Fluffing, Shivering, and Entering Torpor
Feathers as Insulators
When it’s cold, a hummer often looks fat because it’s fluffing its feathers for extra insulation. It’s body temperature is about 105°F (40.5°C). By fluffing its feathers, the hummingbird is able to trap the warm air and protect its body from the surrounding cold air.
When fall temperatures are dropping and there is frost in the forecast, how do hummingbirds cope? At night, and when it is too cold or rainy to find food, hummers go into a sleep-like state called torpor. The bird’s body temperature drops and their heart rate slows down. This adaptation allows them to keep warm and conserve energy.
During torpor, the tiny bird’s body temperature can drop almost 50 degrees. The heart rate may slow from 500 beats per minute to fewer than 50, and breathing may briefly stop. Over the years, some concerned observers have reported seeing a hummingbird still and lifeless, hanging upside down on a branch or feeder. No need for worry, just wait and let the hummer come out of this torpor state at its own pace so that no harm is done.
There is also no need to worry about a hummer falling from a branch during torpor. Its feet get a good grip! When the hummer settles and bends its legs to perch, the stretched tendons of the lower leg flex the toes around the branch and clamp in place. This perching reflex is automatic.
In the morning, a hummingbird has to warm up its body the moment it awakes. To do this, the hummer shivers — making its muscles contract and release rapidly. When a hummingbird’s body temperature is about 68°F (20°C), it takes about an hour for it to warm up enough for normal activity. During this time, the bird’s stomach is empty and shivering is using up stored energy. The hummer is sluggish, groggy, and hungry. As soon as it can, it flies off to eat breakfast.