Providing Birds with Cozy Winter Roosts

Birds that nest in cavities or birdhouses will also use boxes in winter

02-01-2004 // Cynthia Berger

ONE MORNING last winter I glanced out the window just in time to see a bird poke its head out the round entrance to our backyard birdhouse. It was a surprise to see the wooden box occupied. I’d put it up the previous spring, hoping Eastern bluebirds would nest there, but the local bluebirds had ignored my offering. Grabbing binoculars, I could see a red patch on a black-and-white head—my first tenant was a male downy woodpecker. He wasn’t building a nest in the dead of winter, though—he was using the box as a snug bird bedroom.

Birds often seek protected places to roost or sleep. Dense vegetation found in thickets or the interior branches of evergreens serve as a windbreak and conceal the birds from night-prowling predators. A few species of songbirds—the ones that nest in tree cavities or birdhouses in spring—will also use roost boxes in winter. Among them: bluebirds, chickadees, titmice and some woodpeckers. Screech owls, another box-nesting species, will also roost in boxes. But don’t expect to see such familiar backyard visitors as cardinals, jays or juncos in your bird hotels. Species that build cuplike nests in trees and shrubs generally don’t enter boxes.

Gary Springer, a bird enthusiast in Carnesville, Georgia, has 35 bluebird boxes on his 50-acre property, and several double as winter roost boxes. “I’ve got a tufted titmouse that comes like clockwork—I’ve seen it every cold night for the past two winters,” he says. The bird pops into the box about half an hour before sunset, pokes its head out for a last look around, then settles in for the night.

Just as some college kids love roommates and others demand a single, different birds like different sleeping arrangements. Woodpeckers tend to want a private room; Eastern bluebirds like the concept of a “family bed.” Keith Kridler, an environmental educator in Mount Pleasant, Texas, watched a bluebird pair raise three clutches in a backyard box one summer. When winter rolled around and temperatures dipped to freezing, the whole extended family—nine birds in all—crowded into the box to sleep at night.

And, if your children have flown the coop, hosting some bird guests in winter can bring back those days when your teenagers wanted to stay out late. “You’ll see chickadees and tufted titmice at the feeder, eating sunflower seeds until dark,” says Kridler. “Finally, they beeline over to the box and zip inside. You just feel like, ‘Phew, they’re safe for tonight.’”

Luring Them In
There’s not much research on which roost boxes work best, but prefabricated boxes are worth a try. Some look like jumbo nest boxes fitted with interior perches except that the entrance holes are near the bottom of the boxes instead of in the middle or near the top. The idea is that hot air rises, so a low entrance hole keeps warm air from leaking out.

Don’t go with prefab boxes if you want to attract roosting bluebirds. For one thing, bluebirds don’t need the perches because they sleep in a heap on the floor. Bluebird enthusiasts also worry that the low entrance hole creates a draft for floor-sleeping birds and allows predators to reach in and grab a bird.

“Experiment!” encourages Keith Kridler, an environmental educator in Texas. Many people attract roosting birds simply by leaving nest boxes up all winter. Anecdotal evidence suggests roosting birds prefer boxes mounted 10 feet high or more in winter—perhaps because birds feel safer up high. (Nest boxes are usually mounted at eye level, so it’s easy to check on nest progress.) Use a telescoping pole to put the box in place and take it down for cleaning and repositioning in spring.

Should you winterize? If you opt for using your spring nest boxes as winter roosting boxes, some people recommend blocking the large ventilation holes—which keeps the summer sun from overheating the interior. Foam weatherstripping (the kind sold for air conditioners) works well, and it’s easy to remove when spring rolls around. But don’t seal the box up tight. Usually birds like to peek inside a box before entering—possibly to check for danger—and seem reluctant to enter pitch-dark boxes.

Try building a roost box using plans available on the Internet. Bird enthusiasts recommend a box with a larger-than-usual entrance hole—it’s easier for birds to get a good look inside and also easier to enter and exit. Studies in Belgium show that blue tits, presented with a choice of large-holed and small-holed roost boxes, preferred the larger entrance even though they fit just fine through the small one.

When you hang out your box, don’t forget to ensure it’s protected from predators. Mount the box on a metal pole—cats, raccoons, weasels and rats can climb trees and wooden fenceposts. As heartbreaking as it is to lose a nestbox full of chicks to a predator, says Gary Springer, “it is much more dreadful to think of losing 10 to 20 mature bluebirds in a single winter night.”

Pennsylvania journalist Cynthia Berger wrote about  ruffed grouse in the December/January 2004 issue of National Wildlife.

Birdy, It's Cold Outside 
It may seem obvious that a box is a warm place for a bird to sleep, but scientists like to confirm these things. Zoology professor Chuck Kendeigh did just that. More than 50 years ago, he noticed a house sparrow on the University of Illinois campus roosting each night in a box under the eaves of a building. Kendeigh rigged two recording thermometers to measure air temperature inside and outside the box, 24 hours a day, from December 20, 1949, to January 11, 1950—the coldest days of the year.

By day, when the bird was away, the temperature was the same inside and outside the box. When the bird went in at night, its body and its exhaled breath heated the small space. The colder it got outside, the greater the difference became inside versus out. At 18 degrees F outside, for example, the inside temperature was 29 degrees F.

That may not sound exactly toasty, but it makes a big difference to a bird. At those temperatures, Kendeigh calculated, the sparrow would burn 11 percent less energy sleeping indoors as opposed to outdoors. “The amount of energy thus conserved may make the difference between survival and death during periods of extreme weather during the winter,” he concluded.

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