I never managed to grow one big enough. But i do know a little (literally, a little) about birds.
The back door is for feeders, not nesting boxes. For feeders, a back door is often a good idea - and it should be smaller than the front door, so a little shy bird can get away from a large aggressive one. I guess. Actually, on my feeders, everybody tends to be polite in winter when the orderly chickadees set the pace and it's a free-for-all in summer, when the uncouth grackles take over.
For nesting, you need one entrance, of a different size and placement for each kind of bird. The best way to decide which to make is to look what the birds you're hoping to accommodate build for themselves.
http://www.amishgourds.com/sto...
