Saturday, February 06, 2016

Philippine Eagle Puppet Feeding

How do we learn to become human? From fellow humans, of course! We are raised by humans, fed and cared for by humans, and taught language by humans. Parents, relatives, friends, teachers, and other people around us teach us language, cultural norms and values, and the ins and outs of social interaction. From them, we learn the information and skills we need to function as members of our society at any given time in our lives. As we grow, we gain companions, friends, enemies, co-workers, rivals, and perhaps mates who will shape our lives and help us raise the next generation of children to instill with our cultural norms and values. 

The same is true of animals. Peregrine falcon and bald eagle parents spend a lot of time interacting with their young: brooding them before they old enough to thermoregulate, stuffing them with food at every opportunity, vocalizing (think of a peregrine falcon chupping encouragingly at a hatchling), and playing post-fledging chase and hunt games in the first few weeks of flight development. Birds learn to be birds from other birds: the vocalizations, songs, or calls of their species, flight skills (think of fledgling peregrine falcons chasing one another, or a flighted predator chasing down flighted prey) and, among birds that are social, the intricacies of social interaction. As they grow, they learn how to acquire food, avoid danger, find mates, and raise the next generation of young. 

In birds and mammals, this process starts with imprinting, defined as "Rapid learning that occurs during a brief receptive period, typically soon after birth or hatching, that establishes a long-lasting behavioral response to a specific individual or object, as attachment to parent, offspring, or site". In short, we first recognize our parents, which lays the groundwork needed for us to recognize and interact with our species and eventually find mates. This type of imprinting is referred to as "filial imprinting". 

But what happens when a human is responsible for raising a bird? If humans are a bird's only role models and source of food or protection, than it will imprint on humans for species recognition. As a sexually mature adult, it will reject its own species and seek the attention of humans. People raising birds for release in the wild take steps to prevent human imprinting from happening, including foster-parenting and puppet-feeding.

When Bob was raising falcons for release, he turned young birds over to a female 'foster-falcon' at around 10 to 12 days of age. By feeding and caring for them, "Freddy" imprinted the young falcons with a falcon model. As juveniles and adults, they sought a bond with their own kind rather than with us.
Philippine Eaglet feeding. Photo by Kike Arnal.
However, foster parenting isn't always an option, especially among critically endangered birds. Kike's photo shows Philippine Eagle Center staff puppet-feeding a Philippine eagle nestling. The person behind the mask imitates a Philippine Eagle parent: vocalizing, interacting with the eyass, feeding it, and providing protection. The eaglet's room is painted to look like the forest it will eventually live in and contains the sounds that will surround it as an adult. These steps imprint the young eaglet on a Philippine Eagle model, establishing a behavioral response to other eagles and forest habitat, rather than human beings and an urban habitat. The puppet is our way to teach a Philippine eagle that it is an eagle, not a human.

Imprinting is a fascinating topic and we'll write more about it later this year. There is a reason that adult eagles chirp to hatching eggs! 

Things that helped me learn and write about this topic:

Learn more about the "Father of imprinting", Konrad Zacharias Lorenz. 

Monday, February 01, 2016

Sounds of the Eagles

I just finished the book Birdsong by Don Stap, a book which seeks to answer the question Why do birds sing and what do their songs mean? Most studies of bird song have been done on oscine birds, which are a subset of passeriformes with a complex voicebox. Many oscine birds such as chestnut-sided warbler, brown thrashers, hermit thrushes, and starlings sing complex sounds that might include, depending on the bird, buzzing, clicking, trilling, and warbling. Non-oscine birds like bald eagles (an accipitriform) might fascinate us with their striking colors, amazing flights, and sheer size, but their vocalizations don't tend to overwhelm us with their beauty or musicality.

After reading Birdsong, I decided to download some spectrogram software and run a few eagle vocalizations through it. The software I chose was Praat, a powerful free acoustical analysis program available from the University of Amsterdam. I wanted to see how 'questionable' copulatory vocals (are they mating?) appeared next to known copulatory vocals (they are mating!).

