Recently, we focused on avian metabolism and thermoregulation, but some birds have additional mechanisms to cope with more extreme, colder temperatures. Most commonly, birds migrate south during the winter to seek warmer environments with more food. We can easily observe this phenomenon in the Keys; during the fall, thousands of migratory birds ranging from tiny warblers to large raptors fly overhead on their way to warmer points further south.
The main method of collecting data regarding these feats is setting up at a point with high visibility and counting the birds that fly by during bird watches. Technological advances have allowed us to learn more about these journeys. Just this past year, using tiny data logging devices, scientists discovered that the common swift, a small (35-56g) bird with a deeply-forked tail and sharp, sickle shaped wings, spends up to ten months continuously in the air. During their non-breeding season, including a migration from Europe to sub—Saharan Africa and back, some individuals were recorded to be airborne for the entire 10-month flight, and the majority were airborne for over 99% of that time (i). These birds literally spend their life on the wing; mating, foraging for food and nesting material, and even sleeping while in the air.
Other types of seasonal migration occur as well. For instance, spectacled eiders congregate around boggy tundra along the coasts of Alaska and Siberia in the summer, and then move to the edges of ice packs at the southern edge of the Bering Sea in the winter. This sea duck only weighs around three pounds, yet survives with its legs and lower body submerged in 29°F water all day, air temperatures around the teens, and winds ranging from 35-60 mph. The counter-current exchange between arteries and veins in their legs, as well as their down feathers, minimizes overall heat loss. Though there are several breeding populations, they all spend the winter in an area below the island of St. Lawrence in the Bering Sea. Though cold and barren, this area also provides shelter from large waves and most predators (ii). With its physiological adaptations, short migration, and grouping behavior, these sea ducks survive seriously cold winters.
Some birds do not migrate at all, and instead tough it out through the winter. The black-capped chickadee, a small songbird with an average weight of ~11g (~ 4 pennies), does not migrate, and instead overwinters in areas as far north as Fairbanks, Alaska. Fairbanks has 18-hour winter nights, with temperatures averaging -16°F and as low as -40°F. During the short 6 hours of daylight, these small birds stuff themselves full of frozen insects and seeds to gain 10% of their body weight in fat (iii). Using tiny radio transmitters, scientists found that chickadees roost individually in birch tree holes, puff up their feathers, lower their body temperature by 15°F, and burn up fat reserves all through the night to survive (iv). These chickadees feed on caches, food stored in hundreds of locations during the autumn, and their incredible memory is a result of the hippocampus, a part of the brain responsible for spatial organization and memory, growing by as much as 30% with the addition of new nerve cells (v). Although chickadees are smaller than both a swift and a sea duck, these songbirds manage to stay alive through harsher winters.
Although we have to rely on warm layers of clothing and heaters to survive cold weather (or permanently move to warmer locations further south), a variety of birds have amazing adaptations to cope with the winter. Some other species of interest that you can look up include the gray-crowned rosy finch and the rough-legged hawks.
Elsa Chen, Sanctuary Intern, 2017
(i) Current Biology: “… Common Swift Apus apus.”
(ii) National Wildlife Federation: Finding Comfort in the Cold
(iii) Alaska Department of Fish and Game: Alaska’s Backyard Birds
(iv) National Wildlife Federation: How Chickadees Weather Winter
(v) Lehigh University: …This Chickadee’s Brain Begins to Expand.