Study Estimates Clean Air Act Has Saved 1.5 Billion Birds

Over the last 40 years, bird populations across the U.S. did the best in places with the most stringent air pollution regulations

Great blue heron and oil refinery
A great blue heron seen wading in front of an oil refinery. Burning and producing fossil fuels are major sources of air pollution. A new study estimates that over the last four decades environmental regulations aimed at improving air quality have saved the lives of some 1.5 billion birds across the United States. Gerrit Vyn / Courtesy of the Cornell Ornithology Lab

Over the past 40 years, the improved air quality demanded by the United States’ Clean Air Act saved the lives of roughly 1.5 billion birds across the country, according to a study published last week in the journal the Proceedings of the National Academy of Sciences. That whopping total equates to roughly one fifth of all birds fluttering in U.S. skies today, reports Sara Tabin for Forbes.

“Our research shows that the benefits of environmental regulation have likely been underestimated,” says Ivan Rudik, an economist at Cornell University and co-author of the study, in a statement. “Reducing pollution has positive impacts in unexpected places and provides an additional policy lever for conservation efforts.”

Rudik and his colleagues delved into the relationship between bird populations and air pollution by comparing bird observations recorded using the citizen science program eBird with measurements of ground-level air pollution and the policies aimed at limiting harmful emissions.

The team focused their attention on air pollution caused by ozone. While the ozone layer of Earth’s upper atmosphere famously protects us from ultraviolet radiation, ozone gas closer to the ground forms smog and is harmful to breathe. Cars, power plants and heavy industry are some of the main human-caused sources of ground-level ozone pollution.

In humans, ground-level ozone pollution can damage and inflame the lungs and worsen respiratory conditions, including asthma, bronchitis and emphysema. But this “bad” type of ozone can also harm animals’ respiratory systems and degrade whole ecosystems by making it harder for plants to photosynthesize.

The new study used the bird observations to approximate the animals’ relative abundance around the U.S. and used air quality data to track levels of ozone pollution over 15 years in 3,214 U.S. counties. For those same counties, the researchers also recorded air quality regulations over the same 15-year span.

The team found that significant ground-level ozone pollution was strongly associated with declines in bird numbers in the U.S., reports Alexandru Micu for ZME Science. In particular, ozone pollution hurts the small migratory birds such as sparrows, warblers and finches that make up 86 percent of all North American land birds, writes Chrissy Sexton of

“Because flight is so physiologically demanding, it’s not surprising that ozone pollution can directly harm birds–especially small migratory species,” Amanda Rodewald, a conservation ecologist at the Cornell Lab of Ornithology and co-author of the study, tells Forbes. “At the same time, ozone can cause indirect harm by damaging plant health and reducing numbers of the insects that birds eat. When birds can’t access high-quality habitat or food resources, they are less likely to survive or reproduce successfully.”

Per Forbes, concentrations of ozone in the lower atmosphere fell by roughly ten percent between 1980 and 2018, largely due to environmental regulations like the Clean Air Act. It may not be surprising that the biggest local drops in air pollution came in the places with the strictest regulations, but it’s a reminder that properly calibrated policy can do significant good.

The study’s biggest caveat is that it can’t definitively prove that declines in ground-level ozone pollution caused bird populations to do better in certain regions, because other factors may have been in play. However, Rudik tells Forbes that the consistency of the negative correlation between ozone pollution and bird populations across a wide variety of locations makes him more confident about suggesting a causal connection.

“This is the first large-scale evidence that ozone is associated with declines in bird abundance in the United States and that regulations intended to save human lives also bring significant conservation benefits to birds,” says study co-author Catherine Kling, an environmental economist at Cornell University, in the statement. “This work contributes to our ever-increasing understanding of the connectedness of environmental health and human health.”

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