The shear number of pigeons in Duomo square is impressive... and it's one of the reason why the palaces and the cathedral are always under renovation/cleanup. The other is no doubt the air pollution, which is particulary bad in Milan since it sits in a a flat plane where air does not move much. Milanesi have a say for something that never ends: "La fabbrica del Duomo". This is because the Fabbrica del Duomo is the organization that was setup to mantain the Duomo cathedral, and it's essentially in perpetual motion. When they reach one end of the cathedral with the cleanup and repairs, it's time to start all over again..