It is well known that during the winter eastern bluebirds sometimes roost together in the same cavity or nesting box. Although much is yet to be learned about this behavior, observations of bluebirds roosting in a nest box in Indiana during a period of extremely cold weather provide us with insight into the fascinating roosting habits of this popular bird. The event I am describing here took place in Indiana during a period of extremely cold weather.
Throughout this spell of frigid weather, as many as 14 bluebirds used a single nesting box as their nighttime roost. It is interesting to note that while a number of other apparently suitable boxes stood nearby, all of these birds used the same box night after night.
The roosting birds did not simply pile atop one another after they entered the box. To the contrary, they carefully arranged themselves in two to three layers. The birds comprising each layer positioned themselves so they looked like spikes on a wagon wheel. Once each bird settled in, it faced toward the outside of the box.
Birds arranged in such a manner would have little chance of suffocating during the night. By so doing, the warm air expelled by each bird would also help keep the interior of the nesting box warm.