Bird food and beak deformities
Feeder offerings aren’t to blame for abnormalities in Alaska’s chickadees
Published: October 26, 2012
In 2010, biologists with the U.S. Geological Survey’s Alaska Science Center reported that something was going terribly wrong with populations of Black-capped Chickadees, Northwestern Crows, and other resident birds in Alaska, British Columbia, and Washington. They are developing grossly overgrown and often crossed beaks. (See “Birding Briefs,” February 2011.)
A Black-capped Chickadee with a deformed bill rests in the snow in Fairbanks, Alaska.
Photo by Mary Zalar
Two years later, the causes of the affliction, known as avian keratin disorder, remain mysterious, but thanks to a recent study of chickadees captured at three sites in south-central Alaska, we know that the food birdwatchers provide isn’t to blame.
Researchers compared the ratio of carbon and nitrogen isotopes in natural food, in human-provided foods, and in blood samples taken from chickadees with normal and deformed beaks.
Birds with normal beaks consumed primarily sunflower seeds and insects and spiders, mainstays of a chickadee’s typical diet. Peanut butter, suet, and natural seeds and berries made up a smaller proportion of their diet.
Chickadees with deformed beaks ate a smaller proportion of sunflower seeds and insects and a larger proportion of natural seeds and berries and peanut butter, foods that can be consumed without cracking hard shells or probing bark. The group with more severe beak deformities had the most pronounced decrease in the proportion of arthropods.
The researchers, biologists Caroline Van Hemert, Colleen M. Handel, and Diane M. O’Brien, found no evidence of a human-provided food shared by all affected birds. What’s more, birds with moderate-to-severe deformities had the most dramatic shifts in their diet, suggesting that such changes are a consequence rather than a cause of avian keratin disorder.
The study appeared in the July 2012 issue of The Auk, the quarterly journal of the American Ornithologists’ Union.