|
| Black-capped Chickadee, photo by Joy Geiselman |
Overgrowth of the beak can be caused by nutritional deficiencies of vitamin A, vitamin D3, or calcium, or by an imbalanced ratio of calcium and phosphorus (Altman 1986, Harrison and Harrison 1986). Although we have no direct evidence that nutritional deficiencies are responsible for Alaskan deformities, probable mechanisms warrant further consideration. In particular, calcium or vitamin D3 deficiencies may contribute to abnormal beak growth.
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, 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.
Nutritional deficiencies may also be compounded by other physiological problems, 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. Potential combined effects require additional research, and will be tested in a controlled setting with captive birds. |
