As long as you can find enough food to stay active, your body can generate enough heat to stay alive. But you can only eat when it's light. What happens when you go to sleep at night? How can such a tiny warm-blooded animal keep up its body temperature without running out of energy? This experiment can help you figure it out:
Try This! Experiment
You will need a small piece of modeling clay, a balance, a thermometer, and a refrigerator.
Your experiment should have shown that hummingbirds lose their body heat a lot faster than Blue Jays and other large birds. If they needed to keep their nighttime body temperature at even 90 degrees, over 10 degrees cooler than it usually is, they'd have to waste a lot of food energy and could starve overnight!
If you needed to save energy in your house, you could turn the thermostat down at nighttime when people were sleeping. And hummingbirds do the same thing, thanks to a special adaptation. They don't have a thermostat, but they can allow their body temperature to drop sometimes more than 35 degrees F (20 degrees C) at night, from a daytime temperature of over 100 degrees F (over 37 degrees C)all the way down to 60 or 65 degrees F, so they won't need to burn nearly as many food calories to stay alive during the many hours they can't eat. This temporary drop in body temperature is called torpor. Humans normally die if our body temperature drops that low, because we are not adapted for torpor.
Dropping its temperature slows down a hummingbird's breathing and heart rate. One Blue-throated Hummingbird whose daytime, active heart beat 1260 times per minute had a heart rate when torpid all the way down at 36 beats per minute!
The trickiest part of torpor isn't letting the body temperature fall at night, but getting it back up in the morning. A hummingbird has to warm up its body the moment it awakes. To do this, it makes its muscles contract and release very fast; it shivers! The problem is this: the hummer's stomach is empty and it must use up stored energy. When a hummingbird's body temperature is about 20 degrees C, it takes about an hour for it to warm up enough for normal activity. During this time, the bird is sluggish, groggy, and very hungry! As soon as it can, it flies off to eat breakfast.
Try This! Journaling Questions