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The deformity often appears as an oversized upper beak, making it more difficult for the birds to eat and preen themselves, said wildlife biologist Caroline Van Hemert at the U.S. Geological Survey Alaska Science Center.

The problem can be particularly life-threatening during wintertime cold spells.

While the cause is unknown, Van Hemert said initial studies point to environmental contaminants.

It's not the first time Alaskans have been asked to help. The study began in 1999, but the mystery remains.

"A lot of people think it is a problem that has come and gone," Van Hemert said. "We are still really interested."

Since the project was launched by center wildlife biologist Colleen Handel, roughly 1,500 black-capped chickadees have been reported, most of them in Southcentral Alaska - and many of them by citizens.

The numbers indicate that about 10 percent of the chickadee population is affected, making it the highest concentration of deformities ever observed in a wild bird population, Van Hemert said.

Deformities are typically seen in birds at a rate of less than one half of one percent.

Similar beak abnormalities have also been noticed in northwestern crows - a species more common to Southeast Alaska.

"We have received more than 150 reports accounting for approximately 60 individuals in Alaska and at least 20 additional birds throughout the rest of the Pacific Northwest," Van Hemert said.

Roughly one-third of the Alaska sightings have been in the Juneau-Douglas area. Among other affected species are downy woodpeckers, Steller's jays, and black-billed magpies.

"There have been a few birds seen. I know some northwestern crows and some ravens. I've seen one raven, I believe," said Mark Schwan, vice president of the Juneau Audubon Society. He sees the abnormalities themselves as just part of the puzzle.

"I think what concerns me more is that it seems like a fairly widespread problem. It makes you wonder what is going on," he said.

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Biologists also plan to survey remote areas to see whether the condition has a human connection.

Van Hemert said that since chickadees and crows have such a different diet and other characteristics, the problem may stem from both terrestrial and marine systems where they get their food.

The deformity is primarily seen only in adults, Van Hemert said. The youngest deformed chickadee on record was six months old.

"It is not clear if there is underlying susceptibility or external factor, or if it just something they just grow into," Van Hemert said. "We are looking more into environmental contaminants and possibilities of disease. Genetics don't seem to be a likely cause."

In some birds, center biologists have discovered low levels of polychlorinated biphenyls or PCBs, industrial toxins banned in 1977 but still found in the environment.

"They come from byproducts of transformers, and in the electronics industry they are used. They are nothing that would be produced locally," Van Hemert said.

Research on the northwestern crows is funded primarily by the U.S. Geological Survey. Previous funding for the chickadee research also came from the U.S. Fish and Wildlife Service.

• Brittany Retherford can be reached at brittany.retherford@juneauempire.com.