You don’t need a PhD in ornithology to frighten these fellas. A decoy owl or a plastic snake can work wonders—for about a week. Then pigeons realize, "Hey, that owl hasn’t blinked in six days."
Solution? Move the decoy around every few days. Give it a new hat. Tie a balloon to its beak. Keep them guessing.
Another winner: shiny, reflective things. Old CDs, aluminum pie plates, or those spiral wind spinners catch sunlight and spook pigeons like a bad mirror selfie. Hang them near your plants, and watch them do a double-take.