I have read this too. The article I read was talking about parrots, vit d production and preening oil.My understanding is that the oil from the preen gland contains the precursors to vitamin D and those gets converted to vitamin D when exposed to the sun. The bird then ingests the vitamin D when they preen.
They can also make vitamin D in exposed skin (as we do) but there is only a little bit of that. I think unlike us their eyes can also make vitamin D.
I can probably dig up the reference to this, I came across it when researching vitamins in chickens because of Bernadette who I believe has a vitamin B2 issue.