While I haven’t made much progress in figuring out why birds where I live seem fewer in number this winter, and keep disappearing and reappearing for no clear reason, I did do a bit more reading on their winter survival strategies.
Bernd Heinrich, a University of Vermont professor emeritus of biology, wrote a fantastic book on animal survival in winter — “Winter World: The ingenuity of animal survival.” Heinrich not only studies and writes about nature, he spends a lot of time in the field, often discovering more on the subject. He also wrote an article on how birds survive in winter for the AllAboutBirds website. Between these two sources, he gets into a lot of the nitty gritty about birds’ winter survival adaptations and strategies.
As Heinrich points out, wild birds that spend their winters in areas where the season can be harsh, even life threatening, need to solve two problems simultaneously: maintaining an elevated body temperature (about 105°F for most birds) and finding food, which is key to internal-temperature maintenance and to other important life functions. “Although some species have devised the evolutionary equivalent of proprietary solutions,” he writes, “most birds follow a simple formula: maximize calories ingested while minimizing calories spent.” Birds have many strategies for both, depending on the species, and some are quite surprising.
To conserve energy in severe cold, birds can go into torpor for brief periods, slowing their metabolism. And some birds can divert energy from one body part to keep their core warm, as Heinrich points out. The feet of chickadees, for example, don’t freeze; their temperature “is regulated near the freezing point and may stay cold most of the time all winter, even as core body temperature stays high.” When resting, birds can also trap body heat by fluffing up their feathers, limiting the rate of heat loss. In observing golden-crowned kinglets, Heinrich found that they not only puff up their feathers but pull their feet into the inch-thick layer and tuck their head under a wing, turning the birds into tiny, fluffy balls.
Sheltering also helps birds keep warm, and in Virginia, they have a lot of options: birdhouses, grass thickets, shrubs, evergreen trees, rock crevasses and even human structures. Woodpeckers are among the few birds that make their own overnight shelters in winter, as Heinrich points out. They excavate in rotting snags rather than in the more-solid ones they use for holes in which to raise their young during the breeding season. Grouse may burrow under the snow, which insulates them and may offer protection from winged predators.
Roosting together with others of the same species is another survival strategy. Even species that do not normally flock up during the breeding season, such as bluebirds and turkey vultures, do so in winter. Heinrich found a group of golden-crowned kinglets huddled together in a pine tree. He also notes that the always-sociable crows gather by the thousands in winter to keep warm, probably sharing information about food and helping to warn off predators in the process.
Keeping the fires stoked
“While physiology is a key component of surviving the cold by temperature regulation, the more critical factor is food input,” Heinrich writes. To find food, especially food rich in the fat birds need in winter, many birds change their foraging and hunting habits and their diet. Insect eaters, for example, may not find their food flying or crawling around and start looking under bark, in the ground — or even underwater, as winter wrens demonstrate.
Golden-crowned kinglets were long thought to survive winters from Alaska to the Appalachians by eating “snow fleas” (Hypogastrura tooliki), Heinrich writes. By observing kinglets and dissecting a specimen, he found they were instead eating loads of a geometrid-moth caterpillar, Hypagyrtis unipunctata (the one-spotted variant Hodges #6654). This unassuming little brown larva feeds on a wide range of plants, including alder, willow, birch, oak and balsam fir, and is found throughout the eastern United States.
The truly remarkable thing about this caterpillar, Heinrich found, was that it survives winter frozen to a branch, in plain sight, where kinglets easily pick them off. Other lepidoptera larvae are available in pupa form throughout the winter, and some plant galls also store a rich feast of some other insect larvae in winter.
Ruffed grouse switch from eating plant material on the ground to taking flight to feed on buds high in aspen, poplar, birch and hophornbeam trees. “A grouse in the top of a tree can pick enough buds in about 15 minutes to support its overnight needs,” Heinrich reports, thanks to a large crop (“a pouchlike extension of the esophagus where food can be stored”) that few other winter birds have. He adds that ravens, which hunt pretty much anything they can bring down, may take to scavenging more often on large animal carcasses and caching food in winter.
Forming groups in winter not only helps keep some birds warm, it also offers advantages in finding food. Chickadees, for example, use trial and error when foraging for food, and learn from each other where and what to eat, Heinrich writes. When one finds food, “its neighbors notice and join in.” For some birds, feeders put out in winter can boost their chance of survival. Studies show that some species get up to about 25 percent of their food from feeders, finding the rest in the wild (see sidebar online at rappnews/wildideas).
These are just some of the strategies birds use, depending on the species, to endure winter. Not being a fan of cold and dark, I could use a few more myself, beyond a heated mattress pad and lots of hot Ovaltine.
© 2019 Pam Owen
The idea of feeding wild birds, which dates back many centuries, has been growing in popularity in the United States. In 2016, for the 81.1 million Americans 16 and older who reported watching wildlife around their homes, the most popular activity was feeding birds and other wildlife (73 percent), according to the U.S. Fish and Wildlife Service’s 2016 National Survey of Fishing, Hunting, and Wildlife-Associated Recreation. That year, Americans spent more than $4 billion on wild-bird food and close to $1 million on nest boxes, bird houses, feeders and baths.
Small bird species, particularly, benefit from food in feeders, as a team of Stanford researchers led by biologist Paul R. Ehrlich reported in 1988: “Small species, which are more constrained energetically, benefit greatly from feeding. In one experiment, chickadees raised their daily fat deposits by about 4 percent of their body weight when offered sunflower seeds in place of their normal diet of conifer seeds, berries, etc. During extreme cold spells, juncos, finches, and other winter residents unable to find sufficient food before sunset often will not survive the night.”
Some hummingbirds, particularly the rufous hummingbird —first reported in Virginia a few decades ago — have extended their winter range through taking advantage of nectar feeders maintained by some bird lovers through the winter. The Stanford team also found that feeders do have their drawbacks, including helping to spread disease (which can be ameliorated by regular cleaning) and attracting predators.