Wednesday, September 26, 2012

Pale Birds at the Feeder

Our friend Jane Ann Johnson reported that she has been seeing two white birds with yellow beaks at her bird feeder the last week.  Initially she couldn't get pictures of it through the screen.  I suggested that a Master Naturalist would simply cut a hole in the screen.  Since they aren't members, her husband John simply adjusted the focus and managed to get a few good pictures.

Charley Burwick and associates have reviewed the photos.  The first guess was that they are amelanistic House Sparrows.  Their "host" Jane Ann replied that they appeared much smaller than sparrows they also see at the feeder.  The votes are still pending, but the species isn't that important to the story of color.

Bird scientists might debate the exact biologic causes of marked color variation in an individual bird.  Obviously genetics control the normal color of birds which are felt to be important in attracting mates.  It is not for nothing that a peacock drags around the heavy plumage which is metabolically expensive to maintain and increases its risk from predators.

"Cropped" photo- pun intended
There are several genetic variations to explain color change.  First there is amelanism that refers to the lack of melanin in skin and feathers.  This is generally attributed to a genetic abnormality leading to the loss of tyrosinase.  Since the only pigment mammals produce is melanin, loss of this pigment creates albino animals.

On the other hand, other vertebrates such as birds, reptiles and fish produce other skin pigments in addition to melanin.  Amelanism in these animals usually lack the total white and red eyed features.  Look again at the cropped picture and you will notice the yellow beak and legs, slight pigmentation on the trailing wing edge and the dark eye.  The eye is important as pink eyed albino's have vision problems in bright sunlight.

Piebald leucistic Rock Pigeon
Another pigment variation is leucism,  This is a decrease or lack of all pigments due to a defect in the development of all the pigment cells.  Since the animals above have their other pigments manufactured in the same pigment cells, all colors are affected.  This may be their entire body or be partial, as seen in piebald animals with patches of white amid other normal pigmented surfaces.

While having two pale birds show up at the same feeder seems unlikely, Charley reports that others have noted this from time to time.  This takes me back to my days in medical practice when weird symptoms or findings were explained to patients with the vague phrase "We see that sometimes."  Maybe a better explanation comes from the wisdom dating back to 1599, "Birds of a feather, flock together."

What ever you want to call the phenonoma, in the words of our resident birder Charley, it is  "really, really cool." 

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