Monday, January 06, 2014

Eagles and Cold Weather

-13F in Decorah at 1:54pm on January 6, 2014. Yikes!
We are getting a lot of questions about Mom, Dad, and cold weather. I've written about feathers and weather, but overwintering bald eagles also protect themselves by maximizing energy gain and minimizing energy loss. Every calorie is made to count.

Maximizing Energy Gain
Bald eagles maximize energy gain during periods of cold stress by foraging in groups, gorging food, and assimilating more energy. As we know from watching Mom and Dad, some eagles remain on their territory year round. But many others leave their territory to roost communally during the winter, especially along stretches of open water. Communal roosters often forage in smaller groups of less than 20 eagles, increasing the likelihood they will find food with less energy expenditure than if they hunted alone. Bald eagles also steal prey from other eagles and birds (kleptoparasitism), an optimal behavior, at least during periods of food scarcity, for animals that forage together. When one bird finds food, all of them exploit it. This social behavior increases efficiency and decreases energy stress.

Mom and Dad don't communally roost, and we don't know whether or not they forage with other eagles away from their home territory. The abundant, high-quality food provided by the trout hatchery, stream, and road may be an important enticement to stay on territory year-round. However, Mom and Dad are probably gorging the food they find. This helps them calorie load quickly, reduces the amount of time they need to spend looking for food, and helps prevent food stealing. The more quickly an eagle eats its food, the less likely it is something else will steal it!

Finding food extracts an energy cost, so Mom and Dad need to make the food they find count! During periods of cold stress, bald eagles assimilate more food energy than they would otherwise. This may be because in extreme cold weather, blood flow is decreased to the skin and extremities and increased to the visceral organs, including the stomach. Greater blood volume in the alimentary tract might increase digestion of food, leading to greater assimilation of food energy when the eagles need it most. A similar mechanism has been found in snowy owls, who live and hunt in even colder weather.

Minimizing Energy Loss
Bald eagles minimize energy loss by becoming sedentary, seeking protective microclimates, reducing night-time body temperature, and reducing foraging costs.  This time of the year, we don't see Mom and Dad at the nest as often during periods of extreme cold. Flying, stick procurement, and nestorating all have a high energy cost, and overwintering eagles need to minimize energy loss. So what are Mom and Dad doing right now? Probably nothing at all. In extreme cold, minimizing activity maximizes fitness.

When overwintering eagles do fly, they don't tend to make long trips. They also seek protective microclimates. Needled conifers hold more heat than leafless deciduous trees, so Mom and Dad might seek shelter in a pine grove. There may also be some sun-warmed, wind-protected pockets next to the bluff, or some protected space between the trout hatchery buildings and the bluff. These areas will reduce wind exposure and hold heat more efficiently, which helps the eagles minimize energy loss. Like mammals, birds are endotherms - that is, they maintain an internal body temperature. Maintaining body temperature takes energy, so anything that helps an eagle maintain its temperature reduces energy loss.

Although eagles are endotherms, they don't have to maintain a given body temperature all of the time. Bald eagles reduce their body temperatures at night an average of 1.8 degrees. This slight hypothermic condition reduces the temperature gradient between their body and the environment, letting them burn fewer calories to stay warm. Mark Stahlmaster estimates that this saves about 4.6% of an eagle's daily energy budget - a very big deal during periods of cold stress. Once again, we see Mom and Dad doing more by doing less.

It's hard not to be concerned about eagles and other wildlife during extreme cold events. But eagles and other animals that live outdoors are well-prepared to deal with them. 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:

For more on feathers and weather, read this blog post:


Ecological Energetics and Foraging Behavior of Overwintering Bald Eagles
Mark V. Stalmaster and James A. Gessaman
Ecological Monographs
Vol. 54, No. 4 (Dec., 1984), pp. 407-428
Published by: Ecological Society of America
Article Stable URL:

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:

A great read

Winter World: The Ingenuity of Animal Survival by Bernd Heinrich

Did you know?

Temperature Rhythms Keep (Human) Body Clocks in Sync:


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