It was minus frigid today. The thermometer showed a high of -8 oC (18 oF) in the afternoon, and a brisk wind dropped it to the teens. Actually, that’s not terribly cold; yesterday it was -17 oC (0 oF), with a wind chill of -29 oC (-20 oF). I had the pleasure of walking home from the subway in that, and the bits of my face that were exposed, my cheekbones and the bridge of my nose, were aching by the time I got inside. The ten minute walk probably only took me seven because I was hurrying so much! The good news was that my new down jacket performed fabulously in its first serious field test. Average temperatures for this time of year run about -1 oC according to weather.ca, so we’re in a bit of a cold snap.
Today, back out in the country at my parents’, I couldn’t help but think of all the little critters who didn’t have the ability to hide indoors beside the wood stove. Those juncos looked a little cold out there. They turned down my offer to come in, though. A lot of animals suffer through conditions that would freeze us solid quickly. All those poor male Emperor Penguins incubating their eggs in the Antarctic winter (seen March of the Penguins?) comes to mind. More locally, the chickadees and sparrows in the backyard often make it through long stretches of sub-freezing temperatures and deep snow.
One of the keys to their survival is their full-body down jackets (or, in the case of mammals, fur coats) that keep them well-insulated. I reflected, as I was hurrying down the street yesterday, that if I had a down jacket for my legs and face I’d be in pretty good shape, since my torso was quite comfy. Part of that was the fact that I was being quite active and producing a lot of heat. This is also a key for birds; they keep relatively active during the day, and the heat they produce then gets trapped under the layers of down. On the above feather, the fluffy bit at the bottom creates many tiny air pockets that trap warm air against the body (this is the bit they stuff in your down jacket or duvet, though less expensive ones include the whole feather), while the smooth upper part streamlines the bird’s body to optimize aerodynamics. Mammals have a similar system of hairs that creates the same effect. Both birds and mammals have the ability, through itty-bitty muscles at the base of the feather/hair, to raise the insulative layer to create a wider air pocket and trap more heat when it’s colder (this is what goosebumps are… we don’t have enough hair for it to do much, though).
But what about those tiny bare feet? Standing on the snow in the winter it would be hard to keep any warmth in them at all. And think of the ducks swimming on the near-freezing water. Well, as the junco above is demonstrating, often birds will tuck one foot up into their breast feathers if they’re resting (I’ve even seen some birds continue to forage for a time, hopping on one foot, while the other’s tucked up). Ducks, gulls and other waterbirds do this frequently, landbirds are less often observed doing it (mostly because they don’t loaf as much as gulls do!).
In addition to that, they’ve evolved an ingenious circulatory system to their legs, called countercurrent circulation. The arteries in their legs line up right next to the veins, so that the blood being pumped down into the legs through the arteries transfers its heat to the blood coming back up from the feet in the veins. This cools the blood down before reaching the feet to minimize heat loss to the environment, and warms it back up before it reenters the body. There’s actually very little tissue in a bird’s foot, it’s mostly tendons, so it doesn’t have the same heat requirements that muscle or organs do (in fact, it can approach freezing with no serious consequences). Mammals have a similar system in their legs, although they have more muscle tissue, so also use a layer of fur for insulation.
Speaking of mammals… look who was out today! I have seen the Red Squirrel more this winter than I can recall in previous years. I have a feeling it’s in part because he’s set up shop in the attic of the house, I think in the area over the room with the wood stove. Judging by the size of the icicles hanging off the eaves there, the attic is probably pretty warm, and I imagine he doesn’t have to work too hard to conserve heat. Plus, with the feeders right there and handy for him, once his cache ran low he didn’t have to go too far for more food. I may have seen more of him this winter than the Gray Squirrels – unusual.
“Oh my GOSH it’s cold!”