Deformed Bill Research
Colleen Handel and her colleagues at the United States Geological Survey’s Alaska Biological Science Center began researching deformed bills, also known as Avian Keratin Disorder or AKD, in birds in 1999. They compiled reports of deformities in at least 28 bird species in Alaska, including year-round residents like Black-billed Magpies, Red-breasted Nuthatches, Downy and Hairy Woodpeckers, Northwestern Crows (formerly a separate species but now included in American Crow), Steller’s and Canada Jays, and Common Ravens. Bill deformities in migratory species, including American Tree Sparrows, Lincoln Sparrows, Ruby-crowned Kinglets, Orange-crowned Warblers, and Dark-eyed Juncos, have also been recorded.
The USGS researchers compiled reports of 2,153 chickadees, representing at least 1,441 individuals, in Alaska with deformed bills from November 1991 through May 2005. Following publicity in Alaska about the bill deformities, reports grew dramatically. However, since 2000-2001, when more than 400 abnormal birds were reported, the number decreased to around 200 per year through 2005. For the rest of North America, they only received a total of 21 reports of chickadees with deformed bills for the same period.
Handel’s team of researchers banded birds to track individuals. They captured 2,186 Black-capped Chickadees between 1999 and 2005. Handel and her colleagues found 178 (8.1%) of the chickadees captured to have deformed bills. By recapturing banded birds, they determined that some birds born with normal bills later develop deformities. The youngest chickadee they captured with a bill deformity was about six months old. The bird had originally been banded with a normal bill. The bird was recaptured a few months later with an overgrown bill. When they captured the same chickadee again in two months after that, the bill had become crossed. Handel documented 54 cases in which an individual that was captured originally with a normal bill was subsequently recaptured with a deformed bill. She found that these deformities were more likely to develop during late winter than during other times of the year. Handel tested DNA and found that chickadees with bill deformities had a significantly greater amount of DNA damage than normal chickadees. This damage could be caused by exposure to contaminants or by disease.
Over the years, researchers ruled out a number of possible causes of AKD, such as bacterial or fungal infections, bird feeders, and contaminants. Then in 2016, a team of researchers from the California Academy of Sciences, University of California San Francisco, and the USGS, used new technology to identify a previously unknown virus, the Poecivirus, that could be linked to AKD. Following that finding, researchers tested 28 birds with bill deformities and 96 without deformities, and they found that 100% of the birds with deformities had the Poecivirus while only 9.4% of the birds with healthy bills had the virus. They don’t know yet if the virus is causing the problem, but it’s a promising new lead. Researchers are now trying to determine how the virus might be causing the deformities. Read more about their research.
Learn more about the USGS research and report any birds with deformed bills directly to the USGS team on their website.