How do birds survive the cold?

There's been a lot of talk about how humans and pets are surviving the bitter cold that has descended on Minnesota this week.

But what about wild birds?

It might not seem as though feathers could be enough to keep a bird from freezing when temperatures dip to double digits below zero. But the birds during winter in Minnesota are surprisingly well-equipped to survive the extreme cold.

About half of the roughly 300 bird species regularly seen in Minnesota head south to warmer climes for the winter, said Andy Forbes, deputy chief of the U.S. Fish and Wildlife Service's Midwest Migratory Bird Program based in the Twin Cities.

The ones that remain behind are "incredibly tough," Forbes said.

"As long as they can find food, they're generally able to cope with the cold temperatures," he said. Common Minnesota species like the black-capped chickadee and Canada jays also store food for times when it's not readily available, he said.

Minnesota birds staying during winter have high metabolisms and generate a lot of body heat. Their feathers are great insulators, Forbes said, which is why their feathers are often puffed up on a cold day.

"That traps a layer of warm air right next to their body," Forbes said. "That prevents them from direct contact with that cold air."

Water-dwelling birds, such as gulls and ducks, are built to walk on the ice covering waterways. Their webbed feet don't contain a lot of muscle or nerve tissue. But they do have an "amazing network" of blood vessels, Forbes said, which keep the blood moving so the feet stay cold — just above the air temperature — but don't freeze off.

"They're able to just focus on keeping their bodies warm, and that helps save energy," he said.

Some birds, like the chickadee, can actually lower their body temperature at night — and survive. They go into a state of torpor — not quite hibernation, but close — to save energy and reduce their need for food.

Still, it's unavoidable that some birds won't survive a brutally cold winter, or even a mild one. Certain southern species that have expanded into Minnesota in recent decades, such as the red-bellied woodpecker, aren't as adaptable to the cold as others like the chickadee, Forbes said.

"When it gets particularly cold like this, the energetic costs increase. They have to find more food," he said. "Those that can't do that are more likely to die."

Humans can help by filling feeders with high-energy offerings such as black oil sunflower seeds and suet, Forbes said. Birds also will make use of nesting boxes this time of year to stay warm during winter nights.

But Forbes cautioned that if you see a bird that appears injured, it's probably best to leave it alone. It's possible the bird is in that torpor state or trying to conserve energy, he said.

"The bird's probably fine," Forbes said. "As long as they're able to eat and get away from you, you'll cause more stress to it by trying to capture it."