I suspect the culprits are gulls, so they've got some kind of tree hugging hippy crap protection granted by the man. It's not like the fat f*****s are endangered around here, gorging on 2 metric tonnes of Blackpool's finest kebabs and gutters full of chunder on the streets each weekend. If it was a tiger s**t I'd probably just say fair enough, they're endangered like, but gulls make a right racket and dirty protest on my Zed. I saw one sitting on her recently, good only knows what it's stupid feet do to the paint. Maybe some dry rice or a can of smash "accidentally" spilt on the floor might work.
There was a story in the local rag about a bloke who chased after some gulls for pulling apart his bin bags. A few days later one went for him when he was out and about in town. It happened again a while after and he reckons it's the same one, a sort of ASBO avian, he's started wearing a hat when he goes out now, not sure if it's protection or a disguise.