In the winter 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. Some cavity nesting birds like House Sparrows, bluebirds, chickadees, titmice and some woodpeckers will also roost together in nestboxes (birdhouses) in the winter. And Screech owls and squirrels are some other box-nesting species that like to stay warm in boxes.
The National Wildlife Federation wrote in one article:
Birdy, It's Cold Outside http://nwf.org/providing-cozy-winter-roosts
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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