[ses4j]

Cities tend to have a concentration effect on birds, especially during migrations. Central Park in NYC, for instance, is one of the best places in the world for birdwatching in spring and fall. Birds migrate at night, and at first light, gravitate to the first tree they can find. Since NYC is concrete for miles and miles, a very wide area's worth of birds all funnel into Central Park (and other parks, of course) and you get amazing quantities and varieties. The same is true for parks in other cities.