North American bird declines are driven by reductions in common species

Biodiversity loss continues to occur globally. However, the long-term dynamics of the redistribution of abundance within declining communities remain poorly resolved. We quantitatively dissected the major decline of North American bird abundance by rigorously analyzing 38,854 surveys that quantify over half a century of annual dynamics for 648 populations of 244 bird species from six regions, each encompassing 140,000 to 370,000 km². Despite regional variation in geography, habitat, and species composition, a consistent pattern emerges: Biodiversity loss predominantly stems from declines in the most abundant bird species, whereas less abundant species show increases. We demonstrate that species once abundant and near their carrying capacity have declined, while less abundant species farther from their carrying capacity have increased. A path analysis identifies the key role played by anthropogenic land-use changes in causing the observed declines. Conservation efforts should place stronger emphasis on the abundance of common species as a central ecological indicator.