This is a very complex subject, and honestly one that I am not fully qualified to discuss. However, I will attempt to share with you what I have learned, from articles as well as from people in the online avian community.
Our birds need Vitamin D – specifically D3. It is a crucial component in the calcium-phosphorus-D triangle. A problem, whether too much or too little, of any of these, can cause issues with the other two. Not only does it help with calcium metabolism, it is also important to cellular function, bone mineralization, and helps regulate other body functions. Vitamin D3 is manufactured by the skin; thus it is not found in plant based sources (aside from lichen, I’ve recently learned). It can be found in egg yolks, beef liver, fish oil, and a few other sources. Because D3 is 30-40 times more potent than D2, it is considered insufficient in the diet, as a bird would need to eat more than they are physically able to in order to make up the deficit.