Last week the winds howled and temperatures fell below freezing. Trees and branches cracked and sprawled across the ground. Leaving my house, I pulled my coat tight, drew my hat over my face, and in my fuzzy boots scuttled the 15 feet to my car. I blew a gust of frozen breath in the safety of the front seat. Then I looked out and saw a dark eyed junco, a tiny songbird pictured above, sitting on a branch, calm as can be.
How can a wee bird weighing no more than three pennies spend 24/7 in such cold temperatures without freezing to death?
Down, down, down
That jacket or blanket that you may have for extra cold days is stuffed with down for a reason. Unlike the long flight feathers on a bird’s wings and tail, the down feathers are short and can be fluffed up. This fluff creates air pockets that trap the warmth escaping from the bird’s body. The specialized down feathers can keep a bird’s body temperature at 104 degrees even in freezing weather!
Birds also undergo a fall molt in which they grow up to fifty percent the number of feathers on their body for greater insulation. That fluff is what makes these tiny birds look round as tennis balls in winter.
Have you ever seen birds poking their beak into the base of their tail, then rubbing their beak all over their bodies? They are collecting oil from the uropygial gland and spreading the oil over their feathers. Not only does the oil keep feathers preened and clean, but it also adds a weatherproofing layer that keeps them from getting wet. Can you even imagine the misery of having wet feathers in winter?
Wait, what about my feet?
No matter how warmly I dress, my feet and hands are usually the first thing to freeze in winter. So how do those featherless, little reptilian bird feet stay warm in winter?
For one thing, those sinewy feet have very few nerves, which reduces the cold that the birds feel through their feet. The scales are specialized to minimize heat loss. Birds also take turns lifting one foot and tucking it into their feathers to warm it up, or crouching down over their feet to warm them.
But they have another brilliant adaptation as well: rete mirabile, which is Latin for “wonderful net.” In the rete mirabile, arteries that carry warm blood away from the heart intersect with veins that carry cold blood from the feet. A heat exchange occurs, and the blood from the feet warms before reaching the bird’s heart.
And then there is shivering
Birds are warm blooded creatures and, through a high metabolism, maintain a body temperature at about 105 degrees Fahrenheit. How challenging it must be to keep such a high body temperature in extremely cold weather! When all else fails, birds will start shivering their powerful chest muscles. The pumping of their strong flight muscles sends blood and heat surging through the body. But this relief comes at a cost. Half the calories that a bird needs to survive a wintry day could be depleted in the act of shivering. A chickadee, for example, requires the equivalent of 65 sunflower seeds to survive a cold winter day. Food to replace those lost calories is scarce in winter. Most insects and trees are dormant.
Birds are highly resourceful creatures, and they have adapted to find sustenance in seemingly barren environments. They have exceptional eyesight, and can tease out tiny cracks in tree bark serving as insect hideaways. As gardeners, we can help them by not removing our dead plantings in the fall. Those browned flower heads can contain seeds and insects that sustain birds through the winter (see Hidden Provisions blog entry). And if you are feeling extra generous, you can fill a feeder with sunflower seeds or suet, and enjoy the feeding frenzy that ensues.