Andreas Kappelmann is removing furniture from a balcony in Frankfurt-Sachsenhausen. Old furniture and a television were placed there and are covered in several centimeters of pigeon faeces. The birds had been using the balcony of the abandoned apartment for years. City pigeons' populations are mainly fed by wild breeding places like this one. Some ask why pigeons choose to live under these conditions, why the live on top of their faeces. The answer is simple: In captivity, their bred ancestors lost their instinct to clean their nests.