Black-capped Chickadee - photo by Laurie Green

Prevalence, Distribution, and Timing

The prevalence (8.1%) and concentration (over 1,400 individuals) of beak deformities among Black-capped Chickadees in Alaska are the highest ever recorded among adults within a wild bird population anywhere.  Since the 1990s, prevalence of beak deformities has clearly increased in the Anchorage area among Black-capped Chickadees.

Abnormal beaks are relatively rare among adult birds, and background levels are less than 0.5% in normal populations.  The prevalence among adult chickadees was four times higher than the maximum rate documented for any passerine (Pomeroy 1962).

Although we have not yet measured prevalence of beak deformities among corvids, the large number of sightings of Northwestern Crows, Black-billed Magpies, and Steller’s Jays suggests that prevalence is higher than the 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 woodpeckers have recently been observed with deformities in the Fairbanks area of central Alaska.  In addition, beak deformities among other species appear to be increasing in Alaska and throughout the Pacific Northwest, which suggests that a broad geographic area is affected.

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.