How Hummingbirds Survive Our Cold Winter Nights
The challenges facing a wintering hummingbird are many: fewer food resources, longer nights and colder temperatures. Rufous Hummingbirds solve these problems by migrating south. Their larger cousins, the Anna’s, employ different strategies, allowing them to survive as far north as southern British Columbia.
In addition to nectar in feeders, Anna’s also need protein in the form of small insects sheltering among trees and shrubs. Warmth leaking from human habitations often supports whiteflies and other hummingbird prey.
Anna’s must overcome the obstacle of long, cold nights. It accomplishes this by slipping into a hypothermic physiological state known as torpor, a sort of “mini hibernation.” This adaptation allows the bird to save energy by lowering body temperature from about 104 degrees Fahrenheit to just under 50 degrees. Respiration rate drops from 245 breaths per minute to less than 10. The metabolic rage (energy expenditure) is about 300 times lower than when the bird is executing its remarkable flight.
It takes approximately 20 minutes for an Anna’s to shake off torpor. Heart and breathing rates increase; wing muscles begin to vibrate. Shivering increases body temperature a few degrees a minute. The Anna’s is now ready to face the day.
Continuing to feed wintering hummingbirds is not harmful. Otherwise, the only sugars available to them come in the form of sap leaking from tree trunk wells cut by Red-breasted Sapsuckers.
Seattle Audubon offers tips to help hummingbirds during particularly difficult cold snaps to combat the freeze. For instance, you can rotate two feeders to keep fluids moving or even string Christmas lights around the feeder to provide ambient heat.
Portland Audubon Society advises leaving a hummer found in torpor as the bird knows what’s best and moving it might be harmful.
Wintering hummingbirds are on the verge of perishing each night. Why not migrate? The reward comes from being ready to defend good foraging territories when the breeding season gets started, instead of making a difficult trip south and back again, finding multiple meals along the way.
- The guest author of this blog post is Steve Ellis, a noted Coupeville area naturalist and past president of Whidbey Audubon Society. Steve and his wife Martha have been leading walks and giving talks on a wide range of natural history topics for more than 20 years.