This was the sort of thing I tended to see. Copulation followed a pretty clear pattern, and Dad also tends to have a higher voice than Mom as shown by the sonograms above. 'Was that copulation?' is Mom alone and was not, in fact, copulation: the bars are low (signaling Mom's deeper voice), thick, and pretty distinct. 'That was copulation!' starts with a brief vocal by Mom at left, and continues into Dad's copulatory vocals - high, quick, and a little complex.

Did all eagle vocals look somewhat like copulation? While Dad's copulatory vocals tend to have a unique sound (I listen for a little warble or chirp and count to seven), eagles don't have complex voiceboxes, and the top vocal isn't that different from the bottom vocal. I decided to compare known non-copulatory vocals next.

Look at 'Mom with a Fish'. It was a cool sounding vocalization and doesn't really look much like copulation. If anything, her initial vocalizations look a lot like Dad's in the 'Sound a Warning' spectrogram , although they are lower. She was mantling a fish after what sounded like a couple of warning vocalizations before she came to the nest, and concentrated energy in high and low pitches, just like Dad did with his initial warning to an unknown neighborhood intruder. The second part of 'Sound a Warning' features Mom and Dad rapidly vocalizing together. While this somewhat resembles the 'solicitation' spectrogram, the vocal pulses are a little farther apart and higher. Personally, I think that 'Mom solicits Dad' most closely resembles copulation. Is she vocalizing the 'music of love' to indicate receptivity? Note that the unknown noise in the spectrogram figure prior to this one really closely resembles her solicitation.

Of course, I also had to compare known copulations. Keep in mind that we used different microphones in 2012 than we do now, and they were located in a slightly different position relative to the nest.

Even given the changes since 2012, these spectrograms look quite similar! It appears that copulatory vocalizations are quite distinct, and spectrograms could be used to help determine whether those early to mid-January vocals are copulatory in nature. After all, we can't always see what Mom and Dad are doing in the upper branches!

While this was more of a thought experiment than anything else, it looks like bald eagles have a little more to say than we realize, even if they aren't singing when they do it. I am looking forward to recording their interactions with eaglets later this year.

I am not a sound recordist. I am sure these spectrograms would benefit from professional equipment, but I have a 64-bit ASUS PC laptop with no special audio equipment whatsoever. The spectrograms were made recording youtube videos via praat in the quietest room of my house, with no one home except myself and two dogs. They were taken from the following videos (note: I can't find the 'was that copulation?' from 2016):

Praat audio software:

Monday, January 18, 2016

Martin Luther King Day: Three Conservationists of Color

Dr.Robert Bullard. By Dave Brenner
via Wikimedia Commons
Martin Luther King Day got me thinking about African-American people in conservation and environmentalism. The three people below represent a very small proportion of Americans with African heritage involved in conservation and environmental issues. There were a lot of potential choices, but I chose Dr. Robert Bullard and Rue Mapp because I thought their work reflects Dr. King's legacy of justice and equality (plus I love Mapp's business), while John James Audubon was a surprise and delight to me.

Dr. Robert Bullard, Father of Environmental Justice
Born in 1946, Dr. Bullard began his work in Houston, Texas in the late 1970s. An environmental sociologist, Dr. Bullard identified the siting of garbage dumps in black neighborhoods as part of a wider systematic pattern of injustice. Dumping in Dixie, a book he wrote about his work, is regarded as the first to fully articulate the concept of environmental justice, beginning with the premise that all Americans have a basic right to live in a healthy environment. He has authored eighteen books in total that address sustainable development, environmental racism, urban land use, industrial facility siting, community reinvestment, housing, transportation, climate justice, emergency response, smart growth, and regional equity.

Dr. Bullard remains one of the leading voices of environmental-justice advocacy. He was one of the planners of the First National People of Color Environmental Leadership Summit in 1991, at which the organizing principles of modern environmental justice were formulated. The Sierra Club recognized his work with the John Muir Award in 2013 and named their new environmental justice award after him in 2014. Bruce Hamilton, Bruce Hamilton, the Sierra Club’s deputy executive director, sees Bullard’s work as central to the Sierra Club’s work today. “The fight for environmental justice should be at the heart of the environmental movement,” Hamilton said. Bullard has expressed concern about the diversity of the national environmental movement, which has not addressed the environmental issues that face low-income and minority communities.  “The right to vote is a basic right, but if you can’t breathe and your health is impaired and you can’t get to the polls, then what does it matter?” When asked what kept him going in his quest for environmental justice, Dr. Bullard replied, "People who fight. People who do not let the garbage and petrochemical plants roll over them."

