The thermometer on my car read -18 degrees Fahrenheit 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 partway down, and turned on my camera. The shape shifted, rotated, and two dark eyes came into view.
|Even though snowy owls sport thick feathers all the way down their legs, this one still chose to hunker down against the cold. Photo by Emily Stone.|
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.
The frigid morning, complete with a stiff breeze left over from recent severe weather, didn’t seem to faze the owl. Why should it? Most of his relatives are surviving just fine up above the Arctic Circle. 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. They must be doing something right.
|Common redpolls add 31 percent more feathers for the winter, and then they also fluff out their feathers and tuck up their toes to stay warm on frigid days. Photo by Emily Stone.|
Just like the snowy owl, these little birds have a thick coat of feathers. Redpolls add 31 percent more feathers for the winter. That’s a big increase for a bird that only weighs half an ounce to begin with. Back in January, when Prof. Sheldon Cooper from UW Oshkosh lectured our Winter Ecology participants about the Winter Ecology of Birds, he compared small birds adding feathers to putting a toddler in a snowsuit. A big critter, like a snowy owl or an adult human, can still move pretty well, even if you add some puffy layers. The smaller the critter, though, the more those layers can impede movement.
Redpolls do have one advantage over owls, though, because they can fit into sheltered spaces—snow tunnels for redpolls and tree cavities for most other small birds—that protect them from wind chill even better than feathers could.
|Pine siskins put on more fat than many other little birds. They also fluff out their feathers and tuck up their toes to stay warm on frigid days. Photo by Emily Stone.|
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% body fat, and 12% better feather insulation even in the winter. Pine siskins, though, put on 50% 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. Birds can also store fuel outside of their bodies. One reason those chickadees don’t need to store fat is that they store food—in up to 2,000 little caches.
|Black-capped chickadees don’t put on much extra winter fat; instead they store fuel outside their bodies in the form of food caches. Photo by Emily Stone.|
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. Forage more, shiver less, and you end up warm, with the additional benefit of a belly full of food.
Prof. Cooper’s lecture came back to me as we watched redpolls, siskins, and chickadees hop around the Welcome Center’s feeders. As I photographed a fluffed up siskin perched above the feeder with just its toes exposed, I thought back to a slide Cooper presented to us about “heat conserving postures." The diagram showed a little bird in warm weather standing normally with its legs and feet exposed. With each successive decrease in temperature, the bird squatted lower and surrounded more of its legs and feet with fluffy feathers. Despite his feathered legs, the snowy owl displayed a similar toe-hiding posture.
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’s second book, Natural Connections: Dreaming of an Elfin Skimmer, will be available on March 14! Preorder yours at http://www.cablemuseum.org/books/.
For 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our exhibit: "Bee Amazed" is open through March!