What Is Causing the American Kestrel’s Perplexing Decline?

On a chilly, overcast December morning in North Texas, a car prowls a pitted dirt road. The occupants are straining forward, looking intently at the sky. “Kestrel!” says Maddy Kaleta from the passenger seat, pointing to a tiny black blob on a power line ahead. She whips out binoculars and confirms the bird isn’t banded. “Let’s go,” says her advisor, University of North Texas ecologist Jim Bednarz. Kaleta grabs a ring-shaped wire mesh cage the size of a small throw pillow with two oblivious mice scampering inside. She opens the car door, leans out, and, as the car rolls to a stop, gently sets the contraption on the grassy shoulder.

Kaleta snaps back inside as Bednarz quickly reverses, stopping a few hundred feet away. The American Kestrel bobs its tail as it scans the muddy fields of a future housing development strewn with construction equipment. The humans are watching, willing it to fall for the bait. “You’re an apex predator,” Kaleta says to the distant falcon. Bednarz joins in, urging the bird to strike: “You’re an eagle! A kestrel eagle.”

As they banter, the colorful predator flies to a wire above the trap, then swoops atop it. After trying to catch the just-out-of-reach mice, the kestrel flaps its wings—but goes nowhere. While trying to snatch the mice, it had slipped a foot through a loop of fishing line sprouting from the trap.

Bednarz floors it, racing the short distance to the captive. As the car rocks to a stop, Kaleta and Brooke Poplin, both graduate students who work with Bednarz, leap out and scoop the trap and falcon off the ground. The robin-size bird doesn’t squawk or flail; it just glares indignantly at the jubilant humans as they untangle it from the fishing line. Victory: They caught an American Kestrel.

This time of year, Texas is the place to be if you’re looking for kestrels. The state is thought to host one of the country’s largest winter populations, with the feisty falcons flocking to its vast grasslands, agricultural fields, and booming neighborhoods to hunt for insects, rodents, and small birds. Throughout the winter, Kaleta will scour sprawling Denton County to band, tag, and monitor dozens of kestrels. Relatively little research has been done on the birds up and down North America’s central corridor, or during the winter. Her findings could help answer a question that raptor researchers and conservationists across the continent are racing to answer: Why are kestrel populations declining?

While Bald Eagles, Peregrine Falcons, and other raptor species have rebounded in North America since the banning of DDT in the 1970s, American Kestrel numbers have continued to drop, plunging by an estimated 50 percent. Although the birds are still common, the decrease has scientists worried. No one knows exactly what’s driving the losses or where the birds are running into trouble—making it difficult to figure out how to stop or reverse the trend.

From their perch in northern Texas, Kaleta and Bednarz are perfectly positioned to help find out. They’re working to fill some crucial gaps in the kestrel’s life cycle, capturing valuable information about how birds fare over the winter, their migratory routes, and their survival rates. Still in the early days of their investigation, researchers are making the most of what they’ve got: lots of kestrels, a willingness to drive thousands of miles, a small army of pet mice, and cutting-edge tracking technology.

Unfortunately, the kestrel that Kaleta caught this morning—a male, given his slate gray wings—isn’t going to play a starring role in uncovering the cause of his species’ decline. Once Kaleta frees him from the trap, she keeps him calm by slipping him into a tube made of two frozen-juice containers taped together. The researchers take measurements, inspect his feathers, clip metal identification bands around his legs, and mark his chest with a smear of blue nontoxic dye so they can tell from afar that they’ve already captured him. But Kaleta can’t attach the location tracker that is essential to her research. The bird must weigh at least 123 grams to safely carry the load of the 3.5-gram tracking device. This one is 11 grams shy.

However, Kaleta isn’t worried. This is only day one, kestrel one of the three-month field season. And for now, the tiny raptors abound.

A

merican Kestrels are the smallest, most numerous, and most widespread falcons in North America. They live in semiopen areas ranging from meadows, grasslands, and deserts to parks, croplands, and urban environments. Kestrels are found across much of the continent, from Alaska and Canada down to Mexico; they also occur in Central and South America. Kestrels in warmer regions tend to be year-round residents. Those that breed in colder climates typically migrate south in winter, and many migrants follow a leapfrog pattern—traveling from a more northern latitude, passing another population, and settling even farther south for the winter.

