Published Jan. 2, 2008, in the Wyoming Tribune Eagle, “Birds stay warm, despite cold. Diving into snowdrifts just one strategy used to deal with winter.”
2015 Update: I wonder how bird physiology responds to sudden drops in temperature?
By Barb Gorges
Wool, fleece and down clothes; insulated rubber boots; vigorous shoveling and skiing; hot chocolate and soup; lap cat and household thermostat; quilts and down comforters; and maybe a trip to Albuquerque–that’s how I survive winter.
Birds have similar strategies. The chief one is migration, whether down from the mountains or down to South America. It’s about balancing food-as-fuel availability with air temperature. The colder the air, the more food birds must find to turn into calories to burn.
Insect eaters depart, except for the gleaners, like the brown creepers which are willing to eat frozen bug bodies found in bark crevices.
Seed, berry and bud eaters stay behind, as well as predatory species. Water birds stay as long as ice doesn’t prevent them from getting to the pond weeds and animals.
Migration is a matter of following the food. Snowy owls are perfectly happy wintering in the Arctic unless there aren’t enough lemmings to go around. Then they head south.
About 50 species out of a total of 325 on the Cheyenne bird checklist are observed regularly on the Christmas Bird Count. How do they survive?
First, there’s fat. Songbirds will put on enough on average to weather three days of storm. More fat than that and they would be too overloaded to evade predators. Sea ducks pack on as much as 10 days’ worth. That’s how long they can go without being able to eat before they run out of fuel and die.
Then there’s shivering. When the pectoral (breast) muscles contract and relax quickly, they produce heat.
That heat warms air trapped by feathers which work better as insulation than mammal hair. Most birds have some down feathers and birds of cold climates have especially good down. Think of eider down, which comes from eider ducks. But the more archaic species, ostrich, emu and kiwi, have none.
A cold bird’s other feathers will lift away from its skin and trap more warm air, making the bird a completely different shape in cold than in hot weather. Others, like the common redpoll, have more feathers in winter than summer. Some north country birds have feathers on their legs, like the rough-legged hawk.
Otherwise, birds keep their bare parts warm by burying their beaks under their wings or hunkering down, fluffing feathers over their feet. But you’ve seen the silly Canada geese at Holliday Park, the ones refusing to migrate because people in the past fed them. They walk on the ice. How do they stand it?
Cold adapted birds have a sophisticated heat exchange system. Warm, arterial blood, traveling from the heart to the feet, passes cooled venal blood returning from the feet and warms it before it is pumped back through the heart. Basically, birds have cold feet so we don’t have to worry about them getting stuck on cold metal perches on bird feeders.
Sometimes a sleeping bird’s core temperature will drop 20 degrees from 108 degrees (chickadee body temperature) for a fuel savings akin to turning down our thermostats at night. However, if we humans let our body temperatures drop from 98.6 to 78.6 degrees, we’d die. Somehow birds can pull it off and wake up for another day of foraging.
A few birds even manage to enter a deeper state of torpor for months. A study in the 1930’s of a poor-will over 85 days showed its body temperature fluctuating with air temperatures in the 60’s.
Birds search out warm microhabitats. Grouse dive into snow drifts and stay as toasty as any Boy Scout in a snow cave–unless the surface gets iced over and then they are toast–the birds, that is, not the boys.
Starlings standing around chimneys are smart–until the fumes knock them out and they topple over.
The higher a perching type of bird is in the flock’s pecking order, the closer to the trunk of an evergreen tree or the middle of a thicket it can roost and the more protected it will be from winter storms. So the more dominant a bird is, the more likely it is to survive. Besides, dominant birds eat better too and may have more fat going into a storm.
And then there’s the “dog pile” effect. A researcher in Maryland cut the top of a hollow wooden fence post where eastern bluebirds had nested so he could remove it from time to time over the winter. Once he found 13 bluebirds packed inside, and another time found more, but two of those were dead from suffocation.
Hawks and owls prefer winter solitude since they are in competition with every other raptor for the limited supply of prey animals.
But for some species, especially the songbirds, sociability is the key to survival. Secretive during the summer nesting season, in winter small birds–chickadees, nuthatches, sparrows–form what is referred to as “mixed species flocks.” It means more eyes watching for predators and potential food sources.
I think sociability is a good way for people to survive the winter, too. Members and friends of Cheyenne – High Plains Audubon Society will be gathering for the annual Christmas Bird Count and the tally party afterwards.
If you are interested in helping, the more of us there are, the warmer we’ll be!