For the birds - Valiant chickadees
By Donald Attwood
PHOTO COURTESY SHUTTERSTOCK
Their small size puts chickadees at a disadvantage in winter but these plucky birds have ways of coping with extreme cold.
Black-capped chickadees are ordinary birds; we see them every day around the neighbourhood. But in fact, they’re quite extraordinary. I could say much about their curiosity, their prodigious memories, their communication skills, and their intelligence – all of which have been well studied. At this moment, however, I want to pay tribute to their remarkable fortitude in the face of harsh weather.
On the bitterest winter days, if any birds at all are about, they include chickadees. Few of us have slept outdoors in winter, but chickadees do so all the time. If I were to make the attempt, at least my size would be in my favour. Not so for stout-hearted chickadees. As with any small creature, their ratio of surface area to volume is high, so they lose heat much faster than I do. (A large mug of tea stays warm longer than a small one.) In winter, all creatures face potential harm due to heat loss, and chickadees approach the minimum size limit for warm-blooded animals. (The smallest bird to winter over in this general region is probably the Golden-crowned kinglet, which averages 10 cm from tip of bill to tip of tail, or 3 cm less than the chickadee. A bit smaller than the kinglet is the Ruby-throated hummingbird, which sensibly winters in southern Mexico and Central America.)
Migration aside, birds adapt to winter in two general ways. First, feathers provide excellent insulation, and in cold weather, birds fluff out their feathers. Chickadees look fatter in winter, but they are simply fluffier. Second, birds must eat often to stay warm and active; their high metabolic rates produce lots of energy (for flight) and demand lots of energy (from food). But in winter, many foods, such as insects, fruits, and seeds, are scarce. Intrepid chickadees cope by storing food during warmer months, hiding it in bark crevices and other such places. One chickadee can store up to 1,000 seeds per day, as many as 80,000 in a season. Months later, it remembers where all these items were stashed. According to David Sibley (in What It’s Like to be a Bird), “The hippocampus – the part of the brain involved in spatial memory – is larger in birds that live in colder climates, where storing food is more important; it grows larger in the fall to accommodate multiple storage locations, and then shrinks again in the spring.”
Like most northern birds, doughty chickadees endure the cold with bare legs and feet. Their feet get very cold, almost freezing, but remain flexible and functional. If a chickadee’s feet were kept as warm as its body core, the feet would radiate heat so fast that the bird could perish of hypothermia. So its feet get cold instead. This is made possible by a process known as countercurrent heat exchange: in their legs, warm arteries are meshed closely with veins. The result is that blood flowing away from the body gives up heat to blood returning into the body – so chilly blood circulates to the feet, and heat loss to the outside air is minimized.
Nighttime is the crucial test for winter survival. Most birds cannot forage in darkness, so they cannot replenish their energy then. Dauntless chickadees reduce their heat loss at night by taking shelter in tree holes – just one among countless examples of how dead and moribund trees provide essential services to the living community. Even in their tree holes, stalwart chickadees may shiver through the night, burning up most of their fat reserves – “which then must be replenished the next day in order to survive the next night.” (These words are from Bernd Heinrich, a biologist living in Vermont who has published several books on wildlife in this part of the continent; his Winter World explores how various creatures – birds, mammals, reptiles, and insects – survive our brutal cold season.)
Plucky chickadees have an inquisitive temperament. Their curiosity helps them find thousands of locations for storing food for the winter ahead. Chickadees forage in small groups, so others benefit when one curious chickadee locates a new food source. Curiosity also helps them adapt to human intrusions. I have not yet persuaded the chickadees in my backyard to eat from my hand, but there are courageous chickadees at certain spots in the Morgan Arboretum that approach people for handouts. Put some sunflower seeds in your pocket the next time you go hiking or skiing there.
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