Anyone who's ever had a pigeon problem in their building knows that these birds (sky rats?) are no joke. I once had a family friend whose Hell's Kitchen neighbor insisted on leaving out pigeon food in the building's air shaft. The resulting onslaught of birds—and bird poop—was so bad that my friend would open up his (filthy) window and blast out a mixture of bleach and water with a super soaker, just to get the birds to scatter. Not what you'd call ideal.
So how to keep your building from a similar, feathered fate? Thankfully, there are a lot of options, all of them more humane than the super soaker approach. First and foremost, you and your neighbors should make sure you're not doing anything to encourage a pigeon problem, keeping outdoor trash containers tidy and sealed. (And remember, a neighbor who insists on feeding pests can actually be kicked out of a building for it, if things get really extreme.)
For the next steps, you'll need to involve the building's management or the board. Habitat Magazine recommends heading to birdbarrier.com for devices to install on flat surfaces (think rooftops, windowsills, etc.) to prevent birds from nesting. It may also be worth investing in Ovocontrol, a form of bird birth control that'll keep any pigeons that do show up at your building from multiplying (a similar tactic is currently underway to reduce the number of rats in the subway system). Whatever you do, Habitat cautions against using gel repellants—which can be harmful to the birds—and keeping in mind that it's illegal to kill pigeons in New York. If you've been loading up a water gun of your own, it's time to put it down and step away slowly.