Road traffic has a consistent negative impact on breeding density of a wide range of bird species in different habitats

Road traffic has a major impact on the environment and numerous studies have shown how various species respond to road traffic. However, a clear picture based on large-scale studies of how complete species communities in different environments are affected by road traffic is lacking. We assessed the distance within which road traffic negatively impacts breeding bird densities and the extent to which effect distances differ between species and habitats. We also tested how traffic intensity is associated with effect distances. We used a large dataset from protocolled breeding bird surveys of > 140,000 territories of complete breeding bird communities within 1500 m from motorways across the Netherlands. We examined effect distances of road traffic on breeding density of individual species, for various ecological species groups and for all species together. Finally, we tested how road traffic intensity influences effect distances. Road traffic had a consistent negative effect on territory densities up to approximately 650 m distance from the road for different species and ecological species groups. Within road-effect zones, densities of breeding birds were significantly lower (on average 25%) than at greater distances from the road. Effect distances were positively associated with traffic intensity in various ecological species groups. These findings that road traffic negatively affects breeding densities in a consistent manner across species and that this effect is exacerbated by traffic intensity will allow policymakers to predict and assess the quantitative impact of road traffic with a given traffic intensity on breeding bird populations.