Bluebirds, chickadees, titmice and nuthatches nest in cavities and readily use nest boxes, but did you know they also sleep in natural and man-made cavities on the coldest nights of the year?

On frigid nights, a cozy roost site can mean the difference between life and death for cavity nesters. You can make winter more bearable for these birds by building or purchasing a roost box.

Roosting behavior gives backyard birders another opportunity to provide a critical resource for cavity nesters. A roost box is designed to conserve body heat and minimize heat loss. Approximately 10 inches square and 24 inches high, a roost box has no air vents, and all upper joints can be sealed with a silicone bead to minimize heat loss. The only opening is a 2 1/2-inch entrance hole cut into the bottom of the front panel. Inside, a series of shelves or quarter-inch dowels runs from side to side. The dowels should be staggered to minimize the amount of droppings that fall from birds perched above to those perched below.

Finally, be sure one side or the bottom of the box is removable so it can be cleaned out occasionally. Hang the roost box 5 to 8 feet above the ground in an open southerly exposure to take advantage of solar radiation, and if possible, place it so it is protected from the prevailing winds.

For detailed winter roosting box plans, visit Cornell's Lab of Ornithology Web site (www.birds.cornell.edu) and search "roost box."

If building a roosting box sounds too difficult, many wild bird stores and nature centers sell them. Or visit the Pennsylvania Game Commission's site (www.pgc.state.pa.us), search "winter roosting box," then click on "wood products brochure."

Finally, I must add a word of warning: During periods of extreme cold, especially following an ice storm, birds sometimes die while roosting. This is natural and unavoidable, though it can be shocking to find a box full of dead birds in the spring.