If birds fly south for the winter why are there still pigeons in New York City during winter?

Well, to understand that you have to understand a little about migration. The important question is why do birds migrate? Migration is a long ordeal, and definitely risky. The only reason it would evolve in as many species as it has is if staying behind for the winter is even more risky. Cold is sometimes a factor, but many birds could survive a cold winter. More of a problem is the reduced amount of food. In human cities, however, there's always shelter from the cold, and, more importantly, food is available year round. In this environment, there's a good chance that a resident rock dove will survive the winter, so more rock doves will die if they migrate than will die if they stay behind.


http://www.madsci.org/posts/archives/may98/895879584.Zo.r.html

Good answer, zb, and put simply, it's a matter of food source. I'm curious, though, about just what happens to the birds in the non-human cities. :-)
http://www.backyardnature.net/birdmgrt.htm

Pigeons are not the only birds who refuse to migrate during the cold weather. Canada geese also stay around areas for the entire year and their large droppings are a human health hazard.

I'd never heard of no-migratory Canada geese. They sure migrate from and to Alaska, and the West Texas skies are filled with them in V-formation this time of year. But!

"Like most geese, the Canada goose is naturally migratory with the wintering range being most of the United States.....In mild climates, such as the Pacific Northwest, due to a lack of former predators, some of the population has become non-migratory."

https://en.wikipedia.org/wiki/Canada_goose

So another factor besides food supply is predation.

Now I just have to figure out what a non-human city is.

Couldn't find a nonhuman city but here's a nonhuman farm.

http://www.ant-farms.com/