Here are a couple different bird houses. One is for tree swallows, the other for barn swallows.

Barn swallows will not nest in a normal bird house. They use to nest in the tops of wooden barns, making nests from mud and straw. As we have less wooden barns and areas for them, it is important to try and provide somewhere for them to nest. Here is plans for a barn swallow nest holder:



The non-native sparrow is very territorial and breaks the eggs of nearby birds, and the little tree swallow cannot compete againt the sparrow. The shape of the hole on the tree swallow house is designed especially for them, keeping the sparrows out. Here are the plans for the tree swallow house:


Here is a pic of it in action:


If you aready have a bird house, but want to sparrow-proof it, just get some thin wood and drill out the new-shaped hole, then glue it to the house.