November 10, 2012
How birds survive winter nights

My birdfeeders have been busy this week, and as I watch the nonstop feeding frenzy, I marvel at how birds as tiny as goldfinches and chickadees can survive winter weather. The long, cold nights that lie ahead make day-to-day survival a challenge for all wild birds. Smaller birds face the greatest problem because they lose heat more rapidly than larger birds.

Wherever a bird sleeps, its first line of defense against cold is its feathers. Feathers repel water and efficiently insulate warm bodies from cold winter air. Each feather is controlled by a group of small muscles that can raise and lower the feather. By fluffing their feathers, birds create many tiny air spaces that increase insulation and reduce heat loss. (It's this same principle that makes down jackets so warm in winter.)

On extremely cold nights, birds reduce heat loss further by burying naked body parts into their feathers. This is why birds tuck their bills into their shoulder feathers and why many water birds often sleep with one leg held tightly up against the body.

Birds also have an amazing network of blood vessels in their feet and legs that minimizes heat loss. Chilled venous blood returning upward from the feet to the body passes through a tiny system of arteries that carries warm blood to the limbs. The warmer arterial blood reheats the cooler venous blood as it returns to the body. This counter-current network of blood flow reduces the amount of heat lost through the feet and is particularly valuable to birds that spend a lot of time swimming in cold water or standing on ice. This is one reason ducks and other water birds are unfazed by ice and cold water.

Sleeping quarters also protect birds from the elements. Songbirds such as cardinals, blue jays and finches retire to dense thickets of vegetation. Take a walk at dusk through such habitat, and you'll be amazed at the commotion as birds settle in for the night. Tangles of briars, grape vines, and brambles protect birds from wind, rain, and snow. At dawn each day, bramble thickets teem with birds.

Conifers and ivy-covered walls and fences provide even greater protection. Dense evergreen vegetation protects roosting birds from wind, rain, and snow.  Ring-necked pheasants, ruffed grouse, robins, and house finches are just a few of the birds that roost in evergreen refuges. This is a good ecological reason every backyard wildlife sanctuary should include some evergreens.

Screech owls, woodpeckers, titmice, and nuthatches sleep in cavities much like the ones in which they nest. Out west in the Rocky Mountains, pygmy nuthatches sometimes roost by the dozens in large tree cavities. Other cavity-nesters, such as bluebirds and chickadees, use cavities only during very cold weather.

On mild nights, bluebirds nestle in small family groups under the protection of small clumps of dead leaves that cling tenaciously to their branches. Chickadees also roost amid foliage or even in used bird nests. But on very cold nights bluebirds and chickadees retire to cavities. As many as a dozen bluebirds can cram into a single nest box or cavity; chickadees typically roost singly.

Roosting cavities cannot guarantee survival, however. Sometimes it just gets too cold, and birds freeze to death. And sometimes birds at the bottom of the heap suffocate. Over the years, I have found dead bluebirds several times in nest boxes after particularly frigid nights or ice storms.

Other avian sleeping arrangements are a bit more unusual. Some ducks sleep while floating on the water. Bobwhites sleep in a tight circle on the ground, all heads facing outward. The contact enables them to conserve precious body heat, and the outward orientation allows wary eyes to detect danger in all directions. And when there's lots of snow cover, ruffed grouse sometimes bury themselves in snowdrifts, where the snow itself insulates the grouse from plummeting outside air temperatures.

Despite their relatively small size and lack of large amounts of body fat, birds use quirks of anatomy, physiology and behavior to make it through the coldest winter nights.


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