Tree swallows near U.S. military bases carry alarming pollution levels

Across the United States, tree swallows near bases and factories carry high levels of per- and polyfluoroalkyl substances (PFAS), according to a new study.

PFAS, also known as “forever chemicals,” are a large family of long lasting industrial chemicals.

The heaviest exposures occurred at sites influenced by aqueous film forming foam, a firefighting foam used on fuel fires, and by urban sources. At those spots, PFAS levels were up to 40 times higher than at cleaner reference areas.

Testing tree swallow samples

For the investigation, the researchers collected samples of covered eggs, nestlings, and the insects they ate at 10 locations from the Mid-Atlantic to the Upper Midwest.

The team tracked whether the eggs hatched and whether the chicks survived long enough to leave the nest.

The work was led by Christine M. Custer from the United States Geological Survey (USGS) at the Upper Midwest Environmental Sciences Center. Her group compared places with legacy foam use to cleaner reference sites.

PFAS totals were as much as 40-fold higher at foam sites than at nearby reference areas. Perfluorohexane sulfonate reached up to 9.7 percent in eggs and 9.0 percent in nestlings at those locations.

“Perfluorooctane sulfonate was the only PFAS detected in all samples,” wrote Custer. Egg and nestling survival metrics did not correlate with exposure levels.

Pollution exposure and bird survival

Hatching and fledging are blunt measures of fitness, and they can miss subtle harm. Adult condition, survival across seasons, and behavior can shift without moving those headline outcomes.

The team also measured immunoglobulin Y, the main antibody in bird blood, and a haptoglobin-like protein called PIT54. Those markers did not differ by site in this dataset.

Outside the nest, some PFAS are linked with cancers, cholesterol changes, and pregnancy-related problems in humans. Ecological risks can differ from human risks, yet both matter for communities.

Mixture chemistry and exposure timing matter, and they vary across places. A result showing no association in one year does not prove safety across lifetimes.

How PFAS pollution spreads

On many bases, firefighter drills have used foam with PFOS or PFOA for decades. Foam residues enter soil and water, and they can move through groundwater to streams and wetlands.

Urban waters pick up PFAS from consumer products and industry through wastewater and storm drains. National sampling suggests at least 45 percent of U.S. tap water contains one or more PFAS.

Some sites near manufacturing towns showed distinct PFAS mixes shaped by local histories. Where legacy foam use and urban runoff overlap, exposure levels climbed the most.

Swallows feed on aquatic insects, so the chemicals in water can arrive by way of food. That route binds bird exposure to the chemistry of nearby wetlands.

Human exposure to PFAS pollution

In April 2024, the EPA set national drinking water limits for several PFAS, including 4 parts per trillion for PFOA and PFOS. These limits aim to lower exposure from public water systems over the next few years.

The rule also uses a hazard index, a tool that sums risks from a mixture. That approach reflects the fact that mixtures can add up even when each chemical is low.

The research on swallows does not translate directly to drinking water risk for nearby towns. Bird exposure depends on insect diets and wetland chemistry – not treatment plant performance.

Communities use monitoring and treatment to meet the federal limits as timelines come into force. Private wells near known sources should be tested by certified labs when local agencies recommend it.

Pollution in tree swallows

Tree swallows nest readily in boxes and forage near water, a point emphasized in a USGS overview. They also return to the same sites, allowing teams to track the same families over time.

Wildlife used to flag contamination are called sentinel species – organisms that signal environmental problems. Because swallows feed low in the food web, they reveal local water conditions within a small radius.

By sampling eggs and nestlings, scientists estimate exposure during egg formation and early growth. That timing captures periods when developing tissues can be most vulnerable.

The new dataset adds PFAS pollution to a long-running record built with swallows in many regions. It gives managers a way to compare sites that differ in pollution history, not just geography.

It’s not time to stop worrying

Follow-up work could test adult survival, migration timing, and multiyear breeding performance at the same sites. Researchers could also track whether cleanup actions reduce exposure in step with local interventions.

Given how widely PFAS appear in water and wildlife, field studies that tie exposure to clear outcomes will matter most. Mixture science is improving fast, so better metrics may sharpen risk calls for wildlife.

Policymakers now have national drinking water limits, but ecosystems will need their own yardsticks. Data like this help define where biological thresholds might sit for wild birds.

For now, the message is narrow yet important. High exposure did not map onto poorer hatching or fledging in these swallows. That is a call to keep measuring, not a reason to stop worrying.

The study is published in the journal Environmental Toxicology and Chemistry.

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