http://news.bbc.co.uk/today/hi/today/newsid_9661000/9661104.stm

City birds sing differently than country side birds, at a higher pitch than those who live in rural areas. The result would be that the song of the city birds will be heard at longer distances. Obvioulsly, the birds are heavily affected by the city soundscape and the architecture that surrounds them. It’s a quite a remarkable difference.