Over a Billion Birds Saved by Clean Air Regs

Yay!  Finally, we humans got something right.  It turns out that the Clean Air Act, intended to protect humans from polluted air, actually saved 1.5 billion (with a “B”) birds.  More specifically, researchers involved in a new continent-wide study have found that 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.

Study overview

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 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.

Study results overview

Study results suggest that ozone pollution is most detrimental to the small migratory birds (such as sparrows, warblers, and finches) that make up 86 percent of all North American landbird species. Ozone pollution directly harms birds by damaging their respiratory system, and indirectly affects birds by harming 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. Not surprisingly, 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.”

-Dr. Amanda Rodewald, Garvin Professor at the Cornell Department of Natural Resources and the Environment and Director of the Center for Avian Population Studies at the Cornell Lab of Ornithology


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 (Rosenberg et. al. Science, 2019). 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.

*Ozone is a gas that occurs in nature and is also produced by human activities, including by power plants and cars. It can be good or bad. 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.

Journal Reference: Yuanning Liang, Ivan Rudik, Eric Zou, Alison Johnston, Amanda D. Rodewald, Catherine L. Kling. Conservation Co-Benefits from Air Pollution Regulation: Evidence from Birds. PNAS, 2020.  DOI: 10.1073/pnas.2013568117