Thanks to what was described as a “drunken” meander of the jet stream last week, much of North America experienced some the coldest weather seen in many years. At Voyageurs National Park, the temperature dipped to -42°F the morning of January 3rd , followed by daily highs of -17°F and -19°F on the 5th and 6th. That’s deadly cold to an improperly prepared human. It’s not all that fun even for cold-adapted mammals like moose.
What happens to tiny bird like the Brown Creeper? A moose weighs 800 pounds, give or take a couple hundred pounds. Brown Creepers, at 8.5 grams, can’t even tip the balance against a stack of four pennies. How can an animal that tiny survive such cold?
The unfortunate reality is that many do not. Uncommonly cold events like this month’s take their toll on bird populations, particularly on weaker individuals. Still, the majority of the birds survive, which seems nothing short of miraculous.
A tiny bird like the Brown Creeper has a suite of adaptations in its bag of tricks when it comes to surviving frigid temperatures. The most obvious protection a bird has is its down jacket. Feathers provide phenomenal insulation, particularly in the thick fall layers northern birds add in the fall. Many birds will perch with their outer feathers fluffed up to trap a layer of warm air against the body. Unfeathered parts of the body, like the legs, are covered with scales that provide some insulation, but the bird’s best trick to keep the feet warm is that it doesn’t bother trying. Blood flow through the feet can be greatly reduced, essentially allowing the feet to almost freeze without this chilling the rest of the body. That’s how ducks can stand on ice for days at a time without the aid of boots.
There’s warmth in numbers, and small birds in particular take advantage of this. Creepers and chickadees are often found at night in communal roosts, with birds pressed tightly together in a confined space to conserve heat.
The smallest bird to take advantage of communal roosting for warmth without relying on any metabolic tricks is the Golden-crowned Kinglet. At four inches long and weighing a hair more than a nickel, kinglets survive northern winter nights in groups. What kinglets apparently can do, but commonly chose not to, is enter a state of torpor on frigid nights. In a torpid state, the metabolism is greatly reduced and the bird essentially shuts down many of its functions, akin to hibernation, but lasting only overnight.
In torpor, the body temperature can be safely reduced by as much as 50 degrees. Brown Creepers can be found clinging in roosts of torpid birds in a deep cleft of bark just before sunrise. Chickadees perform the same behavior, though generally in more inaccessible cavities.
No one is certain why kinglets often choose to forgo torpor. It is a dangerous practice, in that a torpid bird has very little ability to escape a predator, should its roost be discovered. A group of torpid chickadees would be a slow-moving smorgasbord for a mink. Shivering, but fully functional kinglets would be much harder to catch.
Perhaps the adaptation to cold that birds are best known for is the good sense to just get out of town. Millions of birds take off for sunny southern homes every fall to eliminate the need to worry about cold. If that was a fool-proof plan, every bird would do it, but migration comes with a whole host of its own dangers. Flying across the continent twice a year is dangerous business, and some birds would just rather stick it out in Minnesota.
So, the next time the mercury drops well below zero, think about the kinglets, chickadees, and creepers before you get too smug about your personal tolerance for cold! Personally, I’m starting to think that migration thing isn’t such a bad idea.