Birds and Winter Survival

by Stan Tekiela
© 2003 NatureSmart
December 12, 2003

Peering into the steel blue stillness of a mid-winter evening, I am struck by the profound coldness. When the temperatures plummet to near zero my mind turns to the amazing ways that animals and birds cope with such extreme weather.

Winter poses several challenges to birds and animals. Scarce food supplies and limited water are just a few obvious challenges to winter survival. Extremely cold temperatures, strong winds, driving snow and nights that seem to go on forever can be deadly for birds. Each winter we loose many of our feathered friends to the rigors of winter. It’s how nature works-- the survival of the fittest.

Birds have many adaptations to survive winter weather. Wintering birds such as the American Goldfinch and Black-capped Chickadee add additional feathers in preparation for winter. The typical goldfinch or chickadee is coved with about 1,000 feathers during summer and over 2,000 in winter. During very cold days, and nights, birds fluff up their feathers, reducing the amount of heat loss by up to 30 percent. However, extra feathers alone are not enough to make it through a winter night.

Birds have a unique circulatory system in their legs to help them cope with cold temperatures. Warm arterial blood from the birds interior that is on its way to the bird’s feet passes through a network of small passages that runs along side the cold returning venous blood from the feet. The network of vessels acts like a radiator and exchanges the heat from the out-going arterial blood to the cold venous blood. This system insures that no heat is lost and the bird’s feet receive a constant supply of life sustaining blood. That’s also why ducks can swim in freezing water and not get cold.

Fat is another important winter weather survival adaptation. Fat acts as an insulator in addition to an energy reserve. During the day, birds eat to build up fat reserves. On average a bird can put on up to 15 to 20 percent of its body weight in extra fat before it has troubles flying.

Birds don’t have brown fat, the kind you and I have. Instead they have white fat. White fat is a high-energy fuel used to power the birds warming process, which is called thermogenisous. Thermogenosis is just a fancy name for the act of shivering. All birds from crows to chickadees continually shiver during cold weather to maintain their core body temperature at about 106 to 109 degrees F, depending upon the species. That is an amazing high temperature when compared to the surrounding air temperature. Only a thin layer of feathers separates what could be a temperature difference of over a hundred degrees from the inside of a bird to the air temperature.

Shivering produces heat five times their normal basal rate and can maintain a normal body temperature for six to eight hours at temperatures minus 70 degrees Fahrenheit. Without shivering the birds body temperature would quickly drop and the bird would become hypothermic.

At night, birds such as chickadees take shivering or I should say the lack of shivering one step further. To conserve heat, chickadees can lower their body temperature by interrupting their shivering. These periods of inactivity allow the bird’s body temperature to slowly cool, until it drops about 10 to 12 degrees. At this point the bird enters a state of unconsciousness called torpor. Respiration and heart rate will also drop during the period of torpor.

Nearing morning the periods of inactivity decrease until the bird is constantly shivering again and the body temperature is back in the normal range and the bird regains consciousness. The result of this controlled hypothermia is an energy savings of up to 20 percent during a typical winter night. Energy conservation is very important when you consider how little fat a bird can store.

Based on a daily increase of body fat of (15%) a typical chickadee has about 16 to 24 hours of energy reserves to carry it through a winter night. That is why it is imperative that a bird gets out early in the morning and finds food regardless of the weather. If it doesn’t replenish its fat reserves during the day, the bird will not have enough energy to make it through the next night.

You could help our bird friends survive another cold winter simply by filling your feeder with some black oil sunflowers and filling up your suet feeders.

Until next time...