Mrs. North incubating in snow
Iowa bald eagles nest in surprisingly cold weather. How do they keep themselves and their eggs warm? Unique body features and changes in physiology and behavior help bald eagles maximize energy gain, minimize energy loss, and incubate eggs in cold temperatures.
To maximize gain, eagles forage in groups, gorge food, and increase the assimilation of ingested food energy. To minimize loss, they become sedentary, seek protective microclimates, and reduce night-time body temperature. Put simply, eagles keep warm by using the least amount of energy to get the most amount of food.
Group Foraging, Changes in Behavior
Eagles reduce activity and seek shelter during cold weather. Non-territorial eagles also forage in groups and roost communally to help conserve energy.
- Group foraging helps eagles find food with less energy expenditure than hunting alone. When one eagle finds food, everyone knows about it. Bald eagles also steal prey from other birds (kleptoparasitism), which is often more energy efficient than catching it in the first place .
- Gorging food loads calories quickly and reduces the likelihood of food theft by another animal. Excess food can be temporarily stored in the crop for digestion later on.
- It takes energy to fly and nestorate. Reducing activity reduces energy consumption and slows metabolism.
- Sheltering in protective microclimates (thick brush, bluff pockets, sheltered branches, coniferous trees) reduces wind exposure. These microclimes hold heat more efficiently than open spaces, slowing energy radiation and minimizing loss.
Eagles have physical adaptations that help them stay warm and incubate their eggs in subzero temperatures.
- An eagle’s roughly 7,000 feathers help keep it warm and dry in cold weather. Stiff exterior vane feathers zip together over fluffy down feathers, providing an overcoat that sheds water and helps keep heat from escaping. Down feathers trap pockets of air next to the eagle’s body, where it is quickly warmed and prevented from escaping. An eagle can change how much air is trapped by moving its feathers to create more or less air space.
- Bald eagles use counter-current heat exchange to control core body temperature. Warm arterial blood flowing from an eagle’s core into its feet passes cool venous blood flowing the other way. Heat is exchanged, warming the blood flowing into its core and cooling the blood flowing into its feet. The cooler blood is warm enough to prevent frostbite, but the lower temperature reduces the gradient between its insides and its outsides, preventing excessive heat loss through its feet.
- An eagle’s leg muscles are tucked up under its feathers, near the warm center of its body. It has very few soft tissues in its long legs and feet, which are wrapped by thick, scaly skin that helps protect them from the cold. If its feet do get cold, it can always tuck them, often one at a time, underneath its feathers.
- Cold weather causes changes in blood flow. Less blood flows to skin and extremities, making more blood available for visceral organs like the stomach. This helps eagles reduce energy radiation and increase the assimilation of ingested food energy at a time when every calorie is crucial.
- At night, bald eagles lower their body temperatures by about 1.8 degrees. This reduces the temperature gradient between their body and the environment, letting them burn fewer calories to stay warm.
It isn’t easy to incubate eggs in sub-zero weather! An eagle’s brood patch, large size, and nest help keeps eggs safe and warm.
- A brood patch is a patch of bare skin on an eagle’s breast. A bald eagle can roll, wiggle, and tuck its eggs against its brood patch, transferring heat from its roughly 105-degree body to its eggs. Since the optimal temperature for incubation is 99 degrees, some heat can be lost. Bald eagles regulate egg temperature by incubating, uncovering, rolling, and re-positioning their eggs.
- A bald eagle’s large size helps it retain heat. Eagles have a higher inside-to-surface area ratio than smaller birds like chickadees. Less surface area means less heat lost to radiation, which is why birds and mammals in cold regions tend to be larger than individuals of the same species in warm regions. Female eagles are larger than males and almost always incubate at night. A female eagle’s size helps her stay warm through the long winter nights of incubation.
- Eagles cover the bottom of their large nests with several layers of soft insulation, including grass and shredded corn husks. Shortly before laying eggs, they build an egg cup that helps protect the eggs from ice, snow, and water.
How Can I Help Wintering Birds?
It’s hard not to be concerned about eagles and other wildlife during extremely cold weather. You can help by keeping seed and suet feeders stocked, keeping water available, and providing shelter for birds. The Minnesota DNR offers these winter feeding tips: https://www.dnr.state.mn.us/birdfeeding/winter.html. Our pinterest also has some cute ideas, with more to come: http://goo.gl/O4eXWx.
More information on Eagles and Cold
Ecological Energetics and Foraging Behavior of Overwintering Bald Eagles
Mark V. Stalmaster and James A. Gessaman
Vol. 54, No. 4 (Dec., 1984), pp. 407-428
Published by: Ecological Society of America
Article Stable URL: http://www.jstor.org/stable/1942594
Food Consumption and Energy Requirements of Captive Bald Eagles
Mark V. Stalmaster and James A. Gessaman
The Journal of Wildlife Management , Vol. 46, No. 3 (Jul., 1982) , pp. 646-654
Published by: Wiley on behalf of the Wildlife Society
Article Stable URL: http://www.jstor.org/stable/3808555
Ask A Naturalist: Why Don’t Duck Feet Freeze?
A Great Read
Winter World: The Ingenuity of Animal Survival by Bernd Heinrich
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