ROOSTING BOXES HELP BIRDS SURVIVE WINTER
An alternative to a bird “house” in winter is a “roost” structure that provides protection from the elements. Designed like a house with a few important differences: 1) a lower entrance hole prevents rising warm air generated by the birds from flowing out from the hole; 2) Using styrofoam panels as a liner helps insulate the structure; 3) perches within the interior gives birds room to utilize the space without crowding too tightly on the floor area only.
Tom outdoors.com 