Many birds will roost in nest boxes in the winter. You need a hole at least 1&1/2 inches for Blue Birds, Woodpeckers etc. I host Blue Birds here in the summer, in the winter my Downy Woodpeckers use the boxes. I put wood shavings or chips in there for them. You need to identify what kind of birds you have so you know what size hole you need......LOL there is a Bird Watching and a Blue Bird forum on Garden Web too... I'm not talking up Garden Web....I just happen to be a Birdwatcher with a pond. ; )