Eats Like Godzilla, Sleeps Like the Mummy!

A flash of scarlet and emerald zooms past me as I poked my sleepy head out of the kitchen door, a vibrant splash of holiday cheer against the sullen winter sky. Suddenly, an indignant Anna's Hummingbird confronts me, beak-to-nose, demanding his breakfast. Shivering, I retreat quickly into the kitchen to prepare warm sugar water.

A hungry female Anna's Hummingbird prepares to sip nectar from a feeder. [Cliff Drake photo.]

Equivalent to the average human consuming an entire refrigerator full of food, hummingbirds eat roughly twice their own body weight of food each day to meet their high metabolic requirements. Hummingbirds, among the smallest of all warm-blooded animals, lack the insulating downy feathers that are typical for many other bird species. Due to their small body size and lack of insulation, hummingbirds rapidly lose body heat to their surroundings. Even sleeping hummingbirds have huge energy demands that must be met simply to survive because they cannot forage during the night.

So, how can such diminutive birds survive the long cold winter nights in Seattle without eating constantly? To save energy, hummingbirds lower their internal thermostat at night, becoming hypothermic. Their night time body temperatures are maintained at a point, called a set point, that is far below what is normal during the day.

"If you try to cool an animal down below this new set point, it will generate enough heat to maintain the set point," says Sara Hiebert, hummingbird expert and associate professor of biology at Swarthmore College in Swarthmore, Pennsylvania. This physiological phenomenon is called torpor. There are several types of torpor, classified mostly by duration and season. For example, when torpor takes place for long periods of time during the winter, it is known as hibernation. Hummingbird torpor can occur on any night of the year so it is called daily torpor or noctivation. Even tropical hummingbirds have rigid metabolic budgets so they commonly use daily torpor to conserve energy, too.

Torpid hummingbirds exhibit a slumber that is as deep as death. In 1832, Alexander Wilson first described hummingbird torpor in his book, American Ornithology. He said, "No motion of the lungs could be perceived ... the eyes were shut, and, when touched by the finger, [the bird] gave no signs of life or motion."

Awakening from torpor takes 20 minutes or more. During arousal, the hummingbird's body can warm up by several degrees each minute and the bird awakens with enough energy reserves to see him through to his next feeding bout. Interestingly, hummingbirds reliably arouse from torpor one or two hours before dawn without any discernible cues from the environment. So, it appears that the circadian clock triggers arousal.

What are hummingbirds doing during those pre-dawn hours when they are warm but not yet active? "One suggestion is that they might be using this time to sleep," explains Hiebert. "Although there is some evidence that torpor is an extension of slow-wave sleep, there is also evidence that the body is too cold during torpor for the normal functions of sleep to occur."

[Cliff Drake photo.]

Torpor is not limited to hummingbirds; it has also been observed in swallows, swifts and poorwills. Additionally, scientists think that most small birds living in cold regions, particularly chickadees, rely on torpor to survive long cold nights. Even though rodents, bats and other small mammals show some form of regulated hypothermia when it is cold, they can only rely upon daily torpor during the winter when they are not breeding. In contrast, noctivation is possible on any night of the year for hummingbirds. Because daily energy balance is progressively more difficult to maintain as body size decreases, hummingbird torpor is finely tuned to preserve their daily metabolic budget.

"Hummingbirds are the 'champions' of this kind of energy regulation because they have to be," concludes Hiebert.