heybud, I built some bluebird houses last year. I installed them on fence posts. A neighbor about 1/4 mile away had them but I had never even seen one. I modeled the houses on some I saw on EBAY. In early Spring they showed up and immediately nested. I was thrilled until the squirrels got them.
This year I put them on T posts and built some sheet metal squirrel excluders. Problem is that the excluder flops in the breeze and no birds go near them. I might try some gourds. At least I could hang them and grease the wire.
