I have a car paint shop,best way to remove bird lime marks is lightly wet flat with 3000 grade wet & dry.Polish the wet flat marks out with a mop,any good paint shop should be able to do this for you,takes about 15 minuets
regards Marcus
It is the Acid in the poo, leathal from Seagulls....pour boiling hot water over the effected area as soon as possible, if marks remain take Marcus's advise