It soars the skies with an effortless elegance. It can catch falling prey mid-flight and even dive-bomb into rivers to snag wriggling salmon. There is no question about it: the American bald eagle is a highly specialized predator. But what is the evolutionary background that allows the bald eagle to pull off these amazing maneuvers?

It may be difficult to believe, but the ancestors of bald eagles have lived on Earth for eons. Evolutionary biologists believe that birds evolved from reptiles. From their skeletal structure to their blood cells, today’s birds share a surprising evolutionary foundation with reptiles. According to biologists, feathers may have evolved from the scales of reptilian skin.

Between 144 million and 66 million years ago, during the Mesozoic era, the first birds began to evolve. Fossil records from this period reveal that the earliest birds had teeth. Eventually, tens of millions of years ago, an ancient group of birds called kites developed. Like today’s bald eagle, early kites are thought to have scavenged and hunted fish.

Finally, about 36 million years ago, the first eagles descended from kites. First to appear were the early sea eagles, which — like kites — continued to prey on fish and whose feet were free of feathers, along with booted eagles, which had feathers below the knee. While no one knows precisely when the bald eagle appeared on the scene, the earliest known fossil remains that closely resemble the bald eagle date back to about a million years ago.

Like the kites, bald eagles have featherless feet, but they also developed a range of other impressive adaptations that help them hunt fish and fowl in a watery environment. Each foot has four powerful toes with sharp talons. Tiny projections on the bottom of their feet called “spicules” help bald eagles grasp their prey. Ospreys, which are also believed to be descendents of kites, have similar projections on their feet. Could these spicules be a remnant of the birds’ shared reptilian ancestry? A bald eagle also has serrations on the roof of its mouth that help it hold slippery fish, and incredibly, the black pigment in its wing feathers has been found to strengthen the feathers against breakage when diving into water.

Obviously, there is much more than their striking white heads that sets these iconic raptors apart from the crowd. Their incredible physiology, built for life near the water, is literally millions of years in the making.

Photo © Laura Johnson

From its headwaters in Minnesota to the mouth of the Ohio River at Cairo, Illinois, the Upper Mississippi River is flanked by diverse terrain sheltering a wide variety of wildlife.  Nearby prairies make perfect havens for rabbits and mice.  Possum, skunks, red fox, deer, woodchucks and wild turkeys scurry amid the trees in the hardwood forests that cover the high bluffs and hillsides overlooking the river.  The waters are filled with fish and overhead, ducks and geese traverse the skies while bald eagles thrive in the upper branches of cottonwoods.

That’s right. Bald eagles have returned to the Upper Mississippi Valley. In the 1960s, ecologist and writer Rachel Carson, who had publicized the claim that DDT was killing bald eagles and other birds, organized a bald eagle count along the entire length of the river that turned up only 59 individuals. Now, up to 2,500 bald eagles spend the winter along the Mississippi, with a smaller — though increasing — number staying year-round to nest.

While most adult eagles that live in southern states will remain in their nesting area through the year, bald eagles that spend the mating season in colder regions must migrate to survive.  In October, juvenile eagles in Canada and the northern reaches of Minnesota and Wisconsin are the first to leave.  Even those migrating for the first time find their way by instinct. By mid-November, adult eagles that nest in these colder areas will also start to head south as their nesting lakes begin to freeze. 

It’s not the cold itself that poses the greatest danger when winter sets in. These birds are actually quite rugged. Instead, it’s the need for a stable food source that drives the migrations. Living in the cold requires more energy, and thus more food.  In winter, an eagle must consume 6 to 11 percent of its body weight each day.  Their primary food source is fish, supplemented by ducks, coots, carrion and small mammals. When ice begins to form their access to food is cut off, and eagles move to where the water is still open.

The migrating eagles make their way by riding thermals, currents of warmer air that rise from the bluffs and hillsides lining the river corridor. When conditions are right, bald eagles can travel up to 270 miles in a day.  But, when tailwinds dissipate or thermals are scarce, bald eagles will linger.  In time, many will find their way to the Upper Mississippi River Valley, where stretches of the river are fed by tributary waters and do not freeze.

Elsewhere on the river, human intervention has unintentionally created a favorable environment for raptors. At one time, the entire Upper Mississippi flowed freely from its source all the way to where it joins the Ohio River. In the 1930s, however, the US Army Corps of Engineers began constructing what is now a system of 29 locks and dams on the Mississippi.  Where surging rapids and waterfalls once prevailed, humans created a navigable transportation route that supports both recreation and commerce.  As it turns out, these changes were not only good for cargo barges that carry petroleum, coal, and agricultural goods. They were also good for the eagles.  In winter, when bald eagles spend upwards of 90 percent of daylight hours roosting in order to conserve their energy, the fish that have been stunned or killed in the turbulent waters around dams provide an easy meal.  Elsewhere, power plants use river water for cooling, then pump the warmed water back into the river. In these places, large groups of eagles can be seen gathering around the open waters, freed from the ice, where they can take advantage of plentiful, and accessible, supplies of gizzard shad and other fish. 

Near the end of winter, as early as February, those eagles that have migrated south begin to follow the thaw back to their northern homes. Others, like the nesting pair at the Iowa hatchery in American Eagle, never migrate. They stay within several square miles of their nesting site throughout the year. During the spring and summer, all the eagles in the area become very territorial, protecting the resources that will sustain their young.  But at least with the migrating  “visitors” gone, the riches of the Upper Mississippi River Valley belong to its year-round residents.

