An ethometrical analysis of street pigeons showed that most social interactions under conditions of overpopulation are of an aggressive nature. Through various mechanisms this frequent aggressive behaviour leads to "slumlike" living conditions characterized by intraspecific stress. It is possible to characterize street pigeons in their urban environment by calculating the frequencies of the most important aggressive and sexually-motivated social interactions. Behavioral patterns adapted to their living conditions result in misbehaviour in an anthropogenous environment. A high population density in commercial pigeon units should therefore be avoided. It is also necessary to consider the typical disposition of a race for aggressive behaviour. The frequent aggressive interactions among street pigeons at their breeding lofts seem to play an important role as a density dependent regulator of population size.