More and more people are living in cities, and cities are becoming larger and more numerous. Some bird species have been able to adapt to this new environment. The great tits of the city of Barcelona, for example, are smaller, have different personalities, and even differ genetically from the woodland birds that live in Collserola. Genetic studies show that this differentiation between forest and city can take place in as little as fifty years. Cities therefore become a great experiment in the study of evolution.