We had a cold snap recently in the Pacific Northwest. It snowed a foot and the temp was subfreezing for a few days. This is nothing compared to many parts of the world where it’s below freezing for months at a time and snow or ice carpets the ground. But such times are tough for birds, short or long, and watching my local birds got me to thinking about how they manage.
The basic problem for birds is that they are warm-blooded. They require plenty of regular fuel to keep warm and to support their fast metabolism. Birds have both physiological and behavioral strategies to deal with the challenges of cold. A good example combining those is the puffed up birds we see so often at this time of year. Layered feathers and down trap warmed air.
You might look at that fluffed up junco and wonder about his feet, which have no warm covering and are in contact with a frozen branch. Ducks and other aquatic birds who dangle their legs in cold water have the same problem, as do birds like swans walking around on snow or ice.
But bird feet have several advantages over ours. They are covered with hard scales that slow heat loss. Birds also have a “countercurrent heat exchange system” in which their leg veins are in close contact with arteries allowing warmth to transfer between them. Cold blood returning from the feet gets warmed, and warm blood cools before reaching the feet and doesn’t get wasted, lost to ice or cold water. A third reason cold feet aren’t as dangerous for birds as for ours is because there’s very little muscle or nerve tissue in bird feet, mostly bones and tendons which are less vulnerable to cold. So birds can have exposed feet in freezing temperatures without much problem.
They can also lie down or tuck a leg under a wing to help conserve warmth.
Birds will tuck their head under their feathers for warmth too. Often the birds I see with tucked heads have their eyes open, but when they are in a flock they can really burrow under. Somebody is on the lookout for predators.
Harlequin ducks tend to gather in flocks in winter, rather than the pairs or handful I see in summer. This difference in flocking behavior is common among birds in winter, for several different reasons according to Bernd Heinrich, a biologist/professor who lives in Maine. Heinrich has written many books about nature, based on his observations and field work, including a few focused on winter survival. He posits this shift in behavior helps bird survival as lessening predation (many eyes), improving communication about food resources (in a season when short days requires maximizing foraging efficiency) and in conserving body warmth by huddling together. He tells of birds who live on the knife edge of survival for months of subfreezing weather, who wouldn’t last til morning otherwise. In an article for Cornell’s site, Heinrich describes what he’s seen among Golden-crowned Kinglets on -40°C Maine nights:
On one evening I saw four kinglets disappear into a pine tree. Later that night, with extreme caution and armed with a flashlight, I climbed the tree and spied a four-pack of Golden-crowned Kinglets huddled together into one bunch, heads in and tails out, on a twig. One briefly stuck its head out of the bunch, and quickly retracted it—indicating it was staying warm, and not in cold torpor.
- Bernd Heinrich
Some of the birds I see flocking more in winter than summer are oystercatchers, ducks, robins, blackbirds, juncos. You probably have your own set of winter flockers.
Finding an adequate food supply in winter is more difficult, so it pays to pool your efforts. There are fewer foraging hours with short day lengths. Food also tends to be more hidden: under snow or ice, burrowed under bark or underground, covered by winter daytime high tides. Birds need to use all their versatility and experience to negotiate changing conditions on the ground and from the weather.
Larger birds like ducks and turkeys lose heat less readily than smaller birds like kinglets and hummingbirds because they have less surface area exposed to cold air relative to their volume. As a consequence they don’t need to eat as much, relative to their body size, since they don’t have to generate as much heat. The metabolic rate of small animals is faster. Food becomes a critical challenge for birds, especially the smaller ones.
Seasonally speaking, birds tend to eat voraciously after breeding season through fall building up fat reserves for migration or winter cold. The amazing Bar-tailed Godwit puts on enough weight before migration to enable it to fly 7,000 miles in 8 days, losing half its body weight. When I see migrating shorebirds stop on the beaches here they are eating constantly all day, replenishing energy for the next leg of their journey. I try to explain that to beach walkers who let their dogs loose to chase the birds: they are likely sentencing the shorebirds to death since not only is their foraging interrupted but they are wasting precious energy flying away.
For birds who stay in cold climates over the winter, when the various physical and behavioral adaptations aren’t quite enough, there’s an extreme strategy some kinds of small birds use to get through cold winter nights: torpor. Hummingbirds and chickadees are among the birds around here who reduce their nighttime energy expenditure this way. Hummingbirds will latch onto a branch at dusk, then allow their temperature to drop as much as 35°F and their heart rate to slow from ~1300 beats/minute to ~36 beats! Like suspended animation. Before it gets light, birds will come out of torpor by shivering. They’ve used up their fat reserves from the previous day and need to fuel up immediately, which is why it’s critical to keep feeders available in freezing weather once your local birds are accustomed to feeding there.
There are other ways we can help birds deal with the challenges of cold. Providing high-energy food like seeds, nuts and suet is much appreciated. My black-oil seed feeder is always busy in winter but over the cold snap it seemed like everybody in the neighborhood came, including a Varied Thrush! On and under the feeder I counted juncos, house finches, spotted towhees, golden-crowned, fox, song and white crowned sparrows, redwinged blackbirds, starlings, downy and hairy woodpeckers, flickers, chestnut backed chickadees, redbreasted nuthatches, and collared doves on those cold days.
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I use an upside down suet feeder to cut down on mobbing by starlings and blackbirds. Everybody can manage this feeder, including flickers.
Another way to help out birds in our yards is to plant shrubs and trees that bear fruit into winter, and that provide shelter out of the weather. Trees near the feeders are staging spots and give some protection from predators swooping in.
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And one more way to help out our winter birds — provide water! Birds can eat snow for their water but it uses precious body heat to melt it. A heated birdbath doesn’t really warm the water but keeping it above freezing makes a big difference. We use a heated dog dish with a rock for birds to perch on, and it gets lots of visitors.
The weather has warmed up now in the maritime Pacific Northwest, but we could get another cold snap any time. The birds who spend the winter here have a variety of adaptations to deal with cold and snow and wind, but even so, survival at this season is marginal. Given how many ways humans make life difficult for wildlife in general, it’s good to be able to help out our local birds a bit if we can.
What’s it like for the birds who spend the winter in your area? How do they manage the challenges of cold weather?
Dawn Chorus is now open for your birdy reports of the week.