Like you say, he's got a fully stocked bird table and so he's encouraging the birds\vermin (depending upon your point of view) to sit around and do what they do.
So you could conceivably argue that as stated he's encouraging them. As such he's partly liable for any costs incurred as a result of them being there. I am sure if they were soiling his car, clothes and household he wouldn't feel the need to encourage them, so why then should he feel that you should put up with it?
Perhaps have a quiet word with him and point out that you feel he's contributing to the birds adding their corrosive droppings to your car and as such you feel that he's jointly contributing to this vandalism by the nature of his encouragement. So you feel that he should meet your costs of repair, or clean-up for as long as he feels the need to encourage the birds\ vermin's presence in both your lives. If he feels that this is unacceptable then he will agree to put up with whatever methods you feel are warranted to employ to rid the pests from the local area. Perhaps nip out and grab a copy of 'Farmers Weekly' Or 'Farmers Guardian' and look out for something like the 'Vari Scarey Gas Gun', the occasional 'BOOM' which may wake both you your neighbour and those in a 10 mile radius up, but at least it will rid you of the birds
Or perhaps he may prefer the tinsel hanging from the line or something you could make like described alongside the aforementioned gas gun:
http://www.techneat-birdscaring.co.uk/ or perhaps some of those mentioned here:
http://en.wikipedia.org/wiki/Bird_scarer may catch your eye. The mere mention of such may fill your neighbour with outrage, though when he eventually calms down he may see your point of view.
Oh yes and as others have said, unless the tinsel is causing some technical malfunction. Then it's doubtful that BT will have an opinion on it. If the pole is sited on your land, then BT should pay you a nominal 'rent' for it being there, if this is the case you can also request that it be re-sited which may eliminate all problems associated with its present location. Indeed if things escelate between your neighbour and yourself, you could even write to BT and advise them you think they're jointly liable as to the birds using it as a perch and them soiling and the corrosive nature of their droppings and involvement in your life. They can't argue they've never seen birds resting on their lines can they, so surely they're aware of the situation and so may enlist some device to rid you of the birds themselves.
HTH