Pigeons are scared of sonic or loud squawking noises, images or silhouettes of hawks or other flying predators and iridescent Mylar streamers. The streamers are moved easily by the wind and scare off pigeons and other birds flying in the distance.
Sonic bird control systems emit distress sounds or predator calls that sound like warning sounds to pigeons and other birds. Scared pigeons naturally fly away from the source of the sound. The sounds may or may not be audible to human ears.
Silhouettes of hawks on kites are equivalent to scarecrows and is one of the most inexpensive and humane ways to scare pigeons away. Mylar streamers deter pigeons by flashing brilliant light that creates a confusing disturbance in its field of sight which birds naturally avoid.