Thanks to Robert Bullard, the father of environmental justice, we have begun to understand and articulate the concept of environmental justice. I believe it is time for all of us to work harder at realizing it.

Rue Mapp, Entrepreneur and Outdoor Enthusiast
Rue Mapp grew up in Oakland, California, but loved to spend time on her family's ranch north of Napa, where she hunted, fished, biked, and spent hours exploring the woods. In 2009, she rejected the path of business school to begin Outdoor Afro, a business reconnecting African-Americans with the outdoors. The business grew in part from Mapp’s dismay at finding herself among relatively few people of color who embraced the great outdoors. “I didn’t see enough people who looked like me. There was a huge number of people missing out.”

Outdoor Afro currently has 30 trained leaders and over 7,000 active members. Mapp's volunteers lead hiking, climbing, rafting, and camping trips, connecting participants with black history and nature on each one. For now, she is working hard to reconnect people of color to bigger outdoor spaces. But in twenty years, she is “...hoping we’ll be able to go out and experience nature, and it’s no big deal.”

Rue Mapp is leading the way in getting people outdoors while helping us understand that outdoor spaces and experiences are for everyone.

John James Audubon
I suspect most readers will be as surprised by this as I was. John James Audubon, famous wildlife artist and the inspiration behind the National Audubon Society, was born in 1785 in Haiti (then called Saint Domingue), the illegitimate multi-racial son of Jean Audubon, a French sea captain, and Jeanne Rabin, a black Creole slave woman from the Congo who was Audobon senior's chambermaid and mistress. While not all accounts of his parentage use the word 'black' or 'african', Creole is defined as a person of mixed European and black descent, especially in the Caribbean. Audubon is famous for his illustrations in "Birds of America", an anthology of more than 435 species. He also expressed concern about the destruction of birds and their habitats in his later writings and is credited with conducting the first bird banding experiment on the North American continent, which makes him the father of bird banding in the US and Canada.

The Raptor Resource Project is proud to be part of his legacy.

I really enjoyed researching this blog and hope to introduce readers to more conservation heroes this year, including some more favorites of those of us involved with the Raptor Resource Project.


Monday, January 11, 2016

Tech Note: Facebook issues, streaming problems, and other things that go bump on your device!

It's only a little over a month before the Decorah Eagles usually start laying eggs, so it seems like a good time to blog about some technical issues and solutions before the season gets going!

Facebook Issues

I followed you on Facebook, but I'm not getting your notifications and posts...
We're hearing from Facebook fans whose news feed is not showing our Facebook page, even though they liked it. Make sure per the image at right that you are signed up to receive notifications in your newsfeed. If you are, comment on one of our threads - even with just one word! - and we'll be back in your news feed. Keep commenting and sharing pictures and posts to keep seeing us. Facebook algorithms no longer show content that people don't interact with, as documented here: and here: Of course, the articles don't talk about Facebook's dirty little secret - bots that pretend to be real people. Limiting your news feed to things you are interested makes your feed more rewarding and reduces your changes of interacting with a bot that scams you, gives you malware, or locks up your hard drive.

This SUPER-ANNOYING Facebook pop-up won't let me see your page unless I have a Facebook page too! It didn't used to be that way...
The all-caps annoyed person is me. I am ALL-CAPS ANNOYED that Facebook is spamming people with a register screen if they aren't logged into Facebook. This is a new thing and not something we have control over, although we did file a ticket with Facebook to complain (in a polite non-caps way). We are a business page and as such are supposed to be available to the public, Facebook or not.

To block the pop-up, add adblockplus to your browser if you don't already have it. Ad-block currently has a version for all of the big browsers used on standard Macs and PCs. Add these two lines to your filter:
Don't know how to add filters? Check this tutorial out: This will not remove the option to register or login at the top of the page, but it will stop the pop-up from covering the page.

