by Bruce Auchly
Tuesday, January 05, 2016
Nights are long, often cold. Days are short, sometimes with a sun that acts like it doesn’t want to come out and play.
Depending on our disposition and age, we might smile or growl about the cold and snow. But if we go outside for any length of time we dress for the weather.
Birds that stay here, that do not migrate, have evolved some ingenious ways to dress for the weather, to stay alive.
Of all the creatures on this planet, only birds have feathers. They can range from about 1,000 feathers for a hummingbird to more than 25,000 on a swan.
Of course feathers are necessary to fly, but they also provide insulation and warmth; they are the original down coat.
Feathers are better insulation than mammalian hair, according to “The Sibley Guide to Bird Life and Behavior.” A bird can even create air pockets between the feathers and the skin that help contain heat.
Sometimes birds will use behavioral methods like clustering together. Grouse will bury themselves in snow. Other birds will roost in tree cavities or dense foliage or brush piles to cut heat loss.
Depending on the species, below a certain temperature birds can shiver specific muscles to increase metabolism and generate extra heat.
Geese, or ducks, will stand or sit for hours on an ice shelf next to a river’s open water. Of course they carry a nice, plump down coat. But what about their exposed feet?
First, their legs and feet have very little soft tissue. Even the muscles that operate the foot are mostly higher up in the leg and connected to the bones of the feet with long tendons. Lack of soft tissue means less need for warm blood.
Second, warm blood flowing through the birds’ arteries passes close to cold venous blood returning from the feet. As arterial blood warms up the venous blood the feet are kept cool, and the small amount of tissue in the feet is supplied with just enough warmth to avoid frostbite.
Another strategy birds use for cold weather survival is simply drop their body temperature. Allowing its core temperatures to drop will help a small bird conserve enough energy to survive cold nights or periods of little food, such as during a blizzard.
In this state, called torpor, these species are inactive and don’t respond to things around them. By the way, this has nothing to do with human teenagers.
When conditions improve, these birds are able to raise their temperatures back to a normal level. Because torpor is usually short term it is not like hibernation that some mammal species use.
Torpor is more commonly used by small birds, like black-capped chickadees.
A chickadee will weigh less than half an ounce, yet as a species survive our northern latitude winters, using a variety of methods.
Studies show chickadees survive cold nights by fattening up on seeds each day, then each night burning up half their body fat, lowering their body temperatures 18 to 24 degrees, shivering, and sleeping – balled up with their head tucked under their shoulder feathers – in a cavity or dense vegetation.
The natural world is an amazing world.