Open windows, direct sunlight NOT through glass.. Necessity?

[fhgwgads]

I agree with debbie276 eggs are a great source of vitamin D.. and some finch foods (such as the pellets I use) also have vitamin D3 added as well as other goodies. Not all of use are capable of leaving our finches by an open window! Especially in the winter months. And I would much rather have my birds indoors than leave them outside... plus I don't have anywhere to build an aviary anyway!