If you aren't interested in blocking the pop-up, you can reduce it. Try pressing the escape key when it pops up. If it doesn't go away, click the x on the right-hand side to reduce it (outlined inside the red circle below). We will also be posting news and video round-ups on our website at

The 'X' is non-ADA compliant for contrast, which is why it is so hard to see. We made sure to notify Facebook of this problem as well.

Computer and Device Issues

I can't see the Decorah Eagles on my desktop computer anymore. What do I do?
  • Step number one: Refresh the page. If that didn't work;
  • Step number two: Clear your cache and cookies. Close the browser, re-open the browser, and try again. 
If neither of these steps work, you probably have a more complicated problem. Back in 2015, Ustream changed their video player from Flash to HTML5, although the video stream itself is still based on Flash in the desktop environment. On balance, the move towards HTML5 is a very good thing, but it has caused temporary problems for some users, especially those with older browsers or those using firefox. 

HTML 5.0 was released in 2014. Those of you with older browsers (for example, AOL Explorer) should still be able to watch the Flash-based stream but will have lost player features like share, embed, and the ability to chose your viewing resolution. I encourage you to upgrade to a newer browser, since you will loose the ability to watch if Ustream makes the jump to an HTML5-based stream. My two favorites are Chrome and Firefox, but any modern browser should work.

I'm not sure why the jump to an HTML 5.0 player seemed to have confused some versions of Firefox so badly. If you are experiencing problems, your browser might be defaulting to Flash not enabled. You will need to manually enable it in order to view streams. Follow this link to find out what you need to do.

Can I watch the Decorah Eagles on my phone, tablet, or iDevice?
You can! I strongly suggest downloading the Ustream app. Go to your device's store, wherever it is located, and search for Ustream. Install the app and start watching!

Ustream is working to improve the device experience, but please be sure to provide feedback (polite, respectful feedback) about any features that are missing.

Miscellaneous Ustream Issues

Why is the stream choppy?
There could be more than one reason for that, but at my house it is usually due to low bandwidth, especially during evenings and weekends. As shown in the image below, the Ustream player allows you to manually select the resolution that best meets your bandwidth. Click the HD button just below the stream to access your options. Drop it down to 360 or even 240p to reduce bandwidth consumption, or throw caution to the wind and see if you can stream at 720p!

I hate ads! Can I get rid of them?
You can! Ustream runs ads to offset the cost of providing the stream. However, they have an ads-free premium membership option that costs just $3.99 per month. Click here to learn more.

I'm okay with ads overall, but I'm not okay with THAT ad!
Ustream ads are supposed to be family-friendly - they shouldn't contain foul language, sexual content, nudity or near-nudity, and so on. If you see an ad that contains problem content, please let them know (you can let us know, too).  To let Ustream know, file a ticket here. To let us know, email Both of us will want to know what time you saw the ad and where you are watching. This will help Ustream trace it down.

We hope you enjoy watching this year! If you have a technical issue you think should be addressed, leave a comment below and we will see what we can do.

Wednesday, December 30, 2015

Birds and Nest-Building

This blog was inspired by the paper 'The design and function of birds' nests', Ecology and Evolution 2014; 4(20): 3909–3928. It can be found here:

Hard at work on N2B
What determines the design of a bird's nest? You know the type of nest I'm talking about, right? A bald eagle's large platform nest, built with sticks, high up in the branches of a tree.  Or maybe a scrape created in dirt or gravel on a shallow cliff ledge by nesting peregrine falcons. How about the burrows created by bank swallows and belted kingfishers, or the cavity nests excavated in trees by woodpeckers? Or am I thinking of the pendulant and sometimes very elaborate suspended nests created by orioles and weavers?

When I think of bird nests, I think of the classic small cupped nests I collected as a child. But birds nest in a variety of ways and places, creating nests from twigs, sticks, moss, grass, fur or hair, roots, bark, leaves, pine needles, feathers, lichens, paper, insect cocoons, snakeskin, saliva, and other materials. They nest high in trees, but they also nest on the ground, under the ground, in the darkness of chimneys and caves, and even in active wasp nests. Some birds construct elaborate nests while others get by with a shallow scrape in a stolen or abandoned nest.

