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

Eggs and Incubation

Why are the eagles off their eggs? Will the eggs survive? What happens during incubation? These are just a few of the questions we get about incubation. I'll see if I can answer some of them here.

An infertile chicken egg. See the blastodisc?

Unlike most birds, chickens will lay eggs even when a male isn't present. We'll start with a quick look at an unfertilized egg, pictured at right. From the Chicken Chick: An infertile egg contains only the hen's genetic material, which means a chick can never hatch from that egg. The hen's genetic material, termed the blastodisc, can be identified on infertile eggs as a light-colored dot with irregular borders. Every egg contains a blastodisc. My breakfast, at right, is an infertile, organic, cage-free 'Egg-Land' chicken egg. The blastodisc can be seen in the center of the egg. If the egg is fertilized, the blastodisc turns into a blastoderm - the first stage of embryonic development. End stage? Hatchling bird!

Let's assume we have a fertilized egg, discussed here. As the embryo develops, it will consume both the fatty egg yolk and high-protein egg white. By the end of day two, our little chicken will have a heartbeat. Its beak and egg tooth will begin to form on day six, and its feathers on day eight. By day nine, it will begin to look somewhat like a bird, and its mouth opening will form. On day thirteen - well, just go look at this blog for the rest.

So what does the developing embryo need to survive? We discussed the importance of temperature here, but incubating birds (or incubationist humans) also need to control humidity, move or turn the eggs, and make sure the eggs are well-ventilated.

Keeping Eggs Alive
Why were the eagles off the eggs so long on March 7th? We were asked this question on facebook, ustream, and via email. I think the eagles may have been giving the eggs a chance to dry out and breathe.

A bird's eggshell has thousands of tiny pores, which allow water and gas to pass through. Mammals like us get oxygen through an umbilicus, but developing birds receive oxygen and remove carbon dioxide through the egg shell. Gases, including oxygen, enter and leave the egg by diffusing through the pores in its shell, across the outer and inner shell membranes, and into the blood in the capillaries of a special tissue called the CAM, or chorioallantoic membrane. As the weather warmed in Decorah, the snow began to melt and the humidity soared. Condensation can form on eggshells exposed to excessive humidity, which clogs shell pores and provides a vehicle for bacteria. The result? Fatal suffocation and/or contamination. Only the eagles know for sure, but I think they may have responded to the threat of rising humidity levels by leaving their eggs uncovered. Standing or leaving entirely allows fresh air to circulate over the eggs, dropping the humidity level and giving the developing embryos fresh air. Turning or rolling assists air exchange, helps maintain an even egg temperature (especially in big egg piles, where outer eggs may be cooler than inner ones), and keeps the developing embryo from sticking to the eggshell.

In short, most bird eggs require specific temperatures, proper humidity, regular movement, and air exchange. Birds provide these by applying body heat, standing and/or leaving for periods of time, and rolling or turning their eggs. In birds, incubation is both an instinctive and learned behavior  - while birds will automatically incubate eggs or egg-shaped things, they get better at incubation with experience. There are no guarantees,  but Mom and Dad are skilled incubators with a long history of brooding together. Watching them meet the challenges of their environment gives me hope for their eggs. Go eagles!

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Resources

Bob Anderson, personal communication.

The Chicken Chick. This is a really neat site for those of us who love chickens!

• Facts and Myths About Fertile Eggs
• Chicken Embryo Development

Brinsea. For all your incubation needs.

• Humidity in incubation

Texas A&M.

• Incubating and Hatching Eggs

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Did you know?

We usually talk about birds, but people also incubate snake eggs! This video from Snake Bytes discusses the ins and outs of snake incubation: http://youtu.be/JlSS5wap-Zc

Eggs and Cold Weather

Mom with eggs on February 26, 2014

Everyone is worried about the eggs in Decorah. Iowa is facing extremely cold temperatures in the weeks to come. While there have been other cold years – 2008, for example, had sub-zero temperatures into early March – this looks like one for the record books. We can’t guarantee that the eggs won’t freeze, but I thought I’d talk a little bit about eggs and cold damage.

