U.S. pollution regulations meant to protect people from dirty air are also saving North America’s birds, according to a new study.

Credit: Brian E. Kushner/Lab of Ornithology

Improved air quality under a federal program to reduce ozone pollution may have averted the loss of 1.5 billion birds during the past 40 years, the study found. That’s nearly 20% of bird life in the United States today.

The study, conducted by scientists at Cornell and the University of Oregon, published Nov. 24 in the Proceedings of the National Academy of Sciences.

“Our research shows that the benefits of environmental regulation have likely been underestimated,” said co-lead author Ivan Rudik, the Ruth and William Morgan Assistant Professor at the Dyson School of Applied Economics and Management. “Reducing pollution has positive impacts in unexpected places and provides an additional policy lever for conservation efforts.”

Ozone, a gas that occurs in nature, is also produced by human activities, including by power plants and cars. A layer of ozone in the upper atmosphere protects the Earth from the harmful ultraviolet rays of the sun. But ground-level ozone is hazardous and is the main pollutant in smog.

To examine the relationship between bird abundance and air pollution, the researchers used models that combined bird observations from the Cornell Lab of Ornithology’s eBird program with ground-level pollution data and existing regulations. They tracked monthly changes in bird abundance, air quality, and regulation status for 3,214 U.S. counties over a span of 15 years.

The team focused on a regulation called the the NOx (nitrogen oxide) Budget Trading Program, which was implemented by the U.S. Environmental Protection Agency to protect human health by limiting summertime emissions of ozone precursors from large industrial sources.

The findings suggest that ozone pollution is most detrimental to the small migratory birds – such as sparrows, warblers and finches – that make up 86% of all North American land-bird species. Ozone pollution directly harms birds by damaging their respiratory systems, and indirectly harms their food sources.

“Not only can ozone cause direct physical damage to birds, but it also can compromise plant health and reduce numbers of the insects that birds consume,” said study co-author Amanda Rodewald, Garvin Professor of Ornithology in the Department of Natural Resources and the Environment in the College of Agriculture and Life Sciences, and director of the Center for Avian Population Studies at the Cornell Lab of Ornithology.

“Not surprisingly,” Rodewald said, “birds that cannot access high-quality habitat or food resources are less likely to survive or reproduce successfully. The good news here is that environmental policies intended to protect human health return important benefits for birds too.”

Last year, a separate study by the Cornell Lab of Ornithology showed that North American bird populations have declined by nearly 3 billion birds since 1970. This new study shows that without the regulations and ozone-reduction efforts of the Clean Air Act, the loss of birdlife may have been 1.5 billion birds more.

“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,” said co-author Catherine Kling, Tisch University Professor at the Dyson School and faculty director at the Cornell Atkinson Center for Sustainability. “This work contributes to our ever increasing understanding of the connectedness of environmental health and human health.”

Other co-lead authors were Yuanning Liang, doctoral student in environmental, energy and resource economics; and Eric Zou, assistant professor of economics at the University of Oregon. Also contributing was Alison Johnston, research associate at the Lab of Ornithology.

Pat Leonard is a staff writer at the Cornell Lab of Ornithology.