Nests traditionally have been thought of as nothing more than places for birds to lay eggs and raise young. It was assumed that natural selection and the need to minimize the risk of predation determined the design of completed nests. However, recent research has found that sexual selection, parasites, and the need to build a nest that can handle variation in weather conditions (which we certainly see in Iowa, Colorado, and Minnesota!) also play significant roles in influencing nest design.

Sexual Selection
How does sexual selection play a role in nest design? It goes something like this: birds that build awesome nests relative to other members of their species must be healthy, high-quality mates. Think about Decorah. Mom and Dad spend a lot of time procuring and hauling wood and soft materials into the nest. This is a costly activity energetically speaking, and it requires both eagles to be in good shape - well-fed (which implies provisioning ability), skilled at flying, and relatively parasite-free. While different things might indicate quality to different species of birds, it appears that large nests signify quality among bald eagles.

Bald eagle nests are bi-parentally built - that is, both parents work on them. However, in Decorah it is Dad who spends the most time getting the sticks just right, scraping in the nest bowl, and preparing the egg cup. Studies have found that female magpies adjust their reproductive effort in relation to the male's nest-building activities. The more time he spends working on the nest, the more time and energy she puts toward reproductive processes, including copulation.

What could we watch for in our eagles? A study of male house sparrows found that they call to females when adding feathers to the nest, suggesting that they wish the behavior to be observed. The volume of feathers delivered by males was positively correlated with clutch size and female provisioning rates. While Dad doesn't usually vocalize when he places a stick, he often interacts with Mom in some way - taking a stick from her, moving sticks while she is in the nest, and so on. His obsession with proper placement might be his way of signaling fitness and getting more tailfeather from Mom to boot! I'm going to pay more attention to Mom and Dad's interactions during and following their sticky adventures!

While the paper doesn't touch on intelligence, it is hard for me to believe that it isn't part of the total sexual selection package. Building a nest is a lot harder than it looks and requires a whole host of things: access to food (which implies hunting ability and understanding of a territory, among other things), strength (which implies access to food and a body relatively free of parasites), and flight skills (ditto). An intelligent bird will presumably make better decisions about maximizing energy gain, procuring food, and hauling and placing sticks. Its intelligence should help it live longer and build a sexier nest, which in theory will lead to more copulation, more fertilized eggs, and offspring that inherit Mom and Dad's traits.

Bird parasites include lice, fleas, flies, mites, ticks, leeches, fungi, and bacteria. We've seen firsthand the damage that blackflies and hippoboscids can do: killing young falcons, driving young from the nest to early, and delaying development. Birds have a wide variety of responses to parasites, including molting feathers, the use of feather toxins, preening, and maintaining their nests.

Watchers might remember Stitch and Spot, the red-tailed hawks from Eaglecrest Wildlife. Male Spot brought green plant material into the nest on a regular basis. The greenery (probably blue oak branches) contained high levels of monoterpenes and isoprene, which helped reduce parasites and fungi and may also have signaled fitness to female Stitch in the same way that a large nest might signal quality to a female eagle [a blog on greenery and parasites can be found here].

However, the bald eagles we watch don't appear to intentionally carry greenery into their nests. So what do they do? Many watchers have commented on the number of mice in the nest. For the mice, an eagle's nest offers food (scraps of protein and stomach and crop contents of prey), shelter, and safety from serious mouse predators like fox and owls. While bald eagles would eat a mouse, it isn't much more than an hors d'oeuvres compared to prey like pigeons, trout, squirrels, and so on, so the risk is well worth the payout! In return, mice devour leftovers and help keep the nest a little cleaner and presumably parasite-free than it would be otherwise. As with greenery and hawks, sexual selection might drive a nest-building behavior that also repels undesired guests!

An early nesting season also helps reduce parasitization of young, although it means parent birds need to be ready for the challenges that winter brings. This leads us to...

Environmental Adjustment
Remember 2013? The birds we watch produced young in the teeth of the worst winter since the 1970s. Our eagles were successful in part because the design of their nest influences the microclimate in which eggs and young are incubated. Dad's attention to the nest bowl - rolling, scraping, nest digging, and the careful arrangement of soft materials to form a cup once the substrate is suitable - may both excite Mom and result in a structure that can weather rain, snow, and serious sub-zero temperatures. Of course, if you watch the eagles, you have probably already realized that the types and placement of materials aren't accidental. We've seen the nest bowl and cup carefully prepared ahead of time, and we've seen Dad bring grasses and husks to tuck around Mom while she incubates eggs.

