Walking down the forest trail, the sting of an arctic wind numbs my exposed face. With all the engineered brilliance of modern clothing, thermals, coats, electric socks, and high-tech snow boots, I am only just able to retain my body heat. I cannot help but marvel at the tenacity of bird life in these conditions.
On my walks, I regularly see chickadees, dark-eyed juncos, pine siskins, brown creepers, red-breasted nuthatches, golden-crowned kinglets, and other tiny birds flitting about the bare winter landscape.
According to Bergmann’s Rule, larger animals are better able to survive the cold because larger bodies are better at conserving heat. The smaller the animal, the larger the proportional surface area that can lose heat.
It seems unimaginable, then, that creatures as small as a golden-crowned kinglet can survive these frigid temperatures. Kinglets are about the size of a large walnut, which means the rate of heat flow from their body is more than a hundredfold more than the heat loss from a human body. The challenge of replenishing this heat loss is why many birds migrate over the winter to warmer grounds.
Every year, an estimated 3.5 billion birds migrate south to their overwintering habitats, often covering incredible distances. Rufus hummingbirds, those tiny, copper terriers of the sky, travel nearly 6,500 kilometres down to Mexico, before turning around and flying back in the spring. Hummingbirds have been known to overwinter if they have a stable food supply.
As it turns out, migratory birds are not so much trying to escape the cold as they are responding to winter food scarcity. Bird bodies are actually quite well adapted to handle the cold. In addition to bulking up with fat reserves over autumn, birds create their own micro-climates through feather insulation.
Puffed-up feathers trap pockets of warm air, helping birds maintain a steady core temperature. Keeping feathers clean, dry, and flexible is vital, because dampness transfers heat much more quickly than dry air. This is why birds dedicate so much time to preening. Some use a special oil secreted from a gland near their tail to waterproof their feathers. Others, like herons, grow special feathers that disintegrate into a waterproofing powder.
Bird legs, of course, are featherless, but since they are mostly tendons and bone, they do not freeze and shred the same way as muscle or nerve tissue. Bird legs also stay warm with heated blood from their core. To minimize the energetic cost of replacing this valuable core heat, birds have a countercurrent heat exchange system. Cold blood returning from the feet runs next to the warmed core arteries, recuperating some of this outgoing heat.
Some birds stay warm by relinquishing their territoriality in favour of shared body heat. Swallows are known to huddle in large flocks.
In extreme cold weather, small birds like chickadees must shiver all night to survive. Shivering produces heat by activating opposing muscle groups to create friction. Since shivering burns up most of their fat reserves, chickadees must spend the day foraging to replenish themselves.
Luckily, the brains of chickadees grow larger in the winter to help them remember the location of food stashes.
As spring finally approaches and the birds begin to return, I am filled with new admiration for the ones that stayed.
Naturespeak is prepared by the Whistler Naturalists. To learn more about Whistler’s natural world, go to whistlernaturalists.ca.
