What about hawk silhouettes flying high? I haven't tried them because I don't want to scare my hens or the wee birds, or acclimatise them to hawks, as we have plenty of real ones they need to run from. I have heard of owl shapes on a pole, with refective eyes as being worth a try too.
I use CDs but eventually the pigeons just duck under them.
There's zip wire which snaps in the breeze.
Last year I hung Tibetan bells all around the place - they sound nice but the pigeons quite like them too.
On a farming scale there are the birdscarers which go off like cannons at irregular intervals, mainly directed to rooks, but if you value your sanity and that of anyone within a couple of miles then you'll come up with something else.
Scarecrows which blow in the wind and twizzle round? The pigeons would probably soon work out that they're all threat and no action.