So the next time you watch a bird build a nest, either online or off, remember that much more is going on than meets the eye. Sexual attraction, courtship and flirtation, proof of fitness, and protection of young from parasites and weather all appear to play important roles in the design of finished nests.

Things that helped me learn and write about this topic:

Did you know that?
There is a bird that nests inside a wasp's nest! Follow this link to read more about the Violaceous Trogon and its extreme housing practices!

Monday, November 30, 2015


We have a day for giving thanks. We have two for getting deals. Now, we have #GivingTuesday, Tuesday, December 1st, a global day dedicated to giving back! If you like the work we do, please consider making a donation. You can donate online via paypal or mail a check to the Raptor Resource Project, PO Box 16, Decorah, IA 52101.

We are celebrating #GivingTuesday with some very special guests on our Decorah Eagles Ustream channel at We'll be starting chat at 8:00am Central Time and running through 8:00pm. Our planned schedule (weather permitting) will be:

10:30  Intro - John Howe
11:00  PE Project Update - Neil & Laura
12:00  Trapping and Monitoring - Dave Kester
3:00  Cameras and Projects Update

We will take questions from chat, although you are also welcome to email questions to I can't guarantee we will get to all of them, but please feel welcome to send them!

What does the Raptor Resource Project do? We are a 501c3 that specializes in the preservation of falcons, eagles, ospreys, hawks, and owls. In addition to bringing you the Decorah Eagles, Great Spirit Bluff Falcons, and other birds of prey, we create, improve, and directly maintain over 50 nests and nest sites, provide training in nest site creation and management, and develop innovations in nest site management and viewing that bring people closer to the natural world. Our mission is to preserve and strengthen raptor populations, expand participation in raptor preservation, and help foster the next generation of preservationists.

As a nonprofit environmental organization, we depend on donors, research, and our other programs for our entire budget. With your tax deductible contribution to the Raptor Resource Project, we can:
  • Continue updating to high definition digital cameras in Decorah and elsewhere.
  • Help develop and realize a dream of Bob's:  establishing a Philippine Eagle cam to save a beautiful bird of prey who's very existence is threatened. We are working in cooperation with the Cornell Lab of Ornithology and the Philippine Eagle Foundation. Neil and Kike will be traveling to the Philippines to scout locations with staff from both organizations in February of 2016.
  • Provide one of the world’s largest public wildlife education programs to countless classrooms through our unequaled Ustream channels, interactive chats, facebook page, and blog.
  • Partner with landowners, private businesses, and government agencies to monitor and band peregrine falcons at over 40 sites. 
  • Continue our collaborative raptor nest-box, trapping, and monitoring programs.
These things all take money. As of 2015, our annual expenses were hovering around $99,000 per year:

  • Staff/Compensation costs were around $55,000. We incurred extra expenses for our N2B build and two camera installs: one at N2B and one at Decorah North Nest. These were intensive projects that required a lot of help. The N2B camera installation alone took five full days of work from dawn past dusk.
  • Equipment – computers, camera upgrades and maintenance, tools, encoders, software, transmitters, and so on – cost around $27,400. 
  •  Supplies – primarily cable, tools, climbing equipment, banding equipment, bands, installation hardware, maintenance equipment, and lumber – cost around $2200 annually.
  • Internet access costs roughly $4,500 annually.
  • Other/Miscellaneous costs around $9,000 annually. This category includes gasoline, electricity, travel-related costs, equipment fabrication, and propane so we can heat the shed!
Our income is generated entirely by donations from viewers of our various cams, and we sincerely appreciate your generosity and support of the Raptor Resource Project mission. Would you please help us make a difference with your donation?

Thank you so much for your support and we hope you enjoy watching in 2016!

Friday, November 13, 2015

How Much Can A Bald Eagle Carry?

