Posted: 22/04/2016 at 21:51
Similar problem at work with pigeons and doves roosting and leaving large piles of muck. Often one bird of a pair will sit away from the nest in a favourite spot while it's mate is on the nest.
In the garden a blackbird often roosts in the same spot and messes on the compost heap. ( yay).
I can only suggest you remove any perch they are using, or make it unusable.
I'm afraid that the wildlife and countryside act makes the shooting of birds in your garden strictly off limits.