Cracking the Red-tail Code: Exploring the Diversity of America’s Most Widespread Hawk | Living Bird

The Red-tailed Hawk Project is a multifaceted research effort that aims to understand one of the most abundant, yet mysterious, raptors on Earth. 

From the Spring 2025 issue of Living Bird magazine. Subscribe now.

We’ve all done it.

Maybe you’re whizzing down the freeway when a bulky shape in a roadside tree catches your attention. Your eye and brain make a quick assessment, then you go back to fiddling with your GPS or playlist.

Maybe you’re out birding when a distant raptor silhouette appears on the horizon. You flick up your binoculars for a quick, confirming glance, then drop them just as fast.

Why did you move on immediately? Because in each case, you thought to yourself: Eh, just a red-tail.

Bryce Robinson would like to explain to you why Red-tailed Hawks are not boring birds. Far from it. Robinson—a doctoral student in Cornell University’s Department of Ecology & Evolutionary Biology, with outsized research goals and a work ethic that would make a certain pink, battery-powered bunny envious—thinks red-tails are not only underappreciated; he thinks they are an ideal study subject for investigating the dynamics of avian evolution, given their expansive geographic distribution and remarkable diversity across 16 subspecies that span North America.

In fact, he thinks that Buteo jamaicensis, in all its glorious ubiquity and complexity, could hold the key to understanding many aspects of raptors overall, and birds in general, that have baffled scientists for generations. Questions such as: How can birds of the same species come in a multiplicity of colors? In some parts of their range, like eastern North America, red-tails are almost always brown above with a pale chest and belly, crossed by a cummerbund of heavy dark streaking and (at adulthood) a tail of brick-orange. Yet in western North America, plenty of red-tails are deep chestnut, or a rich chocolate color. One group of red-tails that nests across the far northwest of the continent are the color of charcoal, and lack even that seemingly mandatory characteristic of a Red-tailed Hawk—a red tail.

As Robinson has found, other questions arise from studying that one: To what extent is such color variation a reflection of the genetic structure within the species, and thus a reflection of its evolutionary history? And how does a species with so much variation maintain that diversity of plumage appearances, when scientists might expect that the evolutionary forces of natural selection would push hawks toward some sort of happy medium?

Before getting drawn into the red-tail research rabbit hole, Robinson says he thought he was pursuing a fairly straightforward doctoral dissertation: “I was thinking I wanted to produce a phylogeny that showed the [evolutionary] relationships between all the different Red-tailed Hawk subspecies, something that had never been done. And I was like, that’ll be my PhD.”

Polymorphism in Red-tailed Hawks

Some Red-tailed Hawk subspecies come in different morphs, or color forms. The calurus subspecies in the western U.S. and Canada varies from light morphs (which account for about 80% of their populations) to rarer dark morphs, with a kaleidoscope of intermediate forms.

“But Irby [that’s Irby J. Lovette, director of the Fuller Evolutionary Biology Program at the Cornell Lab of Ornithology and Robinson’s doctoral advisor] had bigger ideas,” Robinson says. “So I started thinking about the Red-tailed Hawk as a system for answering a bunch of questions about raptor ecology and evolution, while also trying to create a knowledge baseline that will help us understand [red-tails] in general, something that to my surprise, and I think to a lot of people’s surprise, hasn’t been done.”

Robinson’s fascination with red-tails has flowered into the Red-tailed Hawk Project, a collaborative effort based at the Cornell Lab of Ornithology, where he partners with raptor expert Brian Sullivan of the Birds of the World scholarly content platform. Beyond the Cornell Lab, the project involves a growing roster of nine like-minded American and Canadian researchers from five other institutions. The multidisciplinary super group of ornithologists employs a welter of study techniques from full-annual-cycle GPS tracking to mapping entire genomes to analyzing stable chemical isotopes, while recording and archiving detailed photographic data on every one of the hundreds of red-tails they’ve sampled.

It’s all part of what promises to grow into a grand orchestrated effort to, as Robinson puts it, ultimately crack the code of the Red-tailed Hawk—the seemingly everywhere, commonplace raptor that is one of the most mysterious abundant birds on Earth.

Widespread, yet Mysterious

How ubiquitous is the Red-tailed Hawk? For one thing, it’s the most-reported hawk in the world on eBird, with 8.7 million observations. Yet ironically, very few scientists are actually studying red-tails. This arguably most common and widespread North American hawk has for decades been largely ignored by science. That’s not unexpected, says Robinson.

“A lot of common [species] get overlooked because they’re common,” he says. “It’s not the sexiest, most attractive thing to focus on the common birds. People overlook how interesting the common species are, particularly from a research context.”

