If you come up with something let everyone know; but this is their mission in life - eat, breed, crap on everything.
Apart from the usual white/brown gunge I have so far had:
Fish (part and whole), crustaceans of various species, one guillemot egg, mussells - and a suspicious amount of blood...
And when you put your P&J on her mooring early in the season (before the yotties are out of their winter bunks) every pecking oiseau in the Clyde seems to think it would be great fun to eat the upholstery.
I want a 12 bore...
(I did watch "The Human Planet" where the Africans effectively napalmed half a million sparrows - I'm not vindictive, but I may have a job for those lads