This paper estimates the effects of population density on biodiversity, with a focus on bird biodiversity in Spain. My estimates imply that a 1 percent increase in population density reduces the share of municipal area with high biodiversity value by between 0.1 % and 0.17 %. The estimates are robust to alternative specifications, identification strategies, and biodiversity measures. Land use changes are not the main mechanism at work, as biodiversity in densely populated areas is low across all land uses. In a counterfactual exercise, I find that moving people from rural to urban areas increases the overall area of high biodiversity value, as the gains in rural areas dominate the losses in cities.