By Terry W. Johnson
While drinking a cup of hot, steaming coffee on a recent cold winter morning, I noticed that most of the wildlife activity in my backyard centered on my bird feeders. Nearby, a nesting box stood like a silent sentinel watching the parade of birds and squirrels dining at my wildlife café.
While it appeared the box served no purpose this time of the year, I knew better. One late afternoon about a week before, as the sun was seemingly dissolving on the western horizon, I just happened to see two bluebirds fly into the box.
Wow! The bluebirds were using the nesting box as a winter roost site.
Many cavity-nesting birds spend the night in tree cavities and nesting boxes. The long list of birds includes screech owls, bluebirds, brown-headed nuthatches, flickers, woodpeckers and Carolina wrens. Even blue jays, birds that don’t even nest in cavities, will seek shelter in roosting boxes when the going gets tough.
One of the first birds I ever saw using a cavity for a winter roost was a Carolina chickadee. One winter the tiny bird roosted each night in the uppermost pipe on my young daughter’s swing set.
The cold metal pipe was probably less than ideal as a roost. However, the hardy chickadee survived a winter during which temperatures dipped into the teens on several occasions.
While many birds roost alone, bluebirds sometimes roost in groups. If you have never seen a gang of bluebirds pile into a box, you have missed a treat. It isn’t unusual to see a half-dozen or more bluebirds flying into a nesting box shortly before sunset.
As each disappears into something that small, you wonder how they can all cram themselves into such a tiny space. Actually, roosting in a group has survival value. Even on the coldest nights, the warmth of their bodies helps ward off the bone-chilling cold.
The problem with nesting boxes is they aren’t designed to hold in that warmth. As we all know, warm air rises. Consequently, when birds roost in the bottom of a box, much of the warm air produced rises and is lost out the box’s entrance hole.
To solve this problem, the roost box was developed. The first thing that catches your attention when you look at a roosting box is that the entrance hole is at the bottom. The birds roost on perches above the hole. Consequently, the heat generated rises but is largely retained in the box.
There are vertical and horizontal roosting box designs. The boxes also come in a variety of sizes. The size of the box and entrance hole dictates what birds use it. While small birds like bluebirds will use larger boxes, it is impossible for larger birds to use smaller roosting boxes.
An ideal roosting box for smaller birds measures 10 inches square and 3 feet tall. Perches are provided by arranging ¼-inch dowels up the inside of the structure. The perches should not be directly above one another. Why? Think about it: Would you want to roost directly below a bird that has been eating berries and other tasty foods all day?
Place roosting boxes 8-10 feet above the ground. It is also a good idea to face them in a southern direction.
Although winter roost boxes have been around for years, they have never really caught on with folks who put up nesting boxes. These structures are so unusual I can count on one hand the folks I personally know who have erected them. This is unfortunate because the boxes can really help birds survive cold weather.
If you don’t have a roosting box in your backyard, I urge you to put one up. If you do, and happen to see birds flying into it at dusk, you will experience the satisfaction of knowing you are helping these wildlife neighbors survive an unforgiving frigid night.
Terry Johnson is a former Nongame program manager with the Wildlife Resources Division, a noted backyard wildlife writer and expert, and executive director of TERN, the friends group for Wildlife Resources' Nongame Conservation Section. His "Out my backdoor" column is a regular feature in Georgia Wild.