Add to that the fact that funding often flows to species of greatest conservation concern, and red-tails seem to be doing just fine for the most part (though eBird Trends data show a small population decline across its U.S. and Canadian range, especially east of the Great Plains). The lack of scientific inquiry into red-tails is why Robinson’s doctoral research has the potential to be such a game-changer, a directed attempt to pull this bird from the shadows and make sure it remains abundant.

“I’ve seen how important it is to have a deep knowledge of the ecology of a species in order to enact effective conservation,” says Robinson. “For the red-tail, it’s an opportunity to make sure we have that [information] for this very common species, to keep it common.”

While the Red-tailed Hawk has a range that stretches across North America top to bottom and east to west, scientists have divvied Buteo jamaicensis into 16 geographic subspecies. Some of those subspecies are monomorphic, meaning they come in just one color form, or morph. That’s the case with the eastern Red-tailed Hawk, Buteo jamaicensis borealis, which breeds from southern Alberta to the Maritimes and south to east Texas and the Florida panhandle. In a bit of linguistic irony, the subspecies to the north of borealis, in the boreal forest of southern and central Canada itself, is called B.j. abieticola (“dweller of the fir”)—it’s more heavily pigmented than borealis, especially in the belly band and throat, with dark markings at the leading edge of the undersides of the wing. Across much of the West, from British Columbia to northern Mexico, the dominant breeding subspecies is B.j. calurus, which occurs in a variety of dark, rufous, and light morphs that blend and mix almost endlessly.

Other red-tail subspecies include the Krider’s Red-tailed Hawk, B.j. kriderii, which breeds in a pocket of the northern Great Plains and is the palest of the bunch, often showing only a hint of the typical borealis belly band, and a very white head. Fuertes’s Red-tailed Hawk, B.j. fuertesi, nests from West Texas to northern Mexico, and has an almost blindingly white, unmarked chest and belly. Then there’s the most controversial red-tail subspecies of all, Harlan’s Hawk (B.j. harlani). A far-North form that nests from northern British Columbia across the Yukon and much of Alaska, it was considered a separate species by ornithologists until 1973, when the American Ornithologists’ Union (now the American Ornithological Society) decided to lump Harlan’s as a subspecies into the overarching Red-tailed Hawk. Harlan’s Hawks do indeed look dramatically different from typical red-tails. Most adults are so dark they’re almost blackish, usually with white streaking on their upper chest. Harlan’s Hawks don’t have a brick-orange tail; instead their tails come in an astoundingly varied, almost psychedelically mottled mix of grays, blacks, whites, and russets—each tail pattern as unique and beautiful as a snowflake. Given how decidedly un-red-tail the Harlan’s Hawk looks, there have been a few die-hard ornithologists who have refused to accept that particular species lump even 50 years later.

Come autumn, migration shuffles the deck of Red-tailed Hawk subspecies distribution across the continent in complex ways. Some populations move long distances, like harlani birds that fly southeast to winter in the Great Plains and Midwest, or the Krider’s red-tails moving to the southern Great Plains. Others barely budge, staying in the same area all year. It’s a lot to keep track of—and that’s kind of the point of the Red-tailed Hawk Project, says Robinson.

To Catch a Hawk

Bryce Robinson came to Red-tailed Hawk research by a circuitous path. The lanky 38-year-old with a luxuriant dark beard told me all about it one morning in March last year in Ithaca, New York, when I joined him to try to trap a few red-tails. I rode shotgun in Robinson’s Subaru Outback, sitting next to a square, carefully weighted and baited wire cage that’s safe for raptors. Robinson has the necessary permits to trap red-tails for research, and he has the trapping down to a fairly simple process: Cruise the back roads and spot a hawk, drop the trap out an open car door within its line of sight, and wait for the raptor to come down and harmlessly entangle its toes in the trap. Except as fate soon demonstrated, it’s never really that simple.

It was a mild, windy early spring day: the red maple buds swelling, the first flocks of Common Grackles and Red-winged Blackbirds reappearing after winter, a flush of green on the farm fields where Horned Larks flitted across the small country roads we traveled. As we scanned the trees and utility poles for red-tails, Robinson explained how he wound up leading such a big, multidisciplinary project.

“I think really when I sparked on red-tails was when I was counting at the Goshutes,” he says, referring to the fabled hawk migration site in the mountains of Nevada. It was 2012, Robinson was fresh out of college at the University of Utah, and as the season progressed, he became aware of subtle shifts in the plumages and coloration of the red-tails he was seeing, as birds from farther and farther north began to pass through.

