When I was at school, one of my 'friends' painted a mixture of quicklime and caustic soda on the window sill. They never returned after flying away with smoldering feet!
Deny them a flat surface on which to rest. The railway bridge round the corner has newly-fitted, sloping bars placed just where the pigeons used to sit/ squawk/ do their business. And the pigeons have moved on.
Feed them Bicarbonate of Soda. Birds cant break wind (or burp, I forget which), so they will produce all the gas but no be able to release it, so they will eventually explode! If not, buy a BB gun and shoot them. they will soon get the message