I’ve seen a bird with an overgrown or otherwise deformed bill. What is wrong with it?

April 1, 2009
A Cactus Wren with a deformed bill. Photo by Joan Gellatly via Birdshare. A Cactus Wren with a deformed bill. Photo by Joan Gellatly via Birdshare.
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Sometimes birders observe birds with odd-looking beaks. For example, numerous Black-capped Chickadees with greatly elongated and down-curved upper beaks have been reported since 1998 in southern Alaska. The phenomenon has now spread to some 30 species, but seems largely confined at present to southern Alaska. Scientists studying this phenomenon have yet to determine a specific cause. Bird beaks are much like human fingernails—soft structures that grow at a constant rate all the time. Many factors have been implicated in causing beaks to grow abnormally, including disease, parasites, nutritional deficiencies, genetic defects, exposure to extreme heat, exposure to environmental contaminants, and structural damage caused by a collision or other trauma.

A slight malformation may not affect a bird’s survival, but an extreme deformity may make normal feeding difficult if not impossible. Sometimes it happens gradually enough that the bird learns to compensate, but if the excessive growth doesn’t stop, eventually the bird is likely to starve.

Read about progress in understanding the deformed-beak outbreak in this 2011 Living Bird article.

No matter where you live, if you see a bird with an overgrown bill, it is important to report your sighting so that scientists can track the spread of the phenomenon.

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