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Resourceful chickadees triumph over winter
From The Burlington Free Press, Jan. 31, 2002
By Nancy Bazilchuk
Vermont's furred and feathered inhabitants have made an art of escaping or avoiding winter's cold. Birds fly south. Chipmunks and woodchucks hibernate. Bears sleep. Mice and voles create vast warrens and tunnels under the snow. Bats hide themselves in caves. And we huddle around our woodstoves, drinking hot chocolate and watching the worst nature can deliver through our ultra-thermal, triple-paned glass windows.
Then there's the black-capped chickadee. How does a bird so small you could mail two of them first-class with a 34 cent stamp survive winter's dark days and icy blasts?
The answer turns out to be far more complicated than you might think. Chickadees are smart about finding and storing food, and their bodies have adapted so they can endure the cold, dark days of winter. Think about what a chickadee is up against.
This bird feeds in the summer almost exclusively on insects, caterpillars, spiders and other bugs. In the winter, however, it has to eat whatever it finds. That means chickadees eat everything.
Some researchers have found chickadees pecking the fat off a dead deer's body. They'll monopolize suet feeders, clean out your bird feeder, and strip hemlock cones. They feed on maple sap icicles -- the ones that form on the end of a snapped sugar maple twig in spring. The average chickadee needs to eat food that's comparable in energy value to 250 sunflower seeds a day.
Given that grocery list, it shouldn't surprise you that chickadees are good scavengers. It turns out they are also very good at caching and relocating food. One researcher found that chickadees could locate a cache of high-energy food 28 days after the birds had hidden it. Another researcher found that chickadees will remember which caches have the fattest, highest-energy foods and consistently raid those caches first.
They're also clever about hiding their caches, these little black-capped critters. Rather than create one big storehouse that would be easy to remember, they scatter their caches around their territories. They're also very industrious. One Norwegian research studying a chickadee relative found that this bird, the tit, cached between 50,000 and 80,000 spruce seeds in a single autumn.
n the farthest reaches of their range, in Alaska and throughout northern Canada, right up to the tip of Ungava Bay, chickadees must also contend with short days right around winter solstice.
Unlike some birds, chickadees don't hunt for food at night. Researchers have found that chickadees get up earlier than other birds to take advantage of pre-dawn light to extend their foraging time. The early bird gets the black-oil sunflower seed, I guess.
No matter how hard a chickadee works during the short winter days, it can't avoid this fact: The shortest winter days mean as much as a 20-hour fast for the birds. That's where the range of physical adaptations chickadees have evolved to cope with the cold come into play.
The most amazing adaptation is one that is least understood: Chickadees can lower their body temperatures at night and enter a state of hypothermia. Their normal body temperature runs at about 108 degrees. On cold nights they can drop their body temperatures to 85 degrees. The ability to cool their core temperatures cuts a chickadee's rate of fat consumption by about 25 percent. That's a lot of sunflower seeds.
Humans would be hard pressed to survive that low a body temperature, even though our normal body temperature is fully 10 degrees cooler than a chickadee's.
What's even more amazing about this change is that chickadees can fly to avoid predators, even if they're in this hypothermic state.
A lower body temperature is only part of the picture. Chickadees have also evolved so they replace their feathers at the end of the summer. When winter comes, their plumage is relatively new.
This makes more of a difference than you might think. When researchers looked at the difference in weight between worn summer plumage and fresh fall plumage for a number of bird species, they found the new plumage weighed about 25 percent more than the old. Chickadees also have remarkable control over their feathers and can fluff them to increase insulation. Plus, their feathers are dense compared to birds of similar size.
This presents a sort of chicken-and-egg evolutionary question, if you will. The more a bird weighs relative to its size and wingspan, the more difficult it is for that bird to fly long distances. Long ago, chickadees clearly made the evolutionary tradeoff -- a decreased ability to migrate, but an increased ability to survive the cold of winter. But which came first?
What's most surprising to me about all this is that chickadees do not roost together.
Each goes off its own hidey-hole at dusk to shiver through the night alone. They do typically choose roosts that are so small they can warm the space with the heat lost from their bodies. But still, there's warmth in numbers -- just look at all those big family beds in centuries past. There must be some evolutionary advantage to not sharing a roost, but I can't think of what it is.
As the chickadees remind you they're still here with their late winter and spring territorial call '' fee-bee'' just think what it takes for them to survive each winter day.