Unlike humans, who absorb vitamin D from the sun through their skin, which is then converted into vitamin D3, bird’s feathers prevent the sun from hitting their skin directly. So, in order to absorb vitamin D, they “preen” themselves: this allows them to orally ingest the vitamin. Once ingested, it is then further broken down by the body, where it plays several crucial roles in the birds health, diet, development, and breeding.
Vitamin D is needed for the absorption of calcium, which is especially important for female birds, as they rely on calcium to produce their eggs. When lacking in calcium, their own bones will become brittle in order to compensate for the amount of calcium required to produce viable eggs. In other cases, calcium deficiency can lead to becoming
This is why “avian vets recommend that birds receive 15 minutes of full spectrum light from an avian light bulb that contains both UVA and UVB rays each day.” (http://www.vitamindwiki.com/UV+for+is+for+the+birds)