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.
U.S. Geological Survey