On November 4th, photographer Alex Lamine photographed the female bald eagle at Berry College carrying a very large stick. The Berry College Eagles Facebook page posted: "Today, photographer Alex Lamine caught an extraordinary occurrence at the nest. Around 7 a.m., as morning light began to creep into the sky, Alex watched Mom Berry gnaw a limb off a tree and begin flying it toward the nest tree. She carried the long limb in her good (right talon) and Alex captured the action. Immediately after, Mom dropped the limb and it landed just a few feet from Alex. It impaled the earth on the heavy end and could have caused a serious injury. Eddie Elsberry weighed the limb later today and it weighs 12 pounds! We've seen both eagles drop limbs from time to time but this is the largest by far that we have seen. Thanks, Alex, for sharing your experience with us. It is truly amazing that she could carry such a heavy limb in one talon!"

We immediately started getting questions about the stick. Could an eagle carry twelve pounds? Was she carrying the stick, or was it actually falling as she held on to it? We've speculated quite a bit about how much weight eagles could carry, but I decided to skip the musing this time and ask some experts. I ended up talking with Professor Jim Grier (Grier has studied birds of prey extensively and owned a golden eagle), Brett Mandernack, Neil Rettig, Chuck Sindelar (a good friend of Bob's who was deeply involved in bald eagle recovery), Jon Gerrard (who wrote The Bald Eagle: Haunts and Habits of a Wilderness Monarch with Gary Bortolotti), and Professor David Bird (among other things, the author of The Bird Almanac, A Guide To Essential Facts and Figures of the World's Birds). It was quite a group of experts, to be sure!

Several of them noted that the amount of weight a bird could carry was highly dependent on the situation, including:
  • Wind direction and speed
  • How the object was carried. Was it streamlined or catching the wind?
  • How dynamic the object was. Did it dangle and swing, or stay steady?
  • How the object was captured. Was it caught and carried in flight (with momentum) or dead-lifted from the ground?
  • The direction the object was being lifted in. Was the object being lifted up or down?
Professor Grier compared eagles with aircraft and talked about dynamics, flight conditions, and how the object was carried: "I sometimes got the impression that my golden eagle could carry as much or more in his crop than in his feet, but never got any good measurements or wrote down details. For example, he could capture a large rabbit that he could not fly with...but then he'd eat most of it and be able to fly with a really full crop. I think the trim of the eagle, as in airplanes, and drag/balance of carried items, is important." He added: "Flight conditions make a big difference, particularly at the limit of weight, as with aircraft. The best conditions are high air pressure with a steady wind plus room and conditions for a good take-off, all of which affect the ability to get airborne and then stay airborne. I've seen bald eagles carry large fish under some conditions, for example, that they couldn't under other conditions."

Think of an airliner. Cargo is balanced around the center of gravity inside the plane, not dangling below it. According to the FAA, center of gravity deviations as small as three inches can dramatically change the handling characteristics of some fully loaded aircraft. When Jim's golden eagle ate a large rabbit, it was for all practical purposes 'balancing' the cargo in its crop, which is located in the center of its neck above the top of its chest. In this case, balance was a bigger issue than weight as far as lift and stable flight were concerned. For more information on how the center of gravity affects flight, watch this video:

Chuck Sindelar wrote about watching golden eagles play with sticks outside of nest-building season. "They would fly off with a stick and gain altitude by circling. A second golden eagle would follow the first up into the sky and would get into a position where,  when the first bird with the stick dropped it, the second bird would dive and regrab it, often before the stick reached the tops of the trees below. Sometimes he would have to pull out of his dive and allow the stick to hit and enter the trees, and then both birds world move a bit away and do this all over again with a new stick." He also pointed out that while he had never personally seen a bald or golden eagle carrying sticks that large, he had often seen them in nests.