Much of what we know about eggs and temperature comes from incubationists, or people who incubate eggs. Temperature, humidity, and regular turning are all important to insuring an embryo’s survival and eventual hatch. In 1969, H. Lundy identified five temperature zones characterized by their effect on the developing embryo. These studies were carried out on chicken eggs in an artificially controlled environment, but we can look to them for some idea of how the eggs in Decorah and elsewhere might respond to ambient temperature.  The five zones are:

• The zone of heat injury or death:  Above 104.9°F or 40.5°C.
• Optimal temperature, aka the zone of hatching potential: 84.5° to 104.9°F or 35 to 40.5°C. The American Eagle Foundation lists this temperature as 99° for bald eagle eggs incubated in an artificially controlled environment.
• The zone of disproportionate development: 80.6 to 95°F or 27 to 35°C. Embryos in eggs that spend too much time in this zone can develop unevenly, leading to crippling injuries or death. Successful hatching is greatly reduced.
• The zone of suspended development: 28.4 to 80.6°F or -2°C to 27°C. Eggs at this temperature don't develop at all. Freshly laid eggs can spend a lot of time at this temperature with no harm to the egg or embryo. 
• The zone of cold injury or death:  Below 29°F or -2°C. Although eggs contain a great deal of water, they can get colder than 32°F/0°C without freezing. However, eggs that reach 29°F will freeze, which usually causes death. Some exceptions to this rule have been recorded in Mallard ducks.

So what does it all mean? 
In the last couple of days before egg-laying, we watched Mom and Dad build a wonderful nest bowl or cup of soft dry grass and shredded cornstalks. The grass conforms to the eggs, wrapping around to provide warm, dry insulation below and around them. Mom and Dad transfer heat from their bodies and regulate temperature by alternately sitting on or standing away from the eggs. Of course, this year has been cold enough that they’ve spent very little time standing. Sitting low in the nest helps keep the eggs and incubating parent warm. One of our facebook fans compared it to Tupperware – a great analogy, since the eggs are cupped below and sealed above. We've also watched the eagles bring in extra cornhusky insulation this year, building their pile as the cold deepens. The eagles may be on to something, since in 2009 the US Department of Energy awarded $200,000 to Husk Insulation, a company that makes insulation from cornhusks. The  company's panels are 10 times more effective than conventional insulators like foam.

So how warm are Mom and Dad? In a study of overwintering bald eagles, Mark Stahlmaster found that adult bald eagle average body temperatures ranged from 102.2°F, or 38.9°C, to 106.1°F or 41.2°C. Ambient temperature, wind, precipitation (snow can be insulative, but rain is not), shelter, and time of day all had an impact on body temperature. Since the zone of heat injury can be lower than the body temperature of an incubating adult, it’s easy to see why Mom and Dad might (in warmer weather) spend time away from their eggs. Although Dad has taken his share of egg duty this year, I wonder if larger Mom might spend more time incubating in extremely cold weather or at night, when eagles can drop their body temperature to reduce energy loss. Larger animals have an easier time conserving heat, since they have lower area-to volume ratios. While Mom isn’t that much larger than Dad, it might make all the difference in a cold weather event.

Lundy’s work was based on artificial incubators, where temperature and humidity can be tightly controlled. Mom and Dad live in the real world, subject to real weather events. Yet they have an excellent record of hatching eggs. Can we guarantee that the eggs won’t freeze or suffer cold damage? As much as we’d like to, we can’t. But Mom and Dad are diligent, experienced parents with a well-built nest cup, a long-term territory, and a good food supply. They are spending a lot of time clamped down on their eggs, applying heat from their very warm bodies directly to the eggs. Unlike us, they don’t need an instruction manual to provide information about temperature regulation, egg-turning, or care. We are watching and hoping for the very best. Go Decorah Mom and Dad!

PS: Sherri Elliott has mentioned the difference in the egg ‘line-ups’  - Mom incubates the eggs in a triangular pattern, while Dad incubates them in a straight line. This almost certainly has to do with the size difference between the two (and the overall area of their brood patches), although ‘shapes’ are not uncommon. Like sitting and standing, shapes and positioning can help regulate temperature and keep eggs alive.

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Resources

Stalmaster, Mark V., and James A. Gessaman. "Ecological energetics and foraging behavior of overwintering bald eagles." Ecological Monographs (1984): 407-428.

