Feather chemistry helps track origin of birds

Isotopic analysis allows UC researchers to learn where hawks and falcons likely hatched

Researchers at the University of Cincinnati are using geology and rainfall patterns to track migratory birds of prey across North America.

Brooke Crowley, a professor of geology and anthropology in UC’s College of Arts and Sciences, has used isotopic analysis to track jaguars in Belize and extinct lemurs in Madagascar, among many similar projects. Her father, a retired science teacher and lifelong falconer, inspired her to undertake this study. 

Using an innovative combination of isotopes from the feathers of kestrels, goshawks and other predatory birds called raptors, researchers can narrow down where the young birds likely hatched and learned to fly. This method offers a useful tool to help scientists track elusive, wide-ranging animals, identify critical habitats and observe any changes in migration patterns.

The international collaboration was published in the journal Ecosphere.

Brooke Crowley leans over a map on her desk in her office.

UC professor Brooke Crowley used isotopic analysis to track wide-ranging hawks and falcons. Photo/Joseph Fuqua II/UC Creative + Brand

Strontium is a naturally occurring element found everywhere on Earth. It is released from rocks into the water and soil where it gets into the food chain. By taking the ratio of two types of strontium isotopes found in the secondary flight feathers of the birds and comparing with geology maps of North America, researchers could narrow down where the birds mostly likely hatched.

Researchers also examined hydrogen isotopes in the feathers. Hydrogen isotopes vary depending on summer rainfall, which likewise is geography dependent.

“Because hydrogen and strontium isotope ratios are controlled by very different variables, they are highly complementary,” the researchers said in their study.

Crowley said each map is informative on its own. But by overlaying the two, researchers can pinpoint the most likely origins of the birds with surprising precision.

“When you combine them, you get a pretty close picture of where the birds fledged,” Crowley said.

A similar analysis of feathers collected from young birds still in the nest validated their methods.

A cooper's hawk is hooded next to banding tools and a notebook.

A Cooper's hawk is hooded and wrapped to prevent injury before banding. UC researchers use isotopic analysis to track wide-ranging birds of prey. Photo/Provided

UC graduate and study co-author Kaitlin Sommer prepared samples of the hawk feathers for analysis in Crowley’s lab. She also presented their project at conferences in the United States and France.

“I was drawn to UC’s geology department because of the research opportunities and the professors,” Sommer said. “All the professors were incredibly passionate about their work and were always looking for students to take on projects and help them with existing ones. As someone who had little to no research experience, this was appealing to me.”

Isotopes offer several advantages to track the movement of animals, said study co-author Clement Bataille, an assistant professor at the University of Ottawa. They provide a natural marker, so there is no need to use external devices. Likewise, isotopes are preserved long after an animal dies.

“This is the most important quality of isotopes,” Bataille said. “It provides a way to reconstruct pre-industrial mobility habits of species and to understand if humans have an impact on migratory bird mobility.”

It’s a great way to learn more about species we can’t observe directly because they’re rare, cryptic, nocturnal, wide-ranging or extinct.

Brooke Crowley, UC professor of anthropology and geology

Technology is leading to smaller and smaller tracking devices to follow the migration of living creatures, Bataille said. But these devices can only track the future movements of animals.

“There is really no possibility to reconstruct the mobility of dead individuals other than with isotopes,” Bataille said. “So, I think isotopes will be really critical to understand how breeding grounds might have changed in comparison with present-day.”

And that could help biologists and conservationists make better decisions about preserving critical habitat, he said.

Crowley also partnered with Idaho wildlife biologist and author Bruce Haak, who has banded birds for 40 years.

“I became fascinated with birds when I was quite young. My focus shifted to birds of prey and falconry in high school, which is when I learned to trap various species of raptors safely,” Haak said.

A UC student prepares samples in a lab.

UC graduate and study co-author Ellie Severson prepares samples for isotopic analysis in UC professor Brooke Crowley's lab. Photo/Brooke Crowley

Biologists have placed unique bands on the legs of birds to track individuals for scientific studies dating back to John James Audubon in the 1800s.

“Banding birds is labor intensive and yields small bits of evidence over long periods of time,” Haak said.

Scientists track birds using a variety of methods. Some attach light-sensitive geolocators to continent-hopping migratory birds like bar-tailed godwits that fly nonstop from Alaska to New Zealand. The tags record the hours of daylight, letting researchers know the latitudes of the birds on their north-south migrations.

Haak and other researchers use GPS or radio telemetry. By placing a radio transmitter on captured birds, Haak can follow their daily movements.

“The use of radio telemetry provides intimate data in real time: One learns more about movements and daily behavior from a single satellite transmitter than from decades of banding birds,” he said.

containers of feathers.

Feather samples provide tissue for isotopic analysis. Photo/Brooke Crowley

But there are limitations to this method, Haak said. The equipment is expensive. And the wide-ranging birds can be difficult to track even with the benefit of technology.

“Some of the Cooper’s hawks and red-tailed hawks that I tracked went to Colima, Mexico, which is roughly 2,000 miles south of my Idaho trap site,” Haak said. “While I learned the migration routes and wintering destinations of these subjects, I had no idea where they were coming from.”

That’s why isotopic analysis is especially useful, UC’s Crowley said.

“Unlike telemetry and other tracking devices, when you sample a feather, it records the history of where that bird has been,” Crowley said.

“I think this can be a good tool to consider population connectivity and to identify trends about where individuals come from and how they move across space,” Crowley said. “It’s important to document things like key natal grounds or places where animals have to stop during migration.”

Combining isotopic analysis with genetic markers and habitat suitability models could further help identify the birds’ origins with even greater precision, researchers said. The method could even be used with museum specimens, which would give scientists a virtual time machine to compare animals that lived decades to centuries apart.

“It’s a great way to learn more about species we can’t observe directly because they’re rare, cryptic, nocturnal, wide-ranging or extinct,” Crowley said.

The project was supported by a grant from the North American Falconers Association, a UC Undergraduate STEM Experiences award and a National Sciences and Engineering Research Council of Canada Discovery Grant.

Featured image at top: UC researchers use isotopic analysis to track the migration of hawks and falcons. Photo/Joseph Fuqua II/UC Creative + Brand

A portrait of Brooke Crowley in a lab coat.

UC professor Brooke Crowley has applied isotopic analysis to research and wildlife conservation projects around the world. Photo/JayYocis/UC Creative + Brand

Next Lives Here

The University of Cincinnati is leading public urban universities into a new era of innovation and impact. Our faculty, staff and students are saving lives, changing outcomes and bending the future in our city's direction. Next Lives Here.