I believe the answer is birds.
In Unicorn the man who been shot (don't remember his name, is it Barthold?) points at some bird giving Tintin the clue to the Bird brothers.
In The Castafiore Emerald, the opera The Thieving Magpie gives Tintin the clue about the real thief in the book. Funny that in the book named after her, the clue is not from the opera Faust by Gounod (with the famous jewel song/aria) but Rossini's Thieving Magpie. Though had Hergé used Faust instead perhaps a "cheesy" marquis dressed up as Méphistophélès could have been the thief.
http://en.wikipedia.org/wiki/Faust_(opera)