As American Kestrels Mysteriously Decline, Researchers Look to Their Migration for Clues

We still don't know a lot about the migration of North America's smallest falcon, but these scientists are using tracking technology that will help inform their conservation.

Few birds capture our hearts and imagination like the American KestrelWith a royal-colored mantle, American Kestrels are undoubtedly our most colorful raptor. However, the kestrel does not just draw in its legions of admirers with a refined color palette. Ounce for ounce, the American Kestrel, may also be the fiercest of our falcons, even if it is our smallest. It is also our most widespread. Ranging throughout the Western Hemisphere, it is truly a bird of the Americas. From Alaska to Tierra del FuegoAmerican Kestrels occur just about anywhere that is not dense forest or the Arctic Circle. Due to its ubiquitous nature around open areas, often perched on telephone wires, fence posts, or dead trees, most of us have probably seen an American Kestrel, even if we did not know its identity. 

Sadly, the American Kestrels now face similar threats other raptors did during the second half of the 20th century. Formerly one of our most plentiful birds of prey, the kestrel is declining across portions of its range, sometimes in devastating numbers. In recent decades, more than 90 percent of kestrels have disappeared from New England's pastoral landscapes. These rates of decline are on par with the painful losses suffered by the kestrel’s larger cousin, the Peregrine Falcon, in the decades following World War II. Thankfully, some researchers are starting to break through the fog after analyzing prior data. The American Kestrel Partnership, a project of The Peregrine Fund, is one such group of scientists. Now, it believes the key to understanding the kestrel’s decline lies in their wintering grounds or during migration. 

Despite how much researchers have studied kestrels, we still know little about their migration or their wintering ecology. As such, the American Kestrel Partnership is supporting ground-breaking research in Texas by providing tracking devices that will help researchers better understand overwinter survival, annual survival, and, most importantly, the migratory pathways for kestrels that spend their winter in the Lone Star State. 

One such researcher working with these tracking devices provided by the American Kestrel Partnership is the University of North Texas Ph.D. student, Kelsey Biles. A native to Texas, Biles is spearheading efforts to track kestrels wintering in her home state, around Denton and Gillespie Counties. For Biles and her team of student assistants and volunteers, it is all about being in the right place at the right time. Texas has the largest population of wintering kestrels in the United States. Despite this concentration, very little research on kestrels has occurred in Texas, let alone the entire Central Flyway

"There have been almost zero studies on kestrels during any time of year in the Central Flyway," states Biles. "There's even been minimal band returns [data back from the USGS Bird Banding Lab] for any kestrels banded in Texas and found elsewhere or vice versa. [Fewer] than 100 total recoveries in 50 years.”  


It is worth noting that the American Kestrel, with its 17 recognized subspecies, exhibits a wide range of migratory and non-migratory strategies. Kestrels that reside in colder climates tend to migrate to warmer regions during the winter, while those in sub-tropical and tropical areas are non-migratory. Kestrels also exhibit leapfrog migration, where individuals of a more northern population during the summer breeding season migrate beyond another population, to become the more southerly group during winter. Despite knowing about their overall migration strategies, researchers know very little about the migratory pathways or destinations for many populations of kestrels. 

Attempts to look at the genetic landscape of kestrels to reveal these mysteries of migration have had limited success. By using high-resolution molecular markers, researchers from the American Kestrel Genoscape Project, led by long-time kestrel researcher, Dr. Julie Heath, hoped to use the DNA from a single feather to map wintering kestrels back to their breeding population of origin. This method has proven valuable in the past, where researchers such as Dr. Kristen Ruegg of the Bird Genoscape Project were able to link breeding and wintering populations for other species, such as the Wilson's Warbler. Unfortunately, even with this higher resolution genetic investigation, the kestrel genoscape doesn't show sufficient genetic differentiation to give the level of resolution researchers would need to pluck a feather from a wintering bird, run an assay, and generally know where it bred.  

According to Dr. Chris McClure, Director of Global Conservation Science for The Peregrine Fund, “the genetics for most populations of eastern kestrels are indistinguishable from each other.” Because of this muddiness, McClure and others feel tracking devices remain the best option to understand kestrel migratory connectivity, migratory routes, seasonal survival, threats, and causes of mortality, all of which can better inform kestrel conservation efforts. When asked about threats to kestrels, Biles reflects, "The major answer is we don't know yet. We suspect there may be some reduced survival or increased mortality during migration or on the wintering grounds." Because of this, researchers such as Biles are excited about what discoveries may transpire by analyzing tracking data for kestrels. 

By studying wintering kestrels in Texas, one aim of Biles is to reveal the connectivity with where they breed. This has huge conservation implications when we think about understanding populations such as those in New England that are declining at such a steep rate. It is why her research season is only from December to February, stopping around the first week of March. "We don't want to put trackers on birds that are already migrating, coming from somewhere other than Texas," says Biles.  

Working with her principal advisor and long-time raptor researcher, Dr. Jim Bednarz, Biles started researching American Kestrels in March 2018 after initially studying Northern Bobwhites. Thanks to generous donors, the American Kestrel Partnership provided Biles with 25 tracking devices that she deployed later that year, during the winter of 2018-19. The devices Biles deployed consisted of 10 GPS units and 15 light-level geolocatorsAccording to Dr. Sarah Schulwitz, Director of the American Kestrel Partnership, part of the hope of contributing tracking devices to Biles’ work was to test things out and find the right combination of weight, cost, and the quality of data coming in 

I hope to figure out what works with Kelsey's study, and emulate that with partners in other important wintering areas.” Schulwitzadded, Our long-term goal is to have several studies streamlined with the same methodology developed in Kelsey’s work.” Finalizing this protocol will be critical to the long-term success of this study as tracking the full annual cycle of American Kestrels is fraught with challenges. Because of these inherent challenges, very little tracking data exists for this species, despite our living in the golden age of tracking. 