The exact paths kestrels take and the ultimate winter destinations for many populations are mysteries. Their smaller-scale movements are also an enigma: We know kestrels need open space, but we don’t know enough about where they prefer to hunt, how big their territories are, or what they do when their preferred habitat disappears. Despite advances in tracking technology that have unlocked the pathways of birds as varied as Golden Eagles, Arctic Terns, and Blackpoll Warblers, it’s been tricky to find a gadget that’s a good fit for kestrels. The vast majority of knowledge has come from data collected during a fraction of their annual cycle, when they’re relatively easy to monitor.

Every summer for decades, scientists and scores of kestrel-loving volunteers across the continent have set out to learn as much as they can about the birds. Some take part in the North American Breeding Bird Survey, counting the number of birds, including kestrels, they see on a single day at regular stops along established routes. Over the past half century the number of kestrels that participants have sighted has dropped throughout North America.

Enthralled by the birds’ charisma, some observers dedicate more time. Kestrels are cavity nesters that readily inhabit human-made wooden boxes to raise their young, and some return to the same spots year after year. Across the country, from roughly March through August, hundreds of volunteers regularly check thousands of nest boxes on public lands and in backyards to monitor how many chicks successfully hatch and fledge. Data from these studies indicate the sharpest kestrel declines are in the East. “For the birds that show up to nest boxes, their success rates are very high,” says John Smallwood, a biologist and raptor researcher at Montclair State University in New Jersey, of his study area. A single kestrel pair fledges three to four chicks on average. The puzzling problem: “They’re not showing up.”

In at least some parts of the West, kestrels are showing up, but they are breeding weeks earlier than they did in the 1990s. Curious about the change, Julie Heath, a biologist and raptor researcher at Boise State University, discovered that farmers are taking advantage of significantly warmer winters by planting crops earlier to avoid the hotter summers. The bounty draws insects and rodents—prey for kestrels—to the fields. “It looks like kestrels are really tracking that change,” she says.

Another major source of data comes from decades of fall migration monitoring—also a labor of love. Each autumn, as raptors of all kinds fly south, scientists and volunteers gather at high points, coasts, and lakeshores across the continent to tally birds passing overhead. These migration counts suggest that, as with nest-box studies, kestrels in the Northeast are declining fastest. Counts in the West, meanwhile, seem to point to a shift in behavior, with fewer kestrels migrating in the fall and more staying through the winter, says Dave Oleyar, director of long-term monitoring and community science at HawkWatch International in Utah.

Researchers have spent years looking for a smoking gun that explains the declines. They’ve tested blood droplets and feathers for chemicals, but to date no pollutants have popped up as a prominent culprit. Genetic analysis of samples revealed five distinct kestrel populations—Alaska, Western, Eastern, Texas, and Florida. (This finding opened the door to yet another question: If populations aren’t mixing, how are they different from one another?)

Experts now believe multiple complex factors are likely the problem. Possibilities include loss of insects and other prey due to climate change and habitat alteration. Pesticides may be killing rodents and insects or weakening the falcons themselves; neurotoxic insecticides called neonicotinoids are especially concerning because they’re potent, widely used, and spread up the food chain. European Starlings may be outcompeting the native birds for natural nest cavities, and predation by larger Cooper’s Hawks might play a role. How these factors interplay, and how they might differ across geographic areas, is still unclear.

In total, North America has lost an estimated 2 million kestrels since 1970, says Chris McClure, executive vice president of science and conservation at The Peregrine Fund. No one disputes kestrels have long been declining, but the pace of the drop-off has also been a subject of debate. Some research suggests there’s been a steady decrease, a trend that would ultimately lead to extinction if unchecked. A more recent analysis concluded that the rate has lessened in the past several decades, which suggests a less dire fate.

There’s still time to ensure the kestrel remains the most abundant falcon on the continent, but doing so will require embracing new tools and looking in places—and during parts of the species’ life cycle—that are understudied. “It’s my personal opinion that the problems that are causing the American Kestrel decline are happening outside of the breeding season,” McClure says. “And Texas is a really concentrated wintering area for the American Kestrel.”

