Validation of a globally-applicable method to measure urban tolerance of birds using citizen science data

•Monitoring organism responses to urbanization is critical in the face of increasing urbanization around the world.•Broad-scale, repeatable methods, are needed.•Broad-scale empirical citizen science data highlight the ability to measure urban tolerance of birds. Understanding species-specific responses to urbanization is essential to mitigate and preserve biodiversity in the face of increasing urbanization, but a major challenge is how to estimate urban tolerances for a wide array of species applicable over disparate regions. A promising approach is to assess urban tolerance by integrating geo-referenced information on species detections from citizen science data with estimations of urbanization intensity based on remotely-sensed night-time lights. While such citizen science urbanness scores (CSUS) are cost-effective, intuitive, and easily-repeatable anywhere in the world, whether the scores accurately describe urban tolerance still awaits empirical verification. By analysing >900 bird species worldwide, we find that CSUS correlates well with a standard measure of urban tolerance based on changes in abundance between urbanized and non-urbanized nearby habitats. Our analyses show that there is substantial variability in the relationship between these two metrics, but nevertheless highlights the potential for the CSUS approach in the future. Future improvements to the index, including incorporating rare species, and understanding the influence of intra-specific variability in response to urbanization, will be necessary to maximize the broad utility of the approach.