Jon Gerrard expressed interest in the stick gnawing and how the eagle ended up with the small end. In his observations: " would be more common for an eagle to fly in to a limb, usually a dead limb, and break it off in flight.  Did the eagle gnaw it off at the thickest part of the limb?   If so, how did it end up holding the thin part of the limb in its talon?   Did it gnaw it part way through at the thick end, and then fly to grab the thin end and fly with it to break it off at the thick end?  And how far did it carry it before dropping the limb?" He recounted a story from the book "The Bald Eagle Haunts and Habits of a Wilderness Monarch", which he wrote with Gary Bortolotti (pages 35 and 36)

"It is of a pair of eagles nesting along the Gulf Coast of Louisiana in the 1890s.  The female regularly caught and carried Snow Geese (Blue phase) - probably weight 4.5 to 6 pounds - for up a a mile and a half to their nest.   The Geese were caught high and the eagle was able to glide down to the nest with the goose.   This feat was repeated on a number of days, and there were apparently 35 blue goose heads found in the nest, suggesting it was a common practice.    The feat was likely possible because the geese were caught high in the air and going to the nest was downhill.   Since this eagle was nesting in the southern part of the range, the female's weight would likely be in the 8 to 11 pound range, so the bird would likely have been carrying about half its weight."

Neil Rettig shared some of the observations he made filming bald eagles along the Mississippi river: "Last week I was able to get a nice shot on video of a bald Eagle near Stoddard collecting a big branch from the canopy of a cottonwood tree.  I have seen this many times.  I also filmed a juvenile bald eagle 2 winters ago catching an adult mallard in the air and having a hard time keeping it aloft. In high winds eagles can lift more, as Jim pointed out.  In winds they can hover for long periods 30 seconds or more to attack and work ducks and coots."

In general, the expert panel felt that in most circumstances, it would be unlikely for a bald eagle to carry much more than 50-60% of its body weight. However, it might carry more if the incentives and flying conditions were right: favorable winds, a down-carry versus an up-carry, a momentum capture versus a dead-lift, plenty of maneuvering and flapping space, a well-balanced load, and a highly desirable object like a large stick or a dead fawn. Jon's questions about how the Berry College female ended up with the end of the stick are very interesting when considering load balance. Did BCF end up with the thin end of the stick accidentally or on purpose? It isn't the end she gnawed.

Professor Bird provided a table from his book, The Bird Almanac, A Guide To Essential Facts and Figures of the World's Birds, which I found absolutely astonishing! According to his information, a bald eagle should be able to carry 108% of its body weight. Check out the American kestrel at 145%, the Pallas's Fish Eagle at 160%, or the tiny Calliope Hummingbird carrying its mate - 116% of its body weight!

                                        SUGGESTED WEIGHT-CARRYING CAPACITIES OF BIRDS

approx. body weight (g)*
item carried
approx. weight of item (g)
percent of body weight
House finch
cloth rag
American kestrel
Chestnut-collared longspur
Calliope hummingbird
Pallas’s fish-eagle

Bald eagle
mule deer
Golden eagle   
UID prey item
Harpy eagle
Steller’s sea-eagle

Table used permission of Professor David Bird. Taken from his book, The Bird Almanac, A Guide To Essential Facts and Figures of the World's Birds

* In all cases, a maximum weight was assigned based on the literature
SOURCES: B.P. Martin,  World Birds. (Enfield, Middlesex:  Guinness Books, 1987); J.  Terres,  ed.. The Audubon Society Encyclopedia of North American birds (New York:  Alfred A. Knopf, 1987).

Does this mean that a bald eagle can always carry 108% of its body weight?  No. But if the conditions and incentives are right, yes! The Georgia State Parks website states that bald eagles weigh 8 to 12 pounds and as watchers know, females are larger than males. So based on Bird's table, a 12-pound stick should be within her carrying capacity if the conditions are favorable.

As watchers also know, BCF dropped the stick. I would guess - and this is speculation! - that she had ideal conditions for take-off, including favorable winds, plenty of room, and a good spot for a drop. However, her load was poorly balanced. When she took off, she was carrying the stick with the heavy end up and to one side. It would have swung rapidly to hang below her, causing her to roll and pitch, and decreasing her lift. Sudden center of gravity changes are never a good thing, especially when the object you are flying with could weigh more than you do! She may have dropped the stick in response, or it could have torn itself from her grasp given its momentum. Either way, it was something to see!

Thanks so much to Jim Grier , Chuck Sindelar, Jon Gerrard, David Bird, Brett Mandernack, and Neil Rettig for talking to me! I feel very honored to learn from such accomplished and intelligent people. Any mistakes in the information presented here are mine. Brett also had some interesting thoughts on eagle vision and night flight that I am saving for another blog.

A few links - some for learning and some for fun!