Dickeybirds in the Cold
A sprightly chickadee on my hand in Algonquin Park
This morning as I write this it is only 2¡ Fahrenheit outside, much colder than we usually experience here on the Niagara Frontier. It is at times like this when, sitting at my desk in a 68¡ room, I think of tiny birds like chickadees, titmice, nuthatches and kinglets -- species we often refer to as dickybirds -- outside braving the elements.
And 2¡ is far from the worst temperature they manage. I recall seeing a half dozen chickadees from the ski lift near the peak of Quebec's Mont Tremblant where the temperature was -30¡. Still colder was the day a chickadee took a sunflower seed from my open hand at -40¡ in Algonquin Park. My fingers were tingling by the time I got my mitten back on.
Those temperatures don't even represent the worst conditions those dickybirds have to face. I remember a two week period in Minneapolis when the temperature was -30¡ and the wind was blowing 30 miles per hour. The wind chill must have been about minus infinity. On city streets you felt as though you were walking about among bank robbers as everyone wore face-covering knitted stocking caps. Despite those conditions those tiny birds – chickadees, for example, weigh less than a tablespoon of butter – dashed to and from our feeder.
We also had a period of rain, sleet and branch-covering ice here a few days ago. More trying times for those tiny mites.
How do they manage?
First, they have physiological defenses. Small birds have a higher metabolism rate than we and other larger mammals and birds have. That means that they burn calories faster than we do, in the process generating more body heat. But there is a trade-off for this. They must stoke their furnace to maintain that rate. That's why you see them actively hunting food from dawn to dusk.
They are quite literally burning up body fat. I can attest to this from an experience I had years ago in Rochester. I banded and weighed a song sparrow one evening just before a blizzard struck. The next day I recaptured that bird and weighed it again. It had lost a quarter of its weight overnight.
These small birds reduce this metabolism rate at night. When resting they experience physiologically controlled hypothermia, sending them into a state of torpor.
Feathers are excellent insulators to protect that body heat and the fall molt of these wintering species adds additional feathers to that insulation. The feathers create air pockets that are warmed by the bird's body heat and provide an additional barrier to the cold, exactly like our down-filled sleeping bags.
Unlike us, birds don't sweat and that heat around their body is dry heat.
Feathers don't cover chickadees' legs, but those legs have other defenses. They are mostly bone and scales that don't need warmth. And the arteries and veins in those thin legs are close together, allowing the outgoing blood to exchange heat with the incoming blood. Even with this exchange the temperature of their feet is often only maintained near freezing.
When we hang onto a limb we have to exert energy to maintain our grip. Birds don't have to do this. There is a locking mechanism in their feet that holds their grip. Occasionally birds in torpor are found hanging upside down from a perch, their locked feet holding them in place.
Those physiological defenses are not enough, however. so birds enhance them with behavioral strategies. They fluff up their feathers to add thickness to that blanket. At night they may twist their heads under their wings as we more often see ducks do. This covers their unprotected mouths. They can squat down on their legs or lift them one at a time into their feathers to warm them.
Dickybirds often share heat in a roost. Those birdhouses you erected for nesting birds can often serve them in winter as well. And chickadees are hole-nesting species: they may retreat in family groups to the nest the parents excavated last spring. Even when such roosts are not available, thick shrubbery can also provide them some protection.
Remember how tough the cold is on these dickybirds and give them all the help you can. In particular, keep those feeders full.-- Gerry Rising