F

our hours after Kaleta’s team caught their first kestrel, the gray winter sky has given way to a clear blue, the temperature has warmed into the 60s. They’re still out prowling back roads and eyeing treetops and utility lines for falcon-shaped blobs.

Driving past a field of long golden grasses and live oak thickets, Kaleta spots one. She pulls over and checks on her furry accomplices, Chimay and Charcoal Cheddar. These two mice, and the four others in a glass habitat in the trunk, are her permanent pets. She’s named all six after cheeses and jokes about their distinct personalities: Blue has a mild temperament. Manchego is curious and quick. Today, wily Chimay and fleet-footed Charcoal Cheddar soon prove their value once again, this time luring a female hefty enough to be fitted with a tracker.

To attach the device, Kaleta loops a harness made of thin black Teflon around the kestrel’s wings and secures it and a thimble-size tracker with two antennae high on the bird’s back. She fixes the knot with waxed cinnamon dental floss soaked in Krazy Glue. The process takes about 15 minutes. But getting to this point took trial and error—and technological advances.

Bednarz started UNT’s American Kestrel project in 2016 and enlisted undergraduate students to estimate annual kestrel survival by banding and resighting the birds. A Ph.D. student, Kelsey Biles, took over the project a couple of years later. Bednarz and Biles wanted to expand the focus to track where kestrels that winter in Texas go to breed. No one knew.

Just figuring out how to track the birds was a challenge. Unfortunately, satellite tags weren’t an option: They transmit location information, but the battery power they require makes them too heavy for pint-size kestrels. Weight wasn’t a concern with nanotags that emit radio signals, but the birds’ locations would be recorded only when they flew close enough to a stationary antenna site on land and wouldn’t provide the granular picture of kestrel movement the researchers were after. Biles tried geolocators, but they proved to be a bust: The tiny sensors record daylight to estimate location, and kestrels spend too much time in dark cavities to provide useful information. She also tried archival GPS tags, which had been developed only a few years earlier. These satellite tags store position data until scientists recapture the bird and download the information; they require far less battery power than tags that transmit on their own and thus are light enough for a kestrel to carry.

Biles put the GPS tags on 10 birds in the winter of 2019–2020. When eight returned the next year, she was elated. Then dismayed. Although seven of the Teflon backpacks had withstood the kestrels’ razor-sharp beaks, Biles quickly realized she couldn’t get them off—because she couldn’t recapture the birds. The kestrels returned to the same areas where she’d first snared them, but they refused to be enticed by her mice. Apparently you can fool a kestrel only once.

Thankfully, in 2020, The Peregrine Fund supplied the team with archival GPS tags paired with a radio beacon that beams data at a preset time, enabling the scientists to download data from several hundred feet away using a handheld antenna. No recapturing suspicious kestrels required.

With this technological breakthrough, the project could move forward. Biles and Bednarz tagged 20 kestrels from December 2020 to March 2021. Kaleta took over the project that fall, and the following winter she spotted seven birds sporting backpacks. She successfully retrieved data from two (either the others had dead batteries or Kaleta missed the window when they were beaming the data). One bird had spent the summer about 600 miles north in a riverside wildlife refuge in Nebraska. The other kestrel’s tracker battery died in April, but the data Kaleta managed to recover suggested it may have migrated to northern Kansas. As of early March 2023, she had deployed 18 trackers this season. She also found seven birds tagged the previous winter, but she was unable to retrieve their data immediately due to technical glitches, including four failed downloads. “New technology is definitely tricky,” Kaleta says. Her team is still trying to recover the data and hopes that what they learn will help improve the gadgets.

In addition to tracking, the team bands kestrels in winter and surveys birds year-round. Biles, now conservation director at Houston Audubon, determined that 90 percent of the kestrels in Denton County in winter are migrants. Since the start of the project, the team has collectively banded about 400 kestrels. Survey efforts step up in the winter months, with Kaleta and a small team of field technicians scouring the county looking for kestrels that were banded the year before and tracking when and where they return.

