How do birds stay warm??

Redpolls' internal temperature can be 73 degrees warmer than surrounding air, thanks to a neat biological feature. (Denis Doucet/Parks Canada)

Winter is the perfect time to hibernate inside and fatten up on holiday baking — for humans, that is. Wildlife have to come up with their own strategies for surviving the brutally cold season.

Birds have three options when it comes to adapting to temperatures well below zero, naturalist Brian Keating told CBC's The Homestretch on Wednesday: "migrate, hibernate, or tolerate."

• Keating said he was curled up inside his Calgary home with a mug of hot chocolate in hand on Tuesday evening, watching chickadees, nuthatches, woodpeckers and finches flit about his backyard, while the temperature dipped as low as -29 C.

Chickadees dine on fatty sunflower seeds to survive the winter. (Elaine Demers/Facebook)

Chickadees ride out the winter by eating as much fatty foods (like sunflower seeds) as they can, before huddling together in a winter roost at night.

"On the coldest nights when it gets really frosty they enter a nightly hypothermia," Keating said.

"Their body core temperature drops and they tolerate it by allowing themselves to chill down."

On the most bitterly cold nights, when the temperature drops below –30 C, some birds even tuck away in shelters beneath the snow.

Brain gain

Chickadees get smarter — literally — to remember where food is stashed or track down caches of hibernating grubs.

Red Deer naturalist Myrna Pearman explains in her book Beauty Everywhere that the chickadee's hippocampus — the part of the brain that's responsible for memory and spatial organization, said Keating — expands by 30 per cent each fall.

Geese have a counter-current blood warming system that helps keep them warm. (John Rieti/CBC)

Waterfowl like geese and ducks have a built-in feature that prevents their feet from freezing to the ice when tucking them under their body just isn't enough: a counter-current blood-warming system.

"Hot arterial blood wraps the venous (returning) blood with smaller, branched capillaries just like a glove, allowing for that valuable heat to efficiently warm the incoming blood from the legs and feet," Keating said.

"That way they can save that valuable heat energy and keep their core warm."

Cold-weather champion: Arctic Redpoll

But these birds have nothing on the Arctic Redpoll, a tiny finch that's developed some extreme cold-weather tricks.

"They can survive up to 20 hours without access to food, even if temperatures drop to -54 C," Keating said.

Redpolls have specially designed esophageal pouches that allow them to hold on to seeds, then later slowly digest them to provide them with energy to maintain their core at a balmy 40 C — "kinda like throwing logs on a fireplace."

"Their internal temperature can be 73 degrees warmer than the surrounding air, with the two extremes being separated by less than a half a centimetre layer of feathers."