Incubation zones: Brinsea Incubation. Check them out for incubators, hatchers, brooders, and information:
http://www.brinsea.com/

Cornhusk Insulation: http://www.newscientist.com/blogs/shortsharpscience/2009/05/insulation-made-from-corn-wins.html

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Did you know?

Some birds incubate eggs by burying them in mounds. Meet the Megapodes! http://en.wikipedia.org/wiki/Megapode

Does Bad Weather Affect Bald Eagle Egg Laying?

April 20th, 2011: Spring Snowstorm

Facebook fan Dianne Wisse asked the following question about the Decorah Eagles: "With the extreme winter weather predicted to continue and even worsen--more 'polar vortex' cold, snow and dangerous high winds--is there any chance it will affect Mom/Dad Decorah's chances of laying viable eggs, or might nature step in to prevent the loss of eggs/eaglets to the unusual weather?"

We didn't begin tracking the eagles egg-laying schedule until 2009. It is as follows:

• 2013: Egg laying unknown. Hatch believed to have started on 3/25/2013
• 2012: Egg laying began on 2/17/12. Hatch began on 3/27/2012
• 2011: Egg laying began on 2/23/11. Hatch began on 4/01/2011
• 2010: Egg laying began on 2/25/10. Hatch began on 4/03/2010
• 2009: Egg-laying began on 3/02/09. Hatch began on 4/09/2009

2013 was, like 2014, cold and snowy. 2012 was unusually warm, although the first half of 2011 was 'on the cool side', according to the Iowa Department of Agriculture. If weather impacted the timing of egg-laying, we would expect egg laying to happen a little later in cold,snowy years. But that's not what happens. Instead, Mom lays eggs a little earlier every year, regardless of the weather. 

I did a little reading about the regulation of bald eagle reproductive rates. In a study of bald eagles in southeast Alaska, researcher Andrew J. Hansen found that breeding activity is related to food abundance and habitat quality. He wrote: "Laying date was influenced by proximity to spring food supplies. Pairs nesting within 3 km of food patches laid eggs earlier than those nesting farther from food clumps." Hansen also found that laying date was advanced - that is, it happened sooner - where food was abundant. He believed that earlier laying was probably advantageous, since juveniles that fledge earlier have more time to acquire the skills required for surviving harsh weather and unpredictable food supplies. 

So cold and harsh weather don't influence egg-laying dates directly. But both can influence food availability, which does. During extremely cold and snowy winters, rodents might remain concealed under thick snow pack, fish and fish carcasses might be sealed under ice, and roadkill might be plowed under snow drifts, unavailable until the snow melts. In years where food is scarce, egg-laying happens later. If food can not be obtained, the nesting territory might be abandoned altogether. 

Fortunately, the Decorah eagles have an abundant and highly available food supply. It includes dead and live trout from the nearby hatchery, suckers and other fish from the creek, animals attracted to the 'food oasis', pigeons and other non-migrating birds that live in the area, and road kill. The eagles also have a lot of experience living here - they know their territory and its seasonal rhythms very well. The abundance and reliability of their food supply allows Mom to lay eggs a little earlier than she might otherwise. Experience might be the reason her laying date has crept up since we began keeping track in 2009.  

So in short, I don't expect her to lay egg #1 later significantly later than she has the past few years, although I'm not willing to predict a date. We've seen Mom gently footing and pecking Dad, and both eagles have increased their visits to the nest. Watch for copulation - that will help us estimate a time frame!

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References

Regulation of Bald Eagle Reproductive Rates in Southeast Alaska
Andrew J. Hansen
Ecology , Vol. 68, No. 5 (Oct., 1987) , pp. 1387-1392
Published by: Ecological Society of America
Article Stable URL: http://www.jstor.org/stable/1939222

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Did you know?

Latitude also influences nesting chronology. From the US Fish and Wildlife Service:

• In the Southeastern US, egg-laying can begin in mid-to-late October
• In the Chesapeake Bay region, egg laying can begin in mid-January
• In the Northern US (including Iowa) and the Pacific region, egg-laying can begin in late January
• In the Southwestern US, egg-laying can begin in late December
• In Alaska, egg-laying doesn't start until the beginning of April.

Follow this link to learn more. The nesting chronology is on page 6.

