The smallest birds on Earth, hummingbirds may seem frail. Turns out these airborne jewels are as tough as diamonds. In cold weather, hummingbird bodies enter into an 'energy-conservation mode' called torpor.
Birds that stay north for the winter experience a nightly 'mini-hibernation,' in which their 107-degree body temperatures can plummet to 48 degrees. Their heart rate drops from 1,260 beats per minute to 50 to 180 beats.