Western Great Lakes Bird and Bat Observatory

Weathering the Winter - Avian Style

By Kate Redmond

Black capped chickadee Tom Murray

An old Scandinavian saying tells us that “There’s no such thing as bad weather, only bad clothes.” People take advantage of the latest technology in winter gear to get through the season (or we stay inside), but birds weather the cold using a combination of behavioral changes and design features rather than polar fleece and thinsulate.  
Fall migration is about food availability rather than cold. Our summer residents are replaced by northern birds (the old explanation for this was that one species had turned into another – “transmutation”). Permanent residents have a jump-start on establishing new territories in spring, but birds that stay home and avoid the hazards of migration must find enough food to maintain a core body temperature of around 105 degrees, and that’s a full-time job.
Behavioral changes are easy to see. The increasing popularity of bird feeding has a huge impact on overwintering birds, but some species also turn away from their summer reliance on insects (which are still available in winter but are much harder to find) and feed on seeds.  
Birds defend territories during the breeding season, which is very energy-intensive. With few exceptions, they don’t breed in winter, so they don’t have to forage for food for their young at a time when food is at a premium. Barred and Great-horned Owls are very early nesters, often courting and laying eggs in February and feeding young by mid-March, but their prey of small mammals is easier to come by than caterpillars and worms.  
Because they’re temporarily not territorial, birds can form winter flocks with many eyes to hunt for food and many warm bodies that can roost together in a sheltered spot and share body heat.  This is especially important for small birds, which have a proportionately larger surface area from which heat can be lost.  
In the cold, birds fluff their feathers to minimize heat loss, and they groom their feathers with an oil that both waterproofs and insulates them. They sun themselves, and they shiver to raise their metabolic rate, and birds that tuck their bills into their feathers are ensuring that the air they’re breathing is warm. Birds may sit on one foot and tuck the other next to their warm body and then switch, or they may hunker down, covering both feet.  
Most birds don’t visit last season’s nest during winter, but some cavity nesters escape the elements in their old nest sites, and woodpeckers may excavate holes specifically for winter shelter.  
Inactivity is always an option, and many birds hunker down during a bad stretch. A Red-tailed Hawk can sit out a two or three day storm, staying put when hunting would be futile, but then it must find food. Some birds, including nuthatches and chickadees, cache food when it’s abundant for use when it’s not.
But the real magic is in birds’ physical and physiological adaptations.  If birds look like they have more feathers in winter, it’s because most do – they grow extras during their fall molt, and their body heat warms the air spaces between the feathers. They also pack on some fat reserves, and with this extra padding, many are able to lower their body temperature (a chickadee by as much as 22 degrees!) to decrease energy needs at night. It’s a risky ploy because extra energy may be needed to jump-start their metabolism on a very cold morning. 
Most amazing is a bird’s “zoned-heating system.” Birds must eat enough to keep their core temperature steady, but their legs and feet (which are made up mostly of ligaments and tendons, not muscle) are always colder than their body, even though they’re covered with scales that provide some insulation. Most maintain their feet at a colder temperature than their body because keeping their feet toasty would require vast amounts of food, but their feet are kept from freezing by a system called “countercurrent heat exchange.” Arteries carrying warm blood from the core to the feet are situated right next to veins carrying cold blood back into the body, and the incoming blood picks up some heat from the warm blood as it passes so that the core doesn’t get shocked. In addition, many kinds of birds use valves in their leg arteries to ration the amount of warm blood that is sent to their feet.  
Light sleeping, deep sleeping, and true hibernation are mammalian strategies for weathering the winter –- do birds use them? Observations of true hibernation in birds have been elusive, but a number of species, like some hummingbirds, chickadees, doves, swifts, and bluebirds, are able to save calories by going into a state of torpor, a daily form of mini-hibernation, in response to extreme weather.  Poor-wills, a Southwestern Nighthawk relative, may stay in a state of torpor for several months. Birds in torpor have lower body temperatures (by as much as 50 degrees) and lower heart and respiration rates. 
So, the next time you see a chickadee sitting on a branch, fluffed up in the cold sunshine, think of everything that’s going on to make that happen.

Black-capped Chickadee photo by Tom Murray