Besides shrubs, trees, rock and brush piles, nest boxes are a very important addition for shelter. When birds aren't using nest boxes for nesting, they often use them as a place to stay warm and dry. Some birds, like chickadees, will group together in a nest box to stay warm. I've read that there can be as many as 30 birds huddled together.
Size of Entrance Hole
The size of the entrance hole determines which birds can use the nest box. For example, a chickadee entrance hole is 1-1/8 inches. Whereas, 1-1/2 inches for an entrance hole is perfect for swallows and bluebirds.
Competition for a nest box can be fatal so accurate hole sizing is a very important feature. A good source for information on nesting habitat, box height and hole size is Birdhouse Reference at Cornell. If you want to know what birds live in your area, you can go to ZipGuides to find out.
Adding an Easy-Out
We staple plastic 1/8-inch mesh on the inside wall under the entrance hole to allow the babies easier access to get to the entrance hole.
Monitoring Nest Boxes
The Bluebird Monitor's Guide by Jack Griggs, Keith Kridler and Cynthia Berger is an excellent source for information on monitoring nest boxes. Monitoring nest boxes is very important and this book illustrates in great detail how to do it correctly.
Cornell's website, The BirdHouse Network (TBN) is a wonderful place to register and log the results of monitoring your nest boxes.
Photos & Videos About Nest Boxes
There are pictures of Chestnut-backed Chickadee nests and eggs and Tree Swallow nest with eggs in the Nest Materials section. You can also watch a video of a Chestnut-backed Chickadee visiting the nest box. You can watch the chickadee bring in food and then leave with a fecal sac.