The team gathers an unprecedented wealth of information, but they also need to understand it. Kaleta plans to analyze the data on resighted kestrels to determine survival rates, whether birds are dying in high numbers during the winter, and which factors, like age, sex, and rural versus urban habitat, may play a determining role. Tracking more kestrels throughout the year could also help the researchers determine if certain migratory pathways or stopover sites are associated with lower return rates by pointing to areas where the birds are running into trouble.

So far, only the UNT team and a few other scientists have outfitted the birds with archival GPS tags, but Bednarz has heard from other kestrel scientists who are interested in using the same technology. If the trackers prove successful, The Peregrine Fund may distribute them to kestrel researchers in other areas where the birds overwinter and pass along the methodology pioneered in Texas. And as more kestrels are tracked in the coming years, the bigger picture of their migration and how they interact with the various landscapes they inhabit may come into focus.

D

riving around north Texas is a sobering reminder of the fragility of the open-space habitats that kestrels need. Denton is part of what locals call the metroplex: Dallas, Fort Worth, and the surrounding smaller cities. As in much of the state, the population of the metroplex is expanding and sprawling rapidly. What were once prairie and woodland are increasingly big-box stores and cul-de-sacs.

Kestrels gravitate to the edges of these developments. In many ways, they’ve adapted to humans, says Bednarz. Such sites provide seemingly ideal kestrel habitat, with open undeveloped fields for hunting on one side and human structures full of cozy cavities for roosting on the other.

But the development front is constantly expanding. This encroachment creates an ecological trap for the birds, Bednarz says. Kestrels that spend one winter hunting in a field may return to find it has disappeared under concrete and new shopping attractions the next. “My hypothesis is it’s a contributing factor in the decline happening all over,” he says.

A new effort spearheaded by researchers in New Mexico could help identify whether urbanization, which has been on the rise for decades, is indeed a main pressure driving kestrel losses. The scientists plan to feed the growing volume of kestrel data from bird bands, nesting records, and surveys conducted by thousands of experts and volunteers into one giant, continent-wide computer model.

Their work is rooted in the field of “decision science,” a high-concept term created in the business world and adopted by the U.S. Department of the Interior and other wildlife policymakers. It involves integrating everything scientists know about a species into a powerful model. Researchers then work with species experts to develop hypotheses about possible causes of declines and actions that might address them. Finally, they build smaller-scale population models that vacuum up all relevant information—including factors like land cover and climate data—to tease out the most plausible explanations.

As anyone who has ever tried to make a complex decision with limited information and resources knows, there’s usually no single “right” answer, but the scientists hope the model will inform the best possible choices. The project is led by Abby Lawson, a population ecologist at the USGS New Mexico Cooperative Fish and Wildlife Research Unit based at New Mexico State University. She’s led similar work on species including Eastern Black Rails. Though data on the secretive marsh birds are scant, Lawson says, the coordinated effort allowed researchers to identify two credible hypotheses to test in the field—one that targets reducing woody vegetation and another focused on identifying optimal irrigation regimes. They also worked together on several big-picture proposals to win competitive research grants.

For kestrels, possible actions may include putting up nest boxes in new areas, managing or restoring specific kinds of habitat, and reducing pesticide use on farmland. The model can then estimate the impact of those actions on kestrel populations, indicating which measures may slow or even reverse population declines and are therefore worth prioritizing and funding. Scientists hope the model will also point out the more significant causes of decline, says Kristin Davis, a postdoctoral researcher on the project.

Such insights will likely yield widespread benefits. “Birds like kestrels, they are such good conservation tools because they’re colorful; they’re charismatic; people love raptors,” says Chad Witko, Audubon’s senior coordinator of avian biology for the Migratory Bird Initiative. “If we can protect them and protect their open-space habitats, you’re going to protect a lot of other species that fall within those habitats.”

No one knows what happens to kestrels when they return to their hunting grounds after a summer away and find not a field but a gas station or a block of apartments. But by monitoring the birds closely, the scientists in Denton might just be the ones to figure out how adaptable these raptors really are—and what challenges they need our help to overcome.

This story originally ran in the Spring 2023 issue as “The Perplexing Decline of the American Kestrel .” To receive our print magazine, become a member by making a donation today.