I've just been reading this thread, very interesting. Predator protection and ventilation are both so important! I've got two suggestions. The Restore run by Habitat for Humanity has doors, windows, and other useful stuff, very cheap. Also, rather than actual windows, a large opening covered in hardware cloth and maybe also woven wire, works great. In winter, some clear plastic sheeting over the lower section protects from snow and cold winds very well. Maybe not as beautiful, but good for the birds. Mary