How do eagles stay warm in cold weather?

Iowa bald eagles nest in extremely cold weather. How do they stay warm?  Bald eagles maximize energy gain by foraging in groups, gorging food, and increasing the assimilation of ingested food energy. They minimize energy loss by reducing activity, seeking protective microclimates, and lowering their nocturnal body temperature. In short, eagles keep warm by using the least amount of energy to get the most amount of food.
Group Foraging and Changes in Behavior
January 14, 2021: Eagles at Lock and Dam Seven on Lake Onalaska, Mississippi River

Foraging in a group – Eagles at Lock and Dam Seven on Lake Onalaska, Mississippi River

All bald eagles reduce activity and seek shelter during periods of extreme cold. Non-territorial eagles also forage in groups and roost communally to help conserve energy.
  • When one eagle finds food, every eagle finds food. Non-territorial bald eagles often forage in groups, which lets them find more food with less work than hunting alone. They also steal prey from other eagles, which can be more energy efficient than catching it themselves. Why go fishing when someone else can do it for you?
  • Eat it before someone else gets it! When eagles eat more food than they can digest, they store the excess in their crops for later. Gorging loads calories quickly and reduces the opportunity for food theft.
  • It’s too cold to do anything but perch. Eagles are less active during periods of extreme cold. Inactivity reduces an eagle’s energy consumption and slows its metabolism.
  • Stay out of the wind! Eagles reduce wind exposure and slow energy radiation by sheltering in thick brush, bluff pockets, on the lee side of branches, and in coniferous trees: places where wind won’t wick their body heat away.
Physical Adaptations
January 2, 2022: Mr. North looks stylish in his wintery down jacket! Subzero temperatures had every one in winter gear today.

Mr. North looks stylish in his wintery down jacket! 

Physical adaptations help eagles stay warm and incubate their eggs in subzero temperatures.
  • An eagle’s 7,000 feathers keep it warm and dry. Stiff exterior vane feathers zip together over fluffy down feathers: an overcoat that sheds water and keeps heat from escaping. Down feathers trap layers of air next to an eagle’s body, where it warms quickly. An eagle can change how much air is trapped by moving its feathers to create more or less air space.
  • A bald eagle controls its core body temperature with counter-current heat exchange. Warm arterial blood flowing from an eagle’s core into its feet passes cool venous blood flowing back. This warms the blood flowing into its core and cools the blood flowing into its feet. The cooler blood is warm enough to prevent frostbite and reduces the temperature gradient between its insides and its outsides, which prevents 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 in thick, scaly skin that protects them from the cold. If its feet do get cold, it will tuck them beneath its feathers.
  • Cold weather causes changes in blood flow. Less blood flows to an eagle’s 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.
Winter Incubation
03/07/19: Fabulous frosty feathers at the North nest

Fabulous frosty feathers at the North nest

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. The optimal temperature for incubation is around 99 degrees, so eagles have heat to spare! They 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.
More information on Eagles and Cold
  • Eggs and Cold: https://www.raptorresource.org/2019/02/20/eggs-and-cold-weather/
References
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: 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?
http://askanaturalist.com/why-don%E2%80%99t-ducks%E2%80%99-feet-freeze/
A Great Read
Winter World: The Ingenuity of Animal Survival by Bernd Heinrich
Did You Know?