“I guess I didn’t appreciate how much plumage diversity there was in the species until then,” he says.

That interest was heightened by a pivotal friendship that began the year before, when Robinson began volunteering with the Salt Lake City-based group HawkWatch International, which runs the Goshutes count. At a weekend event he met author and renowned raptor expert Jerry Liguori, who lived in the area.

“He took me under his wing, and we started regularly going out to find and photograph raptors around northern Utah,” Robinson recalled. Liguori (who passed away in 2021) had a long-standing curiosity about the almost endless diversity in red-tails, which fueled Robinson’s growing fascination with the species.

“Jerry taught me a lot,” says Robinson. “In conversations with him, I started learning what we do and don’t know. One of the things that we didn’t know was where these dark[-morph] types that winter in the eastern part of the continent, where do they originate?”

Dark-morph Red-tailed Hawks are rare in the East and Great Lakes; fewer than 1% of red-tails passing through Hawk Ridge at Duluth, Minnesota, have dark plumage, for example, and only one to three dark red-tails a year are usually seen in New York. One question Liguori pondered was whether these dark-morph red-tails represented wanderers from farther west, or whether they actually occurred in the Canadian boreal forest, where they weren’t supposed to be.

“That I think is really the question that spurred this formation of the Red-tailed Hawk Project,” Robinson says.

Our conversation was interrupted by the appearance of two red-tails, a pair perched along a power line that cut through a small area of forest, the larger female sitting closer to the road. Robinson did a drive-by to quickly assess the situation and sightlines, something that after hundreds of red-tails trapped and banded—and more than 100 outfitted with GPS transmitters—has become second nature to him. He did a quick U-turn and pulled off the road just far enough for me to ease open the passenger-side door and drop the trap where the hawk had a clear view of it. The female left its perch on a long, shallow glide, but flared off at the last second, pulling up into the wind and then vanishing deeper into the forest.

The East is tough terrain for trapping, Robinson says—the abundance of tree lines and shrubs mean the hawks are usually perched farther from the road. Out on the Great Plains, the raptors mostly use roadside utility poles, readily visible and easy to drop a trap for.

“It’s a dream, really,” Robinson says. He had just returned a day earlier from an extended trapping trip to Kansas and Oklahoma, during which he and his colleagues had deployed GPS transmitters on 14 Harlan’s and one Krider’s red-tail.

As we headed to another red-tail trapping spot, Robinson explained how, after that initial interest in red-tails, his career went in different directions. He completed a master’s degree at Boise State in Idaho, studying Gyrfalcon nesting ecology in Alaska, a state to which he returned after grad school in 2016 to work for the Migratory Birds Division of the U.S. Fish and Wildlife Service. In Alaska he befriended another researcher, Lucas DeCicco. At the time, Robinson didn’t consider himself a raptor biologist—in fact, he still doesn’t.

“I see myself as more of a general ornithologist than a raptor-focused person,” he says. The USFWS job allowed him to do a range of bird work across the immense state, getting field experience and dipping his toe into other research systems, as well as satisfying his desire for adventure. “And Luke and I, we feed off each other, I think, in our interests and our passion for chasing down cool questions in ornithology,” he says.

DeCicco took a position at the University of Kansas in 2017, where he is now ornithology collections manager. As he soon found out, Kansas is a winter hotbed of red-tail diversity. The university’s respected, now-emeritus bird collections manager Mark Robbins—along with Liguori, Sullivan, and other raptor experts—had already been trying to figure out some of the more puzzling questions those winter-migrant red-tails posed. They were especially intrigued by a population of dark ones that appear in Kansas every year—birds that were chocolatey-brown with a wash of rufous on the upper chest. They didn’t appear to be Harlan’s hawks, but no one knew where they came from, or what subspecies they represented. They did look a bit like the abieticola subspecies, which might make sense if the hawks were coming due south from the Canadian boreal forest where abieticola breeds— and if abieticola had a dark morph. Which, so far as anyone knew, it didn’t.

To solve the mystery, DeCicco, Robbins, and Robinson spent three years arranging permits and cobbling together some funding from several sources to purchase GPS transmitters. DeCicco used his personal frequent flier miles to fly Robinson down from Boise, where he was working as a freelance natural history illustrator, and the trio hatched a plan to catch two of those dark mystery hawks in 10 days during March 2020.

The hawks played very hard to get, however. Robinson struggled day after day, with one red-tail in particular that reliably, every time, dropped on his trap, and just as reliably every time, pulled loose. “I started wondering, ‘Why can’t I catch a damn dark bird?’” Robinson recalled.

