Cities alter the latitudinal diversity gradient of birds in North America

The latitudinal diversity gradient (LDG) is a widely recognized biogeographic pattern, yet its persistence under increasing human impacts remains unclear. Leveraging 17 million eBird records, we investigated how urbanization alters LDGs in North America. We quantified LDGs across 662 cities and their surroundings, and found that LDGs vary by season and native status: non-native species exhibited weak or no LDG, whereas native birds showed the strongest LDG during winter. Notably, native birds showed a significantly flatter winter LDG in cities than in surrounding areas. Although human population density generally reduced bird diversity, it exerted positive effects under cold conditions in high latitudes. These findings indicate that urbanization can reshape classic biodiversity patterns, with cities potentially acting as winter refuges for native birds at higher latitudes.