[DesertRatTom]

Those are fun projects. I did some research some time ago and found that the size of the opening depends on the birds you're hosting. They also said don't add a peg to land on because it is then easy for predators to raid the nest. It's supposed to be fairly deep so predatory birds can't reach all the way to the nest. And they recommended using a fairly rough wood so the birds claws can get a grip. Finally, a trap door in the floor so you can clean it out now and again.

I bet they were fun to make too.