Migrating Birds Chill to Conserve Energy

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 Michal 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.

For this study, the researchers looked for a similar phenomenon in blackcaps, a small and common type of warbler that spends winters in Africa and summers in Europe. Among millions of hungry migratory birds, lots of blackcaps stop to refuel in Israel each spring after a long flight over the barren Sahara Desert.

On a University campus in the Negev Desert highlands, the researchers caught eight migrating blackcaps, outfitted them with implantable radio transmitters and put the animals in cages with plenty of fruit and mealworms to eat. For up to 12 days, the scientists monitored the birds' body temperatures and rates of metabolism.

During the day, the study found, the birds maintained an average temperature of about 42.5 degrees C. At night, the scientists report today in the Journal of Experimental Biology, their temperatures dropped to an average of less than 39 degrees C, with one bird dipping as low as 33 degrees C.

The skinniest birds cooled down the most. In turn, they were much more efficient at turning food into fuel. Calculations showed that blackcaps who lowered their temperatures the most boosted their fat reserves by up to 30 percent more than birds that stayed toasty. Bulking up quickly helps birds get back on the road and gives them a better chance of claiming a prime spot for breeding.

The strategy is a creative way for birds to get more bang for their buck, said wildlife ecologist William Karasov, of the University of Wisconsin-Madison. Just like people can spend less money on clothes to save money for a car, he said, blackcaps seem to spend less energy on warmth to bulk up for their migrations.

"Here's an animal making a tactical decision to reduce expenditure in one part of its budget so it can allocate more of its relatively fixed income to fat gain," said Karasov, who suspects that other birds do the same thing. "I'm always impressed with how flexible animals are in meeting objectives."

Descending into hypothermia has its risks, Karasov added. An animal in a cold state of torpor can't react quickly to threats and becomes more vulnerable to predators. For that reason, he said, birds that are already plump probably stay warm through the night, even when they’re in the middle of a lengthy voyage.