To survive a cold winter night a bird looks for a sheltered, warm place to sleep, such as a hole in a tree or a dense evergreen. On really cold nights several birds might all pile into one birdhouse and snuggle up to share body heat. Studies show that when 2 kinglet size birds snuggle up on a 32 degree evening, each reduced its heat loss by 23%. In groups of three, each reduced its heat loss by 37%. So if you are cold, hug a friend.
Large birds have an advantage over smaller birds because the have a relatively smaller surface area for their mass.
Ptarmigan and grouse, birds of snowy areas, will plunge under loose snow and sleep under the snow cover. It is warmer under the snow than in the open air. In Alaska, small Snow Buntings may stay under snow both night and day. Air temperature above the snow might reach – 30 degrees but two feet under the snow it will be a relatively balmy zero degree.
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