How Do Bald Eagles Stay Warm In Cold Weather?

Mom, Dad, and eaglets in a late storm, 2011

We get a lot of questions about bald eagles and cold weather. I've written a few posts on the subject, but thought I'd combine them here. 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, successful eagles use the least amount of energy to get the most amount of food. Here's how they do it.

• Foraging in groups (or observing and following other birds on their territory) increases the likelihood that eagles will find food with less energy expenditure than if they hunted alone. Bald eagles steal prey from other eagles and birds (kleptoparasitism), an optimal behavior, at least during periods of food scarcity, for animals that forage together. 
• Gorging food helps eagles load calories quickly and reduces the likelihood of food theft by another animal.
• 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. 
• It takes energy to fly, procure sticks, 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 also holds heat more efficiently than open spaces, slowing energy radiation and minimizing loss. 
• 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. 

Eagles also have physical adaptations that help them stay warm and incubate their eggs. 

• Feathers are wonderful insulators. The roughly 7,000 feathers an eagle grows help keep it warm and dry. Stiff exterior vane feathers zip together over smaller, fluffier down feathers, providing an overcoat of sorts to shed water and help keep heat from escaping. Underneath, down feathers trap pockets of air next to the eagle's body, where it is quickly warmed and prevented from escaping. 
• However, some heat needs to escape during incubation. A brood patch is a patch of bare skin on an eagle's breast. Eagles can roll, wiggle, and tuck their eggs up against the brood patch, effectively transferring heat from the eagle's roughly 104-degree body to the eggs. Since the optimal temperature for incubation is 99 degrees, some heat can be lost. The eagles regulate egg temperature by alternately incubating the eggs and leaving them uncovered. 
• An eagle's large size helps it retain heat. Eagles have a higher inside-to-surface area ratio than a smaller bird like a chickadee. Less surface area means less heat lost to radiation, which is why, within a broadly distributed group of animals, northern animals tend to be larger than their southern cousins. (http://en.wikipedia.org/wiki/Bergmann%27s_rule)
• An eagle's legs use counter-current heat exchange to control 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 still 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, nearer the warm center of its body, and 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. 

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. http://goo.gl/KRqbrv. Our pinterest has some cute ideas, with more to come: http://goo.gl/O4eXWx 

More information on Eagles and Cold

• Feather and Weathers: http://goo.gl/46BZmA
• Eagles and Cold: http://goo.gl/Bp2Rfy
• Eggs and Cold: http://goo.gl/AJysIZ

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?

Temperature Rhythms Keep (Human) Body Clocks in Sync: http://www.sciencedaily.com/releases/2010/10/101014144314.htm

Kleptoparasitism: http://en.wikipedia.org/wiki/Kleptoparasitism

What happened to SNOW1?

By Kay Neumann, Executive Director SOAR – Saving Our Avian Resources

Just like the other 13 snowy owls that SOAR worked with in 2012, SNOW1 came in starving. Seven of the 14 owls died before they got to SOAR’s facility. Five more died despite our best efforts at rehydrating, warming, and trying to reverse their severe starvation. Most had lost half their body weight. Two owls made it through. One of these had a dislocated elbow and cannot fly. He is now an ambassador for his kind as a SOAR education bird. One female, SNOW1, made it to release. This is her story.

SOAR education snowy and SNOW1 share some together in rehab

SNOW1 was rescued from a farm field by Wright County Conservation Board staff in January 2012 and conservation board volunteers, the Rector family, drove her down to SOAR. She weighed 2.5 pounds. Female snowy owls should weigh between 4 and 4.5 pounds.  She was kept warm (we do not want starving birds to have to spend any of their precious energy trying to keep warm), tube fed high calorie fluids, and slowly force fed easily digestible solid food. She was able to bounce back and started eating on her own! After a month in intensive care she was moved to a medium-sized flight pen to stretch her wings. As you can imagine, she was fairly weak and it took another good six weeks for her to get her muscles back into shape. At this point the snowy migration was over and with a few feather imperfections we decided to intermew (let her molt) here at SOAR. Then several folks worked on a plan to be able to monitor her after release.

SNOW1 gets flight practice with her transmitter on.

Bob Anderson, Raptor Resource Project, and Brett Mandernack, Eagle Valley Nature Preserve, stepped up to provide a transmitter and the expertise to gather and monitor the location data. SNOW1 made a quick trip to Decorah to have the transmitter attached with a backpack type of harness. She spent more time in SOAR’s 100-foot flight area to develop muscle and make sure the transmitter was comfortable and not interfering with any of her activities. She also had live prey practice and made easy work of this, when other food wasn’t provided.