Photo © Neil Rettig

Four years before the bald eagle was listed as an endangered species in 1967 there were fewer than 500 breeding pairs in the lower forty-eight.

Click around the following maps to find out how many bald eagle breeding pairs there were in each of the lower forty-eight states from 1990 to 2000.

Photographed by three-time Emmy Award winning filmmaker Neil Rettig, “American Eagle” is the first HD hour on bald eagles ever. In this behind-the-scenes exclusive, Rettig talks about the making of the film while he captures footage of the spring migration along the Mississippi River. Rettig shares his favorite experiences from the filming and explains why he believes wildlife filmmakers have a special responsibility to move people to think about the natural world, now more than ever.

Thank you for all of the comments so far -– I’m touched to see how the film has affected people so deeply. We’re happy that our story of the bald eagle was able to reach such a wide audience. Neil dreamt for years about making a bald eagle film, so the broadcast of this show (and your positive comments) mean the world to him.

I’ve enjoyed reading your observations and look forward to corresponding with those who felt moved to write a comment on this website. I wanted to answer some of the posted questions and to add feedback on some of the postings to date:

• To answer Ryan’s question, the fish hatchery nest is situated in Decorah, IA. The eagles live on private property though, so no intruding! :-> On a side note, the one-eyed female once caught two trout with one foot. She was a very impressive hunter when she needed to be.
• Our production team was very moved by Path’s poem. The section of the film involving the death of the one-eyed female and egg was (by far) one of the toughest scenes I’ve ever been a part of editing. My wife was pregnant at the time Neil Rettig reported the deaths to us, so I felt the loss on a deeper level than expected. I was happy though that the male was able to rebuild his family during the next season and that we were able to capture his story for our film.
• On the other American Eagle comments page, there have been some posts questioning the validity of the impact of DDT on bald eagle eggs. This allows us to delve into the issue a bit further. Be assured, our team thoroughly vetted all of our sources. We were keenly aware of the dissenting opinions regarding the effects of DDT on bald eagles, so we were especially careful and thorough with research on this point. We reviewed a variety of reports published by well-respected journals and took into consideration not only the information in the reports, but also the sources they cited. When considering the evidence appearing to indicate that DDT had little effect on bald eagles or their eggs, the quality of the information or sources did not meet with our strict standards; therefore, we feel confident that our film’s comments about DDT are accurate.
• I enjoyed looking at Darlene’s photography — especially photos #16 & 19. (The bald Eagle’s wingspan is ridiculously large, and those photos show it off quite well.) Thank you for posting the link to your website. There is nothing like being up close to a bald eagle (although they are not half as scary in person as the harpy eagle — Neil is the caretaker of one on loan from the Peregrine Fund).
• To re-chime the bell (as rung by Tom), Neil is the best raptor cinematographer I’ve ever worked with. He spent almost two years filming this program and his meticulous attention to detail and his tireless work ethic comes across in the final product. The quality of the footage is extraordinary, considering too that bald eagles are shy by nature and do not like getting close to video cameras. Canon deserves a special shout-out for loaning us a 40X Zoom lens with a built-in stabilizer every once in a while. If you’re interested in picking one up, here’s the link: http://www.canon.com/bctv/products/hj40×14b.html
• For Sharon V (and to the other bird rehabilitators who wrote in), good luck with your continued success in raptor rehabilitation. Neil and I worked on a separate program titled Raptor Force, which aired last year on NATURE. I’d recommend watching it if you get the chance. It’s definitely a different film than American Eagle in a number of ways but also goes into great detail on raptor flight and hunting techniques.
• As Deb mentions, there are a ton of webcams that follow nesting pairs of raptors. Bob Anderson maintains a number of such webcams — here is his website: http://raptorresource.org/falcon_cams/index.html

Thank you again for all of your comments. I look forward to checking in every once in a while to answer any questions you may have about our film. And if you feel like a change of pace, please watch our next film on NATURE, Sunday January 25th at 8pm. It’s all about skunks!

Sincerely,

James Donald, Producer

In the 1960s, the bald eagle was on the brink of extinction caused by the pesticide DDT and other human pressures. Following their protection as an endangered species, bald eagles have come roaring back. But even in the best of times, life in the wild for these birds is a surprisingly tough struggle.

From the pristine wilderness of Alaska to the Upper Mississippi River Valley, American Eagle goes behind the scenes and into the nest to provide the ultimate bird’s eye view into the private life of an American icon.

Unique to North America, the bald eagle is the continent’s most recognizable aerial predator, with a shocking white head, electric yellow beak, and penetrating eyes. In the 1960s, this symbol of the United States became an emblem of environmental degradation as the pesticide DDT and other human pressures brought it to the brink of extinction. But following their protection as an endangered species, bald eagles have come roaring back. Photographed by three-time Emmy-winning cinematographer Neil Rettig, this first-ever HD hour on bald eagles is an intimate portrait of these majestic raptors’ lives in the wild. Buy the DVD. This program premiered November 16, 2008.

American Eagle premiered Sunday night (check local listings). If you watched, we’d love to hear what you think!

The bald eagle may be an American icon, but most of us know very little about this aerial predator. American Eagle goes behind the scenes and into the nest to provide the ultimate bird’s eye view into the private life of this striking raptor.

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