A city is a densely populated settlement with a multitude of buildings, streets and human activities. However, cities can also offer surprisingly diverse and valuable habitats. Various animal and plant species feel at home in parks or gardens, but also in or on buildings. Some bird species, such as swifts or jackdaws, return to the same buildings to breed, year after year, for decades. During renovation or remodelling measures, nest boxes can help replacing these important breeding sites. Through sustainable urban planning and the help of their citizens, cities can create habitats for a variety of species and actively support biodiversity.