For more on eagles and cold weather, follow this link: http://raptorresource.blogspot.com/2014/01/eagles-and-cold-weather.html

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: http://www.dnr.state.mn.us/birdfeeding/winter/index.html

For more on feathers and weather, read this blog post: http://raptorresource.blogspot.com/2012/02/it-was-hard-not-to-feel-sympathy-and.html

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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

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

Birds and Weather

We've heard a lot of concerns about weather from nest watchers.  Following the nicest March anyone in the American Midwest can remember, we've had tornado outbreaks, high winds, damaging storms, large fires, and snow. The Decorah Bald eagle nest has been tossed around by wind, the Fort St. Vrain eaglets died of exposure following a spring storm, and the Dairyland Power Genoa cam was damaged by lightning. We get asked daily if we can't do something about the weather: shore up nests, build roofs, and even take young birds from their nests. The short answer: No. I wrote about intervention in an earlier post. We intervene in the case of obvious, acute danger, but weather is a chronic condition that all of us have to live with.

High winds, severe storms, and fire are nothing new. If animals couldn't survive them, they wouldn't be here. The birds we watch are well prepared for the weather challenges they face.

• 
  It's Cold and Wet! Feathers are wonderful insulators and adult Bald eagles have roughly 7,000 of them. Light and fluffy down feathers trap air, while flight feathers hook together like a zipper, forming a continuous 'vane'. Preening releases an oil that coats the outer layer of feathers, which further protects adult birds from moisture: the outer feathers may get wet, but the bird's body does not. We've watched the adult eagles shake off rain and sleep under a blanket of snow. They are well prepared for cold, wet weather.
  
  As of this post, all of the Decorah eaglets have their second down. Unlike the first fluffy white down, the 'wooly' second down has insulative properties. Mom and Dad will still protect the eaglets from rain and snow, but they are able to thermoregulate now.

• 
  It's Hot! We've had a lot of questions about panting this spring. Like dogs, eagles don't have sweat glands. They control heat by panting, radiation through their unfeathered legs and feet, and perching in the shade. March was unusually warm and the cottonwood doesn't leaf out until April, so the eagles spent a lot of time in direct sunlight. We also got to see a little more of the eaglets than we usually do: many cam watchers commented on how lively, naughty, and busy this year's brood has been. The dry, hot weather means that the eaglets spend more time exploring the nest than huddled under Mom or Dad, and everyone spends more time panting.

• 
  It's Windy! Adults and young hunker down against high winds and storms. Some of their behavior appears instinctive and some is learned. The first year Mom nested here, she turned tail to the wind and was blown out, nearly taking a young eaglet with her. That hasn't happened since. The adults minimize their profiles during high winds, keeping wings tucked in, tails low, and young birds protected. Young birds curtail nest exploration and play, sheltering under Mom or Dad until the wind dies down.  When the adults do get pushed around by sudden wind gusts, they don't grab at the eaglets, and the eaglets are not especially aerodynamic at this point.
  
  We've also heard from watchers concerned that the Decorah Bald eagle nest will fall/has fallen. The nest weighs approximately 1300 pounds, is tightly woven of strong wood, and is wedged securely into the limbs of a cottonwood tree. The tree is healthy, with no large dead limbs, and near a good water source.  Can we say that the nest will never fall? No. But is it likely to fall? Also no. The nest is very secure and the tree is healthy and relatively secluded from wind. If the nest or tree did go down, we'd be on site as quickly as we could.

In short, we can't do anything about the weather. Fortunately, the birds we watch are equipped to survive it in all but the direst cases. The Decorah nest in particular benefits from an ample food supply, experienced parents, and a relatively sheltered location. Last year, we watched them respond to cold weather and late snowstorms. This year, we've watched them respond to warmer, windier conditions: they've spent more time off the eggs and young babies, and their 'hunker down' time has been more likely to be a response to wind than rain or cold weather.

