Urban areas affect flight altitudes of nocturnally migrating birds

1. Urban areas affect terrestrial ecological processes and local weather, but we know little about their effect on aerial ecological processes. 2. Here, we identify urban from non-urban areas based on the intensity of artificial light at night (ALAN) in the landscape, and, along with weather covariates, evaluate the effect of urbanization on flight altitudes of nocturnally migrating birds. 3. Birds are attracted to ALAN, hence we predicted that altitudes would be lower over urban than over non-urban areas. However, other factors associated with urbanization may also affect flight altitudes. For example, surface temperature and terrain roughness are higher in urban areas, increasing air turbulence, height of the boundary layer, and affecting local winds. 4. We used data from nine weather surveillance radars in the eastern US to estimate altitudes at five quantiles of the vertical distribution of birds migrating at night over urban and non-urban areas during five consecutive spring and au...