Like jaywalking on city streets, it seems that the anti-bird feeding rules in city parks aren't enforced. According to the NY Post, the people who are supposed to enforce park rules only gave out five $50 tickets last year for bird feeding... out of 18,876 summonses and violations issued in total. And the Parks Department allegedly told the paper, "It's OK to feed the pigeons as long as there are no signs saying you can't."
In 2007, the NY Times wrote that, "Despite a common public perception, there is no law that makes pigeon feeding illegal everywhere in the city. That said, the Parks Department posts notices prohibiting feeding in many areas under its control. If you ignore the sign, you may get an Environmental Control Board summons from the parks enforcement patrols or from the Police Department."
Of course, if it's just going unenforced, bird lovers may want to have their guards up, because some civilians are taking the law into their own hands. Just last week a 63-year-old bird lover was attacked after (legally) feeding pigeons on the sidewalk!