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ALWAYS IN SEASON/ MIKE JACOBS: Chickadees survive the cold to sing another day

Illustration by Mike Jacobs1 / 2
Mike Jacobs2 / 2

The black-capped chickadee is a year-round resident here — and farther north — so it makes a good study in how birds survive the cold. Birds are remarkably well adapted to withstand cold. Some of these adaptations are physical, some are behavioral, and some demonstrate brain power.

Each species has its own adaptations, and the black-capped chickadee has it harder than some. Yet the chickadee has met the challenges of cold weather survival.

Like other birds, the chickadee depends on its feather coating to help retain heat. Feathers are excellent insulation, and chickadees make full use of them. By fluffing the feathers, chickadees create pockets that hold heat against their skin. That explains the roly-poly look of the birds on cold winter mornings. Chickadees are able to constrict the flow of blood to uninsulated areas of the body, such as feet and bills.

Other tactics are at work, too. Chickadees, like other birds, eat voraciously during winter months in order to keep stoking the internal furnace that keeps them warm. This is easier for birds that feed mostly, or exclusively, on seeds, which are high in oil that their bodies convert to heat. Chickadees are foragers, however, seeking at least some animal protein, perhaps as much as a third to a half of their diet, even in winter. Foraging requires more energy, so chickadees are at a disadvantage.

Chickadees overcome this in a variety of ways. Perhaps the most important is shivering, which chickadees do to raise their metabolic rate and generate heat. They also are able to slow down their metabolism, lowering their body temperature — essentially turning down the thermostat to save fuel. It's a dangerous trick for a small bird, but likely worth the risk if danger from cold is greater than danger from predators.

Some northern birds, including redpolls, use snow for insulation, and chickadees may do that, as well, but their usual overnight strategy is to seek shelter in tight quarters. This means heat escaping from the body is trapped in a small space, meaning less heat is lost and less energy consumed.

Chickadees are cavity nesters, but they don't use the same holes for nesting as they do for winter shelter. Nesting holes must be larger, to accommodate eggs and chicks, and the coming and going of adult birds. A chickadee seeking shelter from the cold looks for smaller openings, just big enough to get into, and so less than an inch and a half across.

Birds of some species huddle together, effectively sharing heat. Chickadees may do this, too, but most observations of chickadees in winter suggest they don't share space. Essentially, they are on their own. There's probably survival value in this behavior. Birds roosting together might be more vulnerable to freezing or predation.

These physiological adaptations are important to chickadees, and so are behaviors. Chickadees are sensitive to wind chill; on especially windy days, they forage closer to the ground, where wind speeds typically are less. Another important behavioral adaptation is "scatter storage." Chickadees, like blue jays, are known to store food, and they remember where they put it. Often, they store food in their overnight shelters or nearby, allowing a cold and hungry chickadee ready access to emergency rations.

Still, chickadees leave the shelter both lean and hungry. Studies have shown that the birds have to double their fat reserves during the day. This requires almost constant eating, especially in midwinter when days are short, because chickadees don't forage after dark.

Chickadees have hit upon a survival strategy suited to an ecosystem that most birds abandon during winter, so chickadees have access to the food that's available without serious competition. This is true even in backyards, because chickadees are less fussy than most winter birds, willing to take seeds as well as suet.

This makes them less vulnerable, overall, to shortages. Seed eaters, such as redpolls, have the advantage in that seeds have more intrinsic heat value, but they are an exhaustible food source. A flock of redpolls without access to a seed supply would be in mortal danger. This accounts for the great variation in redpoll numbers from year to year. When food supplies fail elsewhere, redpolls are forced to seek sustenance elsewhere, and in about three years out of five, that means they irrupt southward.

Chickadees, on the other hand, are pretty much place bound. The only exception is young birds looking for places to establish themselves. Overall, this means a smaller number of chickadees in any one location — but that, too, is a survival strategy.

Chickadees will provide proof of their survival skills soon enough. They begin advertising territory — a prelude to courtship — on bright still days in February. Their "phee bee" call is a loud, familiar and welcome sign of spring.