Eastern Bluebird investigating nest box
In cold weather, bluebirds often huddle (roost) together in a nest box using their collective body heat to help them survive. Unfortunately, the thermal characteristics of a well designed nest box makes it poorly suited for winter roosting purposes. A well designed bluebird nest box will have ventilation ports. These ventilation ports help protect bluebirds by allowing heat to escape during the nesting season. As you would expect, these ports also allow the heat generated by roosting bluebirds to escape. To compound matters, wood is a very poor insulator.
We install insulation in the walls and ceilings of our homes to prevent heat loss in the winter and retain the cooling provided by air conditioners in the summer. Insulation can also be added to a wild song bird roost box. The insulation will significantly improve the thermal characteristics of the roost box.
If you decide to build a bluebird winter roost box, don’t add perching rods or ledges. Bluebirds huddle in a pile and take turns rotating positions so they don’t smother but still maintain an acceptable body temperature. Other species of wild song birds like to perch individually and will require perching places.
Insulated Bluebird Winter Roost Box (front view)
Insulated Bluebird Winter Roost Box (inside view)
Building a bluebird winter roost box is a fairly simple task. The outer shell of the roost box shown above is made of 1 X 8. The roof is made of 1 X 10 covered by a piece of Hardie soffit. Notice the entrance hole is set quite low compared to that of a standard nest box. Since heat tends to rise in an enclosed space, the lower entrance hole in the roost box helps to minimize heat loss. The insulated space is made of foam insulation sandwiched between the outer shell and layers of thin plywood. When the door is closed, the roosting area is completely enclosed in insulation.
I put a layer of white pine wood shavings on the floors of my roost boxes. Don’t worry if you put in too many shavings, the birds will eject them until they get the depth they want.
I use a 1 1/4” galvanized joist hanger nail as a side locking pin to secure the door. If you look closely, you can see the head of the nail protruding from the right side in the top photo.
A well constructed winter roost box can help your bluebirds survive during cold spells.