Having grown up in NYC I can tell you that there are a lot of fat overfed pigeons. People often fed them stale old bread at the parks. Also, there are a lot of places for them to make sturdy nests on buildings without too much fear of predators. Then throw in the water fountains around the beautiful parks or elaborate buildings they have a constant water source. So plenty of food, safer surroundings, and water equals the ability to produce more offspring than in an open area where there are more predators and food may be more difficult to come by.
There are also a lot of trash bins and cans throughout the city. The pidgeons know where to find food if someone isn’t handing it to them. They are sky rats.