Black-capped chickadee with a normal beak and an abnormal beak

The photo on the left shows a normal beak and the photo on the right shows a severely deformed beak where the upper beak is elongated and curved down while the lower beak is elongated and straight. These side-by-side photos shows how the disease, avian keratin disorder (AKD), can cause beak deformities among Black-capped Chickadees. AKD is characterized by debilitating beak overgrowth and other abnormalities of keratinized tissues. Affected birds have difficulty feeding and preening, and may suffer high rates of mortality.