Fortunately, luck eventually prevailed. On the evening of his next-to-last field day, Robinson caught one of the dark-morph mystery red-tails, and the next morning, with just three hours to spare before he had to be at the airport for his flight home, he caught a second. That spring, the hawks, nicknamed Trigger and Nutsy after two animated vultures from the Disney movie Robin Hood, migrated away from Kansas—not due north into abieticola country in Canada, but far to the northwest into British Columbia, where they nested about 100 miles from each other.

“They did this westerly trajectory. I don’t know how many birds we’ve tagged there since then, but everything does that,” Robinson says. “They winter [in Kansas], they empty into British Columbia, Alberta, Yukon, Northwest Territories, and Alaska for breeding. There’s nothing we’ve tagged [in Kansas] that has gone straight north.”

It was the first of many surprises from what soon grew into the Red-tailed Hawk Project, attracting additional researchers from Michigan State, the University of Idaho, the Hawk Ridge Bird Observatory in Minnesota, and HawkWatch International. The expanded team has widened the scope of research techniques to include physical measurements of each bird, blood samples that allow for sexing and genetics work, and the development of a catalog of color-balanced digital photographs of more than 600 red-tails from across the species’ range. The project has also gathered tracking data on 109 red-tails in 18 states and provinces that have been fitted with transmitters to monitor their movements.

Trigger and Nutsy really started something.

Digging Deeper Into Subspecies

Over just the past five years, the Red-tailed Hawk Project has already solved a couple of long-standing mysteries and revealed some surprises about red-tails. A recent paper by the project team published in the Journal of Biogeography detailed the movements of 26 light- and dark-morph red-tails tagged in the Great Lakes, in an effort to determine the origin of those mysterious, eastern dark-plumaged red-tails that puzzled Liguori for years. Four dark-morph birds—tagged from Minnesota, Michigan, Illinois, and southern Ontario—migrated north into the boreal forest from central Quebec to northern Manitoba, as did the 22 more widely scattered light-morph, more typically abieticola birds that were also tagged.

The research team determined these dark-morph hawks weren’t calurus red-tails, because they didn’t migrate to the west, but Robinson and his coauthors were careful in the paper not to conclude that these dark birds were definitely abieticola, either (in part because, again, dark-morph abieticola red-tails have never been documented).

In fact, says Nicole Richardson—an independent raptor researcher from Canada who joined the Red-tailed Hawk Project early on—the team prefers to avoid talking about subspecies per se in general.

“On the project, we talk about phenotypes [or how the birds look] a lot more than subspecies, and we’ll often just say, oh, ‘this harlani thing,’ or ‘this abieticola-ish thing,’ or ‘this is kind-of-vanilla-borealis-with-a-hint-of-calurus,’ or something like that,” she says. The idea is not to be too wed to subspecies concepts, since thinking within those rigid lines may blind them to the broader evolutionary history and ecology at play. Also, the deep-dive genomics research into the phylogeny (or evolutionary relationships tracing back to a common ancestor) of the Red-tailed Hawk subspecies complex—analyzing DNA samples from across the entire range of 16 red-tail subspecies to tease out which populations and subspecies are really what—hadn’t been done.

Until now. Many of the details are still under wraps, awaiting publication in a peer-reviewed science journal later this year, but the preliminary results of a sweeping genomic analysis that Robinson has been performing for his PhD dissertation promises to upend much of what ornithologists thought they knew about the genetic relationships between red-tail subspecies. What he is willing to share is surprising enough. There is no Hail Mary for Harlan’s Hawk fans, because B.j. harlani is undeniably a red-tail, not a separate species. But the genomic work also confirms earlier suspicions that the species currently classified as the Rufous-tailed Hawk of southern South America, Buteo ventralis, is genetically a Red-tailed Hawk. In fact, it is more closely related to calurus red-tails of western North American than calurus are to their neighboring borealis red-tails in eastern North America—even though the rufous-tails live 9,000 miles away at the far end of South America.

Robinson says he has an idea how this occurred in evolutionary terms, but he is saving those details for formal publication.

Tracking Hawks, Tracing Genes

A Google Scholar search shows just how relatively overlooked the Red-tailed Hawk has been. The phrases “red-tailed hawk” and “buteo jamaicensis” showed up in 433 research articles in 2023 and 2024, only nine of which dealt solely and directly with red-tails. Most of the rest mentioned red-tails in passing—as part of a raptor community survey in Wyoming, or as victims of rodenticide poisoning. By comparison the Peregrine Falcon, a glamor bird of the raptor world, appeared in more than 1,300 studies over the same two-year period, mostly in a starring role.

