Understanding biodiversity's contributions to human health is the first step toward fostering synergies between biodiversity conservation and health promotion - two major targets of UN's Sustainable Development Goals. The One Health approach acknowledges the health of people and biodiversity are interconnected and facing common threats. In this study, we aimed to unveil the geographical association between avian biodiversity and population health across the US. In this ecological study, we combined citizen science bird data from eBird, population health data from the Institute for Health Metrics and Evaluation, and county-level statistics of population characteristics, including socio-economics, healthcare service etc. Multivariate linear regression analyses were performed between bird biodiversity (measured by rarefied species richness of birds), key indicators of general public health (e.g., cause-specific mortality rate), and socio-economic health determinants of 2751 US counties. We found that a higher number of bird species was significantly associated with longer life expectancy after confounding adjustment (regression coefficient (95% CIs), 0·005 (0·003, 0·008)). Bird species richness calculated using the rarefied method consistently accounted for variance in age-specific mortality risks in both very young and old age groups (R2 from 2% to 4%). Rarefied species richness of birds was negatively correlated with the majority of cause-specific deaths (12 out of 21 mutually exclusive causes of death), indicating a general synergy effect between biodiversity and human health. The associations with the top causes of deaths were regarded as highly significant, with considerable effect sizes, for example, for cardiovascular diseases (regression coefficient (95% CIs), −0·242 (−0·311, −0·174)). Our findings show human health is inseparable from the health of the shared environment and the well-being of all species. Bird species richness offers a valuable means to understand large-scale relationships between human health and the health of the environment. To enable equitable sharing of biodiversity's benefits to human health, more efforts should be made to understand two-way socio-ecological mechanism underlying human–biodiversity interactions.