I think there are 200 in all!!!
First one I found was on the gazebo in the little park near your first hideout. Didn't have a gun that early in the game so I didn't know what to do with it. I flailed at it impotently with my fists and it mocked me with its cooing! Now they all must die!!!
I had some of the little feckers roosting on my bay window last year, waking me up at 4 in the morning. Now it's payback time!
By the way, not a cheat as such, but if you go to tw@ and visit
www.whattheydonotwantyoutoknow.com you can see a map of all the pigeon locations, plus maps for other goodies. It's not very detailed, though.