It's that time of year again when billions of birds fly thousands of miles from breeding grounds in the north to warm winter homes in the south.
A new study helps explain how even small birds manage such impressive journeys twice a year, year after year. When they stop to refuel during their trips, the study found, some birds drop their nighttime body temperatures by as much as 10 degrees Celsius.
By spending less energy on staying warm through the night, these birds are able to put up to 30 percent more energy towards fat storage for the flight ahead.
The study adds hypothermia to the list of strategies that birds use to complete their awe-inspiring migrations.
"It's another answer to the question of how they deal with their environments and how they deal with their energy requirements," said Micha Wojciechowski, a physiological ecologist at Nicolaus Copernicus University in Torun, Poland. "The physiological changes that happen and strategies these birds have are unbelievable."
Most warm-blooded animals, including humans, experience a dip in body temperature overnight, usually by just a degree or two. Some animals, including bats, rodents and chickadees, can send their nighttime temperatures even lower than that. They usually do it to save energy when the weather gets really cold or when energy supplies run low.
Wojciechowski and colleague Berry Pinshow wanted to know if going hypothermic could help birds not just save energy but actually store it, too. In the 1980s, scientists observed that hummingbirds lower their body temperatures during migration.