Alejandro Moya Hidalgo read my item about the cheeky sparrows that insist on perching on the air conditioning unit by the back door of our house.
He has kindly offered me a solution that I think should work. He says that all I need to do is hang something shiny like old CDs or metallic tape by the unit and the birds will stay away. I suppose it is akin to the idea of putting a mirror in your garden to ward of cats. Animals and birds do not like to look at a reflection of themselves, fearing that what they see is an enemy.
Since Alejandro has an allotment where he grows the most amazing fruit and vegetables, I dare say he has faced the problem with birds time and again so he knows what he is talking about.