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Prevalence, Distribution, and Timing

Black-capped Chickadee with upper mandible curved downward - photo by Laurie Green
Black-capped Chickadee - photo by Laurie Green

The rates of beak deformities documented in Black-capped Chickadees and Northwestern Crows in Alaska are the highest ever recorded within a wild bird population anywhere. Since the 1990's, prevalence of beak deformities has clearly increased in Alaska and now affects an estimated 6.5% (±0.5%) of adult Black-capped Chickadees and 16.9% (±5.3%) of adult Northwestern Crows.

Abnormal beaks are relatively rare among adult birds, and background levels are less than 0.5% in normal populations. The prevalence we measured in chickadees was four times higher than the maximum rate documented for any passerine (Pomerory 1962). For crows, this estimate was more than 15 times higher.

Although we have not yet measured prevalence of beak deformities among other species of corvids, the large number of sightings of Common Ravens, Black-billed Magpies, and Steller's Jays suggests that prevalence is higher than normal background level for these species.

Based on reports in the late 1990s, two epicenters of beak deformities among Black-capped Chickadees were first identified: one in the Mat-Su Valley and one in Bristol Bay.  Beak deformities initially appeared to be clustered in south-central Alaska, where most deformed chickadees have been observed.  However, relatively large numbers of chickadees and other species have recently been observed with deformities in the Fairbanks area of central Alaska.  We have also documented chickadees with beak deformities at remote locations in south-central Alaska.

Beak deformities among other species, including crows, appear to be increasing in Alaska and throughout the Pacific Northwest. In addition, recent reports from other parts of North America and Europe, particularly the United Kingdom, suggest that this problem may affect a large geographic area.

Developmentally, birds appear to develop beak deformities as adults, rather than as embryos or nestlings, suggesting that they are not congenital in nature.  Very few young birds had obvious deformities, as compared to high rates in adults.  Most deformities were very subtle in onset, however, with slight overgrowth or asymmetry of the beak.  Therefore, deformities could be congenital and latent, only becoming obvious as the bird aged. 

Based on results from our winter banding study, there seems to be a higher probability of deformities developing during late winter than during other parts of the year.

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