The importance of unsealed areas in the urban core and periphery for bird diversity in a large central european city

Although urbanization poses one of the largest threats for biodiversity, only few studies have so far examined its impact in large Central European cities. Our study aimed at investigating the effects of urbanization on bird diversity using two methods to describe the urban environment. The first measure used the degree of sealing, portion of traffic infrastructure, number of floors of the tallest building at a site scale (100-m radius), and the distance from the city center, while the second one relied on land-cover data at a local scale (1000-m radius). We conducted bird surveys at 761 sites across Hamburg. Bird diversity was assessed as species richness, abundance, and Shannon index. Additionally, evenness was calculated. Both urbanization measures represented a rural-to-urban gradient and were negatively correlated with bird diversity as well as evenness. At the site scale, the degree of sealing had the strongest negative effect on bird diversity followed by the portion of traffic infrastructure and the tallest building. At the local scale, artificial surfaces proved to have the strongest negative effect, while agricultural areas and forests were positively associated with bird diversity. In general, models using the site scale for measuring urbanization showed higher associations with diversity. The results emphasize the importance of unsealed areas in the urban environment and of natural habitats in the periphery for bird diversity in Central European cities with millions of human inhabitants. Considering this for future city planning can help to preserve biodiversity in the urban environment, increase bird diversity in development areas, and thus improve living conditions for people.