It depends on what kind of glass your windows have. Plain glass lets all the UV in, but many modern windows have UV coatings that block UV. If you have an older house with single pane windows, or an attached greenhouse, those windows are probably plain glass. If you have a newer house with double or triple pane windows, it's safe to assume the windows have a UV coating.so if our budgies are placed in doors in a cage they will be sickly and die because they don't get any natural sunlight? i have my guy placed near a window, but apparently the UV rays don't come through the glass? so should i open the window next to his cage?