We needed to decide when and where to release her. We consulted several experienced rehabilitators and thought that taking her further north and waiting for fall, when other snowys would be moving back in would be a good plan. Minnesota DNR agreed to allow transport to their state for release. As we were preparing to do this, snowy owls began arriving in Iowa. A look at habitat areas in western Iowa showed several large wetland and grassland areas that should hold good numbers of small mammal prey items.

SNOW1 was banded, thanks to the Iowa Department of Natural Resources (Iowa DNR), and ready for release. Matt Wetrich with Carroll County Conservation Board and Paul Roisen with Iowa Ornithologist Union released her at Woodbury County Conservation Board’s Owego Wetlands. She flew beautifully and was back in the wild. It was very exciting to get the first data locations to find that she had moved several miles north, then back south, in all traveling over 30 miles to finally spend a week in and around Onawa, IA.

Paul Roisen captured this photo of SNOW1 atop a utility pole on 6 December

Before she left the Woodbury County area, Paul Roisen was able to locate SNOW1 and get a wonderful photo. She was perched on a power pole and looking in command of the situation after about a week in the wild. Then a report came from the Iowa Ornithologists Union birdline of a Snowy along Interstate 29 near Onawa. Fearing she had been hit by a car, Wildlife Lodge and Clinic volunteers were dispatched from Sioux City to try to find her and bring her in. Even with a good visual report and our transmitter data, they could not find her. An Iowa Department of Transportation staff person joined in the search and reported picking up a dead, unbanded snowy owl just a few miles north of Onawa on Interstate 29 a day or two earlier. This was not good news. Snowys were choosing to use the area, but were not fairing well. Iowa DNR biologist, Doug Chafa, stationed near Onawa, joined in the search and got a visual on her spending time in the cloverleaf interchange between Interstate 29 and Iowa Highway 175! Not the spot I would have chosen for her, but she was flying, perching and preening. She was on her own.

We had several more transmitter locations for her around the Onawa area, which seemed to be good small mammal habitat; old hayfields, mowed filter strips, and stream buffers. Then after three weeks in the wild, no movement was detected from her transmitter and we feared the worst. As it would happen, Helen Harvey, a falconer from Sioux City, was en-route near Onawa. She made a thorough walking search of the area of SNOW1’s last signal with no luck. SNOW1 must be out of sight somewhere. Dean, Ray, and Warren from the Siouxland Metal Detecting and Archeology Club volunteered an afternoon with their equipment for a high-tech search with Helen. They found her thin body under the snow. She had not survived well in Iowa. Necropsy (animal autopsy) confirmed that she was not shot, did not have any broken bones, no aspergillosis, her stomach was empty and she had no body fat.

As with most good research, this has answered one question and brought many others to mind. Could we have done something differently? Somehow better prepared her to survive in the wild? Chosen a different release location or a different time of year? As with all of our wild patients we did the very best we could with SNOW1 for her to have a good second chance at being wild.

From band return data we know that many of our released birds survive well. A red-tailed hawk was recovered eight years after release, just seven miles from her release site. An arctic-phase great horned owl that was released in Carroll County in December was recovered more than six months later in Northern North Dakota. A bald eagle was picked up in Mills County with a fractured humerus that was repaired and released in Carroll County. This eagle was found three years after release, she had flown over 100 miles back to Mills County.

Is the landscape (habitat) in Iowa so different from the tundra that snowy owls cannot figure out how to catch prey that is different from the normal diet of lemmings and arctic voles? If the snowy owls were released in March, closer to the time they should be returning north, and released near the Canadian border, would they have better survival? Perhaps the snowy owls that come this far south are not good candidates to contribute to the wild population and should not be released?

The technology to track birds after release is amazing. Even though we all wanted SNOW1 to have a successful winter and migrate back to the arctic to nest, her information will be invaluable to help with future snowy owl rehabilitation. Snowy owls are a very interesting species and may be impacted by climate change. Knowledge about their movements and survival could be an important indicator of what may be happening in the arctic. Stay tuned, we may make plans for SNOW2.

I am always amazed at how many people are willing to help wildlife in distress; it warms my heart and helps me to keep going even when things don’t work out as we would have hoped. A big thank you to everyone who helped with SNOW1’s rescue, recovery, release, and monitoring! She definitely left an impact on many people.