a friend of mine had same prob, neighbour complained of bird crap on her washing so he said tell me which one did it and i'll dispose of it but they are £100 each. If its the rong one and it happens again it will cost you £100, they never complained again!
Do you birds sit in the trees?
Ask them to supply proof it was one of yours and not a wood pigeon or crow etc by means of a photograph, no proof no excuse to refuse!