Whenever I get a series of letters asking the same question, it’s time to address that issue in a column. Over the last few weeks, a handful of readers have asked about winter roosting boxes, so here goes.
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? There is hardly a better place to avoid plunging temperatures, howling winds and blowing snow.
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. Careful observers have reported as many as 10 bluebirds cramming into a nest box at dusk in mid winter.
I have also found reports of as many as 46 winter wrens, 50-plus pygmy nuthatches and 29 white-breasted nuthatches roosting together in single cavities.
They gather to sleep and conserve body heat on the coldest winter nights. On milder nights they perch amidst the protective cover of evergreen vegetation or persistent dead leaves clinging to an oak branch. This pattern holds for many cavity-nesters. On mild nights they roost in vegetative cover; on frigid nights they sleep in cavities.
Woodpeckers and some owls, on the other hand, usually sleep inside cavities, regardless of the weather. 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. Though cavity-nesters often roost in ordinary nest boxes, this may be simply because better roost sites are not available. Nest boxes are typically well ventilated, and the hole is positioned at the top of the cavity. Vents promote heat loss (which is good during the hot nesting season) and the entrance hole allows heat to rise and escape.
Designed to minimize these problems, a roost box measures approximately 10 inches square and 24 inches high. There should be no air vents, and all upper joints can be sealed with a silicone bead to completely eliminate air flow. The only opening is a 2.5-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 5-8 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 and search for roost box.
If building a roosting box sounds too difficult, many wild bird stores and nature centers sell them.
Another great source for quality roost and nest boxes at great prices is the Pennsylvania Game Commission. Search for winter roosting box, then click on wood products brochure.
Another option is to modify ordinary nest boxes for the winter. First, remove all old and soiled nesting material from used boxes and replace with a one-inch cushion of dry, insulating grass. Then plug all vent holes with a pliable, putty-like material that can be easily removed in the spring.
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.
Though roosting boxes have been around for years, surprisingly little is known about how much birds use them. The logic behind roost boxes is sound, but I would love to hear from readers who have experience with roost boxes.
Tell me your experiences, and I’ll review them in a future column.