If you've added any of the birdhouses from the series I've posted to your yard, now you'll want to make sure you protect them from predators.
Here are plans for a guard to add to any houses mounted on poles, to protect them from pests such as outdoor cats.
Schutz, Walter E. 1973. Bird Watching, Housing and Feeding. Milwaukee: Bruce Publishers
On that note - outdoor cats, whether family pets or feral, are devastating to wildlife populations. This link will soon be home to a post on that.