You can poison pigeons using Avitrol. The company that produces Avitrol calls it a flock-frightening chemical. This is because the behaviour of pigeons after eating it scares the remaining pigeons away from the area where the chemical has been distributed. There are other methods of controlling pigeons such as shooting and trapping.
How can you kill pigeons with rat poison? – http://wiki.answers.com/Q/How_can_you_kill_pigeons_with_rat_poison