Since the weather has abruptly changed from being more like autumn than summer, my wife and I have been seeing eastern bluebirds inspecting some of our nesting boxes. I am certain the birds are not checking out potential nesting sites–it is much too early for that. The birds may be just curious, or perhaps the onset of cold nights has triggered a search for suitable roost sites.
In addition to the bluebird, a number of familiar backyard birds also roost in cavities and nesting boxes including screech owls, woodpeckers, tufted titmice, brown-headed nuthatches, and Carolina chickadees. In the case of the bluebird, they typically roost alone in warm weather. However, when temperatures dip below freezing, a cavity or nesting box might harbor anywhere from a couple to more than 20 bluebirds.
The advantage of nesting together is the birds share their body heat. During an extremely frigid night, the additional heat offered by a group of roosting birds may spell the difference between life and death.
With that in mind, as we enter the harshest portion of the year, keep an eye on your nest boxes. There is a good chance one more bluebirds or other cavity-nesting birds are roosting in a box erected to provide a place for them to nest.
The best times to look for such activity is late in the afternoon when the birds are going to roost, or first thing in the morning when they are leaving for a day of foraging.
You can also peek inside a box. If you see some downy feathers scattered about the bottom of a box, chances are birds are roosting there.