WHITER THAN AN EAGLE

Thanks to Rob over at Birdchaser for his post linking to an interesting account of an “albino” Golden Eagle, along with some spectacular photographs. While I would have loved to swipe one and post it, I decided instead on the above watercolor of a white phase Gray Goshawk (Accipiter novaehollandiae).
This common Australasian bird normally looks like the fellow on the right, but a morph occurs with completely white feathers--right down to the wingtips. Not all populations carry the potential for this phase. Usually, white plumage is a liability for a predatory bird, especially in tropical forests. Conventional wisdom suggests the white Gray Goshawks realize a benefit from their similarity to white cockatoos (Cacatua spp.); small prey birds are likely to mistake them for the innocuous psittaciformes until it's too late. In fact, the white phase is not known to occur in any localities where Cacatua species don't.

White phase Gray Goshawks look very much like albinos--right down to the red eyes they share with their entire species, but true albinism is rare in raptors. The North American Red-tailed Hawk (Buteo jamaicensis) is the only species I know of where the trait crops up with any regularity.The plumage of the Holarctic Gyrfalcon (Falco rusticolus - left) ranges the spectrum from nearly black birds to ones that approach the pallor of an albino Redtail or white Gray Gos. Any biogeographical trends to Gyrfalcon color phases are harder to see, although the whitest birds are said to come from Greenland.

Pigment impoverishment can result from several types of genetic mutations: albinism, leucism, hypomelanism, etc. These various conditions differ in the types of pigment cells that are affected, and are especially well-known to herpetoculturists, who, in recent years, have developed dozens of color varieties of numerous reptile species by selectively breeding from rare mutants like the Burmese Python (Python molurus) pictured below, which was wild-caught last year in northern Bali.
Leucism and similar traits are normally thought of as purely genetic in origin, but it seems possible to me that there could be a nutritional aspect to some cases. Back in the '70s, a couple of friends and I trapped an unusual adult female Prairie Falcon (Falco mexicanus) that had been hanging around downtown Salt Lake City. Her plumage was very white--similar in appearance to a light-colored Saker (F. cherrug). Not only were her feathers unusual in color, they were very poorly-formed, with weak barbs. She was in a very emaciated condition when we caught her, and it wasn't clear to us whether her thinness resulted from poor feather quality or vice versa. A pair of jess anklets and a band on her leg though, held a clue. It turned out she had been trapped two years earlier (she was a first-year bird then) by another falconer who had flown her for a couple of months before losing her. He reported that her juvenile plumage had been perfectly normal. The original trapper transferred the bird's permit to my friend, but she unfortunately died of Trichomoniasis before her next moult. A few years later an apprentice falconer of mine trapped a similar Prairie Falcon, this one an adult male (below). He was also quite starved, but since the trapping of mature falcons was illegal, we made the decision to feed him up for a couple of weeks then release him, probably to his demise. I wish we could have kept one of these falcons through the moult to see if the feathers would be replaced by normal ones once their owner was back in good health.

The eagle captured in Colorado is said to have been so thin that he couldn't fly. From the photographs, his feathers look to be in the same shape as these Prairie Falcons. He is being treated at a rehabilitation center, and the report states that he's expected to be returned to the wild in about three months. Should this plan be carried through, I'd predict one dead, white eagle in Colorado by New Year's. I'd expect any zoo director to happily take responsibility of such an unusual and beautiful bird for a year or more, and keeping him through his next moult would not only increase his chance of survival, but possibly add to our knowledge of avian pathology.
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upper: WHITE PHASE GRAY GOSHAWK (1996) watercolor 11.5" x 8.5"
second: SILVER GYRFALCON PORTRAIT (2005) acrylic 15" x 7"
All photographs by CPBvK