ornithology for decades and is highly appropriate, accurate, specific and scientific.
Stan Moore

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