The thermometer on my car read 18 degrees below zero as I turned off the highway onto a snow covered road northwest of Duluth. A leaden sky didn't offer any additional warmth. My parents and I were warm, though, wrapped up like onions in our many winter layers with the car heater blasting.

We were looking forward to a full day of birding in Sax-Zim Bog. This 300-square mile mix of aspen uplands, rivers, lakes, meadows, dead-ending back roads and farms has become famous for its ability to attract and support many species of birds who usually remain farther north.

Our first destination was a windswept field with a gravel road bisecting it and string of utility poles lining the road. Anemic gray light made the scene look barren from afar, but soon a white shape materialized on the top of a particular pole. I pulled up beside it, rolled my window part way down, and turned on my camera. The shape shifted, rotated,and two dark eyes came into view.

Snowy owls can be territorial even in their winter feeding areas, and while there aren't many other owls around to challenge him, this guy - male, as indicated by his very white feathers - seems to have staked out his claim. That's a boon for birders who, like my parents from Iowa, drive great distances to see his species in the bog.

Food and space are more limiting than cold when you have thick feathers all the way to your toes. That substantial winter coat contributes to the snowy owl's status as the heaviest owl in North America, weighing four pounds.

The open car window soon let in enough cold air to get uncomfortable, so we blasted the heater and drove up toward the welcome center run by the Friends of Sax-Zim Bog. Bright sunshine greeted us, as well as a cluster of bird feeders that were just wild with activity.

The stripy little birds with red smudges on their foreheads looked nothing like the snowy owl, but these common redpolls are no less birds of the north than he is. They breed all around the top of the globe, in a circle that borders the Arctic Ocean, and their global population numbers in the tens of millions.

Just like the snowy owl, these little birds have a thick coat of feathers. Redpolls add 31 percent more feathers for the winter.

Just like adding feathers, adding fat has its pros and cons for small birds. Getting too fat can make it harder and more energetically costly to fly, and reduce their ability to escape predators. Chickadees may only achieve 10 percent body fat, and 12 percent better feather insulation even in the winter. Pine siskins, though, put on 50 percent more fat than redpolls do.

Little birds don't store fat in an insulating layer of blubber like penguins and whales; they accumulate stores of brown fat around their wishbone and abdomen as a ready source of fuel for their metabolisms.

One reason those chickadees don't need to store fat is that they store food - in up to 2,000 little caches.

What they do with that fuel is also important. Many birds shiver to stay warm. It's effective, as long as they have plenty of fat to burn. They can also go about their business as usual. One study found that chickadees' feathers can capture the heat generated by hopping around to eat.

Prof. Cooper's lecture came back to me as we watched redpolls, siskins and chickadees hop around the welcome center's feeders.

From inside my warm car, the sight of all these little puffballs made me chuckle. Once the frigid air hit my face, though, I was reminded that fluff is rather serious stuff to a bird in winter.

Emily Stone is a naturalist and educator for the Cable Natural History Museum. Her book, "Natural Connections: Exploring Northwoods Nature through Science and Your Senses," is available to order at Listen to the podcast at