31 January 2018

Have you ever seen a distant white raptor and hoped it was a snowy owl?  I have, but I’m usually wrong.  Snowy owls only visit Pennsylvania in late fall or winter and even when they’re in Erie County they rarely come to southwestern Pennsylvania.

What about a white gyrfalcon? Gyrfalcons are extremely rare in Pennsylvania and in North America they are usually brown, not white. In over 100 years only 41 gyrfalcons were reported statewide, always in the winter (see *1 below). The last gyr to visit southwestern PA was in 1913.

And yet we occasionally see a rare white raptor, even in summer.  What hawk is this?  In nearly every case it’s a leucistic red-tailed hawk (Buteo jamaicensis).

“Leucism is a condition in which there is partial loss of pigmentation in an animal resulting in white, pale, or patchy coloration of the skin, hair, feathers, scales or cuticle, but not the eyes.” (quoted from Wikipedia).  The condition is rare but red-tails are our most common hawk so it’s not surprising to find it in a numerous population.

The whiteness varies from hawk to hawk and even as they molt from year to year.  Sometimes leucistic red-tails are spotted brown, sometimes they’re entirely white.  Pat Gaines photographed a speckled one in Berthoud, Colorado in 2017 and an all-white bird in North Denver in 2010.  Neither bird is albino because its eyes are the normal dark color, not pink.

Leucistic birds, even when apparently all white, have at least one normally-colored feather.  It’s a tail feather on this hawk.

So what makes them white?

A study of color aberrations among birds in India listed six reasons for pale or white feathers. (Download the report here: How common is albinism really? Colour aberrations in Indian birds reviewed.)

• Albino (pink eyes) is a hereditary pigment error. Albinos are rarely seen because they die young due to poor eyesight.
• Leucism (normal eyes) is a hereditary lack of both melanin pigments.  Some feathers are normal color.
• Progressive graying.  Oh my!  A few birds turn “gray” as they age, becoming progressively whiter as they molt each year.
• Brown. Normally black feathers are brown and sensitive to light so they bleach out in the sun.  This mutation is only expressed in females.
• Dilution. Black feathers that are silvery gray, rather than true black, make the bird look pale.
• “Ino” is a condition like albino but not as severe. The bird does not have pink eyes and thus lives longer than a true albino.

Even so, we can’t know why each bird is white without a lot of study.

So who is that white hawk in Pennsylvania?  It’s probably a leucistic red-tailed hawk.

(photos by Pat Gaines via Flickr Creative Commons license)

UPDATE 15 May 2024. An excellent story about a leucistic red-tailed hawk in southwestern PA: Pittsburgh Quarterly: What Was That White Bird?

(*1) How rare are gyrfalcons in PA?   In 1982 and 1984, DVOC’s Cassinia analyzed all the reports of gyrfalcons in Pennsylvania. From the mid 1870’s to 1984 only 41 were confirmed: Gyrfalcon Records in Pennsylvania, Part One, 1982 and Gyrfalcon Records in Pennsylvania, Part Two, 1984.  Most reports were in Schuykill, Carbon, Berks, Lehigh and Lancaster counties with only 2 reports at Presque Isle, Erie County (plus a gyrfalcon at Gull Point on 1 Jan 1994).  As of 1984, the most recent sighting of a gyrfalcon in Pittsburgh’s 11-county metro area was 1 bird in Westmoreland County in January 1913. … Keep in mind that most gyrfalcons in North America are a uniform dark brown, not white.