There's somthing called "HotFoot" that you can load into a caulking gun and then apply to the place where the pigeons sit. Evidently it irritates their little toes, but I don't know if it causes lasting harm. Also, have you seen those strips of close-together, thin pokey metal spines that some businesses nail up on overhangs? Those keep pigeons off, but they might make your place look a litte "Department of Corrections".

perhaps a picture of a cat.. or something like that. maybe a cat stuffed animal that can be seen from where they perch...

what about getting a watergun and spraying them. My dad used to do that to keep squirrels off of our bird feeder.

something i've seen my neighbors do is put up a plastic owl. we have TERRIBLE pigeons at my apt so i want to know what works too!

the best - and most humane - solution i've seen to this problem is stretching out a slinky over the window sill. not sharp enough to cut feet. awkward enough to make landing impractical...