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By Megan Wilde

It’s a cloudy autumn morning. The trees are shrouded with ice, and the air stings with frost. But there, in this suddenly wintery world, you see a flicker of magenta and emerald green, as a hummingbird buzzes to your feeder. How does this hummingbird not freeze to death in such cold? And with winter almost here, shouldn’t hummingbirds be heading south?

Our region’s hummingbirds typically visit from March into October, but some species linger through the winter. Most of these overwintering hummingbirds aren’t accidental tourists, blown off course or too weak to migrate. Though they may look delicate, they’re actually well-adapted to extreme cold.

Consider the fact that some of our hummingbirds migrate through the high Rocky Mountains, where freezing summer nights aren’t uncommon. Our relatively mild winters are a cakewalk compared to a frigid night at 12,000 feet.

Kelly Bryan is an ornithologist in Fort Davis, Texas who studies these tiny migrants.

Bryan: As far as when it freezes at night, these birds are used to that. They just go into a deep sleep called a torpor. Kind of a metabolic shut down. Their heart rate goes from 1200 beats a minute to 50 beats a minute, which is for a human pretty fast.

In this coma-like state, a hummingbird’s body temperature plummets. Its breathing also becomes irregular, with five minutes sometimes passing between breaths. This slumber is so deep, hummingbirds sometimes fall off perches and lay motionless on the ground until they rouse. In fact, being so unresponsive is torpor’s downside. Torpid hummingbirds become vulnerable to predation and changes in the environment.

Despite these costs, torpor isn’t just reserved for freezing weather. Hummingbirds’ hyperactive lifestyle requires loads of energy. And so does maintaining the temperature of a body that weighs less than a nickel. Hummingbirds get this energy by gorging on two to three times their weight in insects and nectar every day. But they can’t sip nectar or gobble up insects while they sleep, and sometimes these foods are scarce. Torpor solves these dilemmas by lowering their fuel needs until they’re able to resume their day-time feast.

Bryan: Even if it gets down to 10 degrees, it doesn’t affect these birds whatsoever. As long as they have a food source the next morning available to them.

That’s why it’s critical to leave up your hummingbird feeders, even in fall and winter. After a freezing night, hummingbirds need nectar. If they’re used to dining at your feeder, it’s the first place they’ll look to refuel. And often it’s the only place, especially after a deep freeze has zapped any nearby flowers.

Equally important is keeping your feeder unfrozen. Sugar water won’t freeze unless the temperature dips below 27 degrees. But on colder nights, you can bring feeders inside and replace them at dawn. Some people warm their feeder by wrapping it in a heating pad or placing a heat lamp nearby.

Experts recommend maintaining at least one feeder until its patrons disappear, or even better, year-round. Don’t worry about interfering with migration. Bryan has found historical reports of hummingbirds overwintering in West Texas from long before plastic feeders became common in the 1980s.

Bryan: Birds are going to do what they want to do. They might stay for awhile. But if they want to migrate, they’re going to migrate.

That instinct is more deeply ingrained than the lure of an easy sugar-water fix. But after a freezing night, an offering of warm sugar water is the perfect way to give thanks to hummingbirds for bringing vibrancy and color to a gray winter world.

Have a question or comment about this episode? Contact Nature Notes Coordinator Megan Wilde at mwilde [at] cdri [dot] org. Or discuss this episode on Nature Notes’ Facebook page. This episode originally aired on November 25, 2010.

References & Resources for Educators

• West Texas Hummingbirds: Kelly Bryan’s research and guide to the region’s hummingbirds
• “Hummingbird Metabolism” from Journey North, including torpor experiment and math questions
• “The Physiology of Hummingbird Torpor” by students in Davidson College’s Animal Physiology Class
• Hummingbirds of Texas: with Their New Mexico and Arizona Ranges by Clifford Shackelford, Madge Lindsay and Mark Klym (TAMU Press, February 2009)

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