Alaska Science Center
Overgrowth of the beak can be caused by nutritional deficiences of vitamin A, vitamin D3, or calcium, or by an imbalanced ratio of calcium and phosphorus (Altman 1986, Harrison and Harrison 1986). Vitamin D3, which is the form of vitamin D used by birds and is necessary for calcium regulation, is derived from sunlight and either absorbed through the skin or ingested through preening. During short winter days, it is possible that birds may not get adequate sources of vitamin D3 from sunlight. They could also develop nutritional deficiencies from incomplete diets, particularly if they are overly reliant on sunflower seeds at feeders or other human sources of food that are low in calcium and vitamin A and high in fat, which can interfere with calcium absorption. However, observed clinical signs in affected chickadees differ from those previously described for vitamin A, vitamin D3, or calcium imbalances, making these possibilities less likely.
Zinc, biotin, and pantothenic acid deficiencies have been associated with keratin abnormalities in other avian species (Fletcher and Abdul-Aziz 2008) and may warrant additional investigation. It also is possible that any underlying nutritional problems could be compounded by other factors, such as exposure to low levels of contaminants. For example, cormorants from Great Lakes colonies that were exposed to low PCB levels developed bill malformations only after being held in captivity for two weeks without natural daylight (Kuiken et al. 1999). Thus, there could be a synergistic condition involving low levels of PCBs and deficiency of calcium and vitamin D3, although this has not been tested experimentally.