Here are a few videos that show the Decorah eagles under various weather conditions:

• April 16th, 2011: Snow storm. http://www.youtube.com/watch?v=ibPf2D87Dg4
• June 15th, 2011: Wind storm. http://www.youtube.com/watch?v=6j90phejogY&feature=youtu.be
• March 10th, 2012: High wind. Mom floats in with crib rail. http://youtu.be/Rpx0bHOwY0I
• March 15th, 2012: Panting. http://youtu.be/X-HBBqYfB_M
• March 30th, 2012: Thunder storm. http://youtu.be/abDevjFZX3U

Things that helped me write and learn about this topic:

• Bob Anderson, personal communication
• RRMod, personal communication
• Feathers: http://askabiologist.asu.edu/explore/feather-biology
• Feathers: http://www.baldeagleinfo.com/eagle/feathers.html
• General Info: http://www.learner.org/jnorth/1997/critters/eagle/AnswerEagle.html#49
• General Info: http://en.wikipedia.org/wiki/Bald_Eagle
• A Model of Energy Consumption in Bald Eagles: http://elibrary.unm.edu/sora/Wilson/v097n02/p0148-p0160.pdf
• Our forum members. Thanks for your notes and caps!

If you would like to learn more about how animals survive cold weather, I highly recommend Winter World: The Ingenuity of Animal Survival by Bernd Heinrich. If you would like to know more about animals in summer, I highly recommend Summer World: A Season of Bounty, also by Bernd Heinrich.

Eagles, Weather, and Feathers!

It was hard not to feel sympathy and concern for Mom last night as she lay in the nest after egg #2, rapidly disappearing under a blanket of snow. Fortunately, she had roughly 7,000 feathers to protect her from the weather.

Different sources provide different answers about how many things birds do with their feathers, but all of them agree that insulation is important. The snow piling up on Mom's back last night provided a clear picture of the insulative properties of feathers: Mom wasn't losing enough heat to melt the snow away from her back. I wish my roof was so efficient.

Feather Anatomy

Image courtesy Ask a Biologist: http://askabiologist.asu.edu/explore/feather-biology

There are two basic types of feather: vane feathers, which cover the birds exterior, and down feathers - looser, fluffier feathers underneath the vane feather. All true feathers are branched: they have a central rachis with rows of barbs on either side. However, down remains light and fluffy, which traps air and helps insulate the bird, while flight feathers hook together like a zipper, to form a continuous 'vane'. Personally, I think of it as dressing in layers: the vane feathers form a sort of 'overcoat' underlain by the soft, insulative down feathers. Given that snow also has insulative properties, Mom was quite warm and cozy under her blanket.

So how else do birds use their feathers? According to 'Ask A Biologist', they help birds fly, keep warm, control body temperature, provide weather protection, aid in swimming, diving and floating (waterbirds and piscivorous birds), snowshoe (Ruffed grouse), toboggan (penguins), brace, feel, hearing (owls, harpy eagles), making sounds, muffling sounds (owls), foraging, keeping clean, aiding digestion, constructing nests, transporting water, escaping from predators, sending visual signals, and camouflage. I think I'm jealous!

Since feathers do so many things, it is unsurprising that they come in more than one type.

Types of Feathers

Image courtesy Ask a Biologist: http://askabiologist.asu.edu/explore/feather-biology

The feathers we find dropped in the woods most often are tail and flight feathers. They seem the same but are actually a little bit different. Tail feathers are balanced evenly left and right of the rachis, while flight feathers have a wider and narrower side. This helps them cut through the air with very little drag. Of course, wing shape also influences flight, but that is for another post. The downy feathers that are good for warmth are shown on the far right. Semiplume feathers are also insulative and help water birds float (although some diving birds are more concerned with going deep - their feathers become waterlogged to help them sink). We think the bristle feathers, found around eyes, nostrils, and sometimes the mouth, help protect those sensitive areas. Filoplume feathers are can be found around the tail and flight feathers. They are thought to be used to sense when the flight feathers need to be maintained.

Although feathers seem light, all of them put together weigh roughly two or three times more than a bird's skeleton does. They also require a lot of maintenance. Most birds have a preen gland near their tails. This gland secretes oil which they spread over their feathers with their beaks. Preening helps remove dust, dirt and parasites from feathers and also aligns them properly. Even with care, feathers eventually begin to suffer damage and must be replaced through molting - an itchy-looking process that renders some birds (but not eagles) flightless. However, the benefits of feathers - flight, protection, insulation, display - far outweigh their costs. We sleep warm in our beds with the benefit of furnaces and blankets. All Mom and Dad need are their feathers.

For more information on feathers, I suggest this website: http://askabiologist.asu.edu/explore/feather-biology. I also took some information from Colin Tudge's book The Bird.