Nothing...but nothing deters voracious,permanently feeding pigeons.
There are but two deadly exterminators.....fox and kestrel.The former now a rare visitor to chez nous ( The last one seen...and disposed of.....a corpse,last October). We think they are victims of two cold winters.
A kestrel flew in yesterday,complete with unidentified kill,regrettably non pigeon,in its talons. More being seen now,as land use locally has changed due to the new,ever increasing National Forest,part of which is but a field's distance away.Once saw one make a 'hit' on a pigeon on the lawn.....awesome explosion of feathers !