Nutcracker
Stop Brexit!
My impression is it only works for a while - Woodpigeons are not used to recognise gulls as potential predators, but they learn fairly fast. After a few weeks, the numbers grazing on the sports field where it happened dropped, and they became more wary of gulls. That meant the gulls stopped catching, and gave up attempting. No doubt the cycle will run again when a new crop of Woodpigeons has forgotten the risks.