How about a battery operated, motion activated contraption that could scare away pigeons?
They roost on a built-in window air conditioner, so something that could fit on a flat surface, or hang about a foot or two above the ac. A simple fan or propeller device that could spin up would probably do the trick - dont want to hurt them though! They also roost on smaller surface areas on corbels - so both a small version and a larger version would be ideal - or even a modular version so you could adjust the size would be great. It would need to be able to survive outdoor elements. Tried screening them off - but then squirrels made the screened off area their home! Im a novice so I could use some help with someone designing and creating, and posting as an instructable or project.