Vitamin D can be produced when sunlight hits the bird's skin. Deficiency leads to rickets. Birds produce thin shelled eggs with reduced hatchability, show leg weakness and penguin like sitting posture. The beak, claws and ribs become very pliable.
This fat soluble vitamin is essential for the utilization of calcium and phosphorus in bone development and egg shell formation. The deficiency can lead to retarded growth, thin shelled eggs, leg weakness, curved legs, rickets and lowered egg production.