By Bauyrzhan Zhaxylykov
Bald eagles are helping scientists uncover where per- and polyfluoroalkyl substances, or PFAS, contamination may be coming from across Wisconsin and the Great Lakes region.
A new study in the journal Environmental Research analyzed blood samples from 115 bald eagle nestlings collected across seven regions of Wisconsin from 2023 to 2025.
The research was led by scientists from the University of Wisconsin–Madison, the Wisconsin Department of Natural Resources and the Field Museum in Chicago.
Researchers say bald eagles are ideal for monitoring PFAS because they feed at the top of the food chain and hunt close to their nests.
“They’re scavengers, which is really good,” said Gavin Dehnert, the study’s lead author and an emerging contaminant scientist at the University of Wisconsin–Madison.
“Their foraging area is about a 5-kilometer (3.1 miles) radius, so it gives us a localized picture of PFAS levels in that area,” he added.
PFAS, often called “forever chemicals,” are a large group of human-made chemicals that persist in the environment because they break down very slowly. Scientists estimate there are more than 15,000 different types, although only a small number have been studied in detail.
The team studies nestlings because the young birds receive PFAS only through food brought by their parents, making them reliable indicators of contamination in the surrounding environment.
“They’re not drinking the water around there. They’re only getting the PFAS from the food their parents bring back,” Dehnert said.
Collecting those samples requires careful teamwork.
Researchers first identify active nests and travel to them by boat or car.
A trained tree climber carefully reaches the nest, places the eaglets in specialized bags and lowers them by rope to scientists waiting on the ground.
Sean Strom, a fish and wildlife toxicologist with the Wisconsin DNR and a co-author of the study, said the team measures each nestling, collects a small blood sample and attaches a federal identification band before returning them safely to their nest.
The PFAS study builds on a long-term bald eagle monitoring program that began in Wisconsin in the 1990s.
Scientists first used the birds to study contaminants such as DDT and PCBs before expanding the work to include PFAS and other emerging chemicals.
Strom has worked on the project since 2011.
“Humans are exposed to the same chemicals that these eagles are,” Strom said.
“They’re just a very good indicator species of overall contaminant exposure.”
The study found that PFAS contamination differed across Wisconsin.
Instead of finding the same mix of chemicals everywhere, the team identified distinct PFAS “fingerprints” in different regions.
Dehnert said those fingerprints may help scientists better understand where contamination is coming from.
“The types of PFAS that we’re finding in Lake Superior and the Apostle Islands have a very unique fingerprint compared to what we’re seeing on the Wisconsin River,” Dehnert said.
Those differences suggest that regions may be affected by different sources of PFAS.
According to the study, some chemical patterns may be linked to paper mills and biosolid spreading, while others could be associated with firefighting foam.
The exact sources are still being studied.
One focus of the team’s ongoing research is understanding how long-term PFAS exposure affects bald eagle health.
While PFAS contamination does not appear to kill bald eagles directly, it may weaken their immune systems and make them more vulnerable to diseases and other threats.
“They’re not falling out of the sky because they’re contaminated with PFAS,” said Emily Ruhs, a postdoctoral research scientist at the Field Museum and a co-author of the study.
“But if they’re highly exposed and then infected with something like avian influenza, they may be less able to survive. Gavin and I like to call it ‘death by a thousand cuts.’”
Ruhs said the findings may also help scientists better understand potential health risks for people because bald eagles are apex predators exposed to many of the same contaminants as humans.
“We see some of the same immune effects in people with high PFAS exposure, such as firefighters, that we’re seeing in the bald eagles,” Ruhs said.
The researchers say they hope eagles will not only help identify contamination but also help guide future cleanup efforts and measure their success over time.