Feathered Trends

London has declared war on pigeons (or "air rats," as some call them). Mayor Ken Livingstone last month imposed an $85 fine on anyone who feeds the birds in Trafalgar Square, where numbers fell from 4,000 to just 200 after a 2001 ban on feed sellers and the recruitment of a harrier hawk. But not all cities are inhospitable.

In Venice , feed sellers can be fined up to $590, but those catering to...