Perhaps the person with the best perspective on that gulf in research interest is Cheryl Dykstra, editor-in-chief of the Journal of Raptor Research and a big fan of the Red-tailed Hawk Project. Dykstra told me she really doesn’t understand why red-tails have been so relatively ignored for so long, but she’s impressed with how Robinson and his colleagues are charging into the breach.

“I don’t know of any other large collaboration like this that is trying to integrate the movement ecology with large-scale genomics, which I think is fascinating, to make a big-data project like that,” Dykstra says.

Dykstra says she’s especially interested in seeing what the project ultimately reveals about how red-tail movement ecology relates to climate change and urbanization, given the extraordinary range of migration strategies that Red-tailed Hawks exhibit across their range.

“Just seeing a species that is such a facultative migrant [meaning different populations and individuals take very different approaches to migration], how it can respond to climate change will, I think, be very informative and instructive for future conservation, not only of this species, but other raptors as well,” Dykstra says. “I think it’s the right species for the questions of the day.”

Irby Lovette, Bryce Robinson’s PhD advisor, says he’s a little in awe of his student’s scope and drive.

“He’s doing multiple things really well that then have an emergent property aspect to them,” Lovette says. “Bryce is pulling together pieces that don’t normally cross-talk to one another because he is really deeply invested in the natural history and the basic biology of the hawks, knowing what makes them tick in the wild.

“For example, normally we think about gene flow happening out there as kind of an abstract concept, but Bryce is actually tracking the hawks. So he’s seeing where their genes go, but he is also seeing where the individuals go.”

By late afternoon on that March day trying to trap hawks with Robinson, we were back near the southern end of Cayuga Lake, having been skunked by the local red-tails up and down the valley.

“I know of a couple of birds near my house,” Robinson says. “They’re not in great spots for trapping, but maybe we’ll get lucky.”

And, at long last, we did. A solitary red-tail was perched along a power line not far from the side of the road. I slipped the trap down as we slowed and then drove away. Before Robinson had even made a quick three-point turn to turn around, the hawk was on the trap, its orange tail flared. Robinson zoomed up, threw the car into park and dashed out the door, picking up the bird and gently freeing its feet.

It was a second-cycle red-tail, a juvenile that had just molted into its first adult plumage. Robinson covered the bird’s head with a falconry hood to quiet it, then carefully wrapped it in an aba, a cloth restraint that works a bit like a gentle straitjacket to keep the bird immobile, while he prepped his equipment.

The hawk received a numbered metal leg band, as well as a colored plastic band—blue with the large code 8T in white, easily readable for scientists and hawkwatchers from a distance. Freeing the bird from the aba, he measured the length of its folded wing, tail, and other physical traits, then weighed it: 1,081 grams (about 2.4 pounds). Based on measurements it was probably a male, which average a bit smaller than females, but a later blood test would confirm that.

Next came a photo shoot. The Red-tailed Hawk Project maintains an extensive photograph collection, archived in the Cornell Lab’s Macaulay Library, as Robinson and his colleagues visually document every bird they catch, posed against a neutral gray background to best capture the feather colors. While Robinson was measuring the hawk, I pulled a large, hinged pair of plywood panels from the car, opened them and unrolled the gray vinyl sheet that formed the photo backdrop. Then Robinson passed the hawk to me, as he mounted a rather roadworn digital camera on a small tripod. Using the self-timer, he began taking a long series of photographs from every angle—the hawk facing front with the right wing open, from the back with the right wing open, the same for the left wing, then the tail fanned front and back, then its wings folded in left and right profile, and so on.

Because understanding the genetic and ecological underpinnings of the red-tail’s remarkable diversity of plumages and patterns is central to the project, these photos are as critical as the telemetry or genetic data.

Robinson’s last step involved taking a blood sample, for the DNA and sexing work. With a quick extraction from a wing vein, Robinson placed a few blood droplets into a vial of buffer solution, a sample that will eventually reveal a great deal about this bird’s genetic makeup, and how it relates to the hemispheric tapestry of its species.

During all this data collection, the hawk remained rather calm. With the blood sample labeled and stored away, Robinson walked the bird out into the old cornfield where it had been hunting, and released it back into the sky—at that point roughly the 600th or so hawk in the project’s rapidly growing database.

Each new hawk is one more set of interlocking data points for this pioneering effort, one that promises to make the Red-tailed Hawk—so long overlooked by scientists and taken for granted by birders—one of the most thoroughly understood raptors in North America.

About the Author

Scott Weidensaul is the author most recently of the book A World on the Wing, and a researcher studying several kinds of owls, hummingbirds, and bird migration in Alaska.