One of the challenges in tracking American Kestrels is their innate ferocity. Even though kestrels are the size of a Mourning Dove, they pack a predator’s punch into their pint-sized frames. “American Kestrels are small, but they’re still raptors. They have sharp bills that can tear off trackers with ease,” states Biles, who has now personally trapped 170 American Kestrels over the past two winters and likens their sass and ferocity to that of small dogs. This fierceness paired with the perfect tools for removing harnesses means researchers cannot use the same elastic harnesses used on similar-sized passerines.  


The other challenge in working with kestrels is their size. Unlike larger raptors, such as eagles, that can support the weight of more substantial tracking devices that transmit movement data in real-time, American Kestrels are too small. With tracking technology, there is a fine balance between cost, size, battery life, and data accessibility. For kestrels, researchers often need to retrieve the tracking devices again to access the data. Fortunately, many migratory birds, including kestrels, return to the same area year after year in a process known as site fidelity. This behavior, combined with kestrels often forming territories on the wintering grounds, has allowed Biles to study some of the same individuals for the past three winters.  


Out of the 25 individuals outfitted with trackers during the 2018-19 winter season, 13 returned the following winter in 2019-2020. Biles was pleased to note that this return rate (52 percent) exceeds the average rate of return for kestrels in the area without transmitters (50 percent). "It means the trackers are not negatively impacting the survival of the birds," states Biles. This metric of survival is an essential goal for all tracking studies, where a standard rule of thumb is to never put a device on a bird that weighs more than 3 percent of its total body weight. However, finding kestrels again year after year and recapturing them to retrieve the data-loggers are two different things.  

During the winter of 2019-2020, Kelsey and her team were unable to recapture any of her birds outfitted with tracking devices the previous year. "They're smart birds. They remember. Once we catch a kestrel, they don't want to look at our traps," says Biles. Their avoidance even prompted Biles and her team to try three different types of traps, several new to the kestrels, but without luck. Adding to the struggle to recapture the kestrels were some external factors as well. "Our winter here in Texas was unseasonably warm, with no day all winter staying below freezing, and many days in January and February reaching into the 70s. Because insect availability was very high, they appeared to have little motivation for coming to our traps.” This choice to avoid the traps illustrates the kestrel's preferred diet of insects over mice, which Biles and her team use in the traps to get the kestrels' attention. 


While Biles and her team were unable to retrieve the trackers from birds that came back this winter, they were able to catch a single individual over the summer. “After expending considerable effort, we did retrieve one intact tracker from a male kestrel on June 13, 2020,” says Biles. “This kestrel happened to be a year-round resident, which was unknown at the time of his initial capture and attachment of his tracker.” According to Biles, approximately 15 percent of kestrels wintering in North Texas remain there year-round, which provided Biles and her colleagues a unique opportunity. “Because of this, we were able to target this kestrel during the breeding season using mist nets and a mechanical owl lure, which is a very different process than the way we capture birds during the winter.” After it was recaptured, the kestrel was re-measured and determined to be in good health, which was also exemplified by his reproductive success; he and his mate fledged four young this summer while his tracker was still attached. As far as the other birds that did not remain during the breeding season, Biles hopes to get some of them next year, owing to their ability to return to the same place. Additionally, Biles hopes to recover as many of the 40 new trackers that she deployed this year around Central Texas.  

Like the American Kestrel Partnership, Biles is ever-looking towards the future, particularly around upgraded tracking technology that would allow her to download data off the tracking devices without the need to capture the birds. "I am extremely excited about the possibility of using GPS/VHF tags. They would solve a lot of our problems," says Biles. "If you get within 200 meters of the tracker, you can download the data remotely.” As one would expect, these units are more expensive at $1200 each. With luck, enough supportive donors will contribute to her research to make this a possibility. Despite wanting to retrieve the data more easily, Biles makes it clear that capturing the kestrels again is not without its merits. “If you capture the kestrels again, you can take measurements to see if their body conditions change.”  

It is clear that for Biles, her interest in studying American Kestrel migration and overwinter survival is not waning, “We’ve done four winters now, and I’ll be doing the fifth. I’m also hoping to do my post-doc for another two years!” She is also hoping that the next generation of students will follow in her wake. “We’re looking to get other graduate students into the program. We are hoping this will be a 10-year-long study on survival. Long-term datasets are surprisingly rare." Biles is also looking to contribute her data to more significant efforts that bring awareness to the migration of American Kestrels and the threats they face, and she expects to share her tracking data with Audubon’s Migratory Bird Initiative after next winter.  

For Schulwitzand the American Kestrel Partnership, once Biles study is replicated elsewhere, she hopes to draw upon the Audubon network for support. "For Kelsey's work, she had a small team of student assistants. However, if we can replicate these studies elsewhere, to get a unified look across multiple areas across the kestrel's range, we need volunteers to help. I would love to have Audubon's members be a part of that."