Significance Understanding the drivers of abundance and biodiversity decline across numerous taxa is imperative for designing conservation policy. We use highly detailed citizen science data to show that there is a strong, robust negative association between bird abundance and ambient ozone concentrations in the United States. In particular, we find that a regulation aimed at reducing ozone precursors has significantly bolstered populations in the eastern United States. Our estimated effects suggest that the large decline in average US ozone concentrations over the past several decades has averted the loss of potentially billions of birds. Environmental policies nominally aimed at humans can also provide substantial benefits to other species.