Mothers usually set about teaching their offspring the moment they're born. But the females of one Australian bird can't wait that long. Superb fairy-wren (Malurus cyaneus) mothers sing to their unhatched eggs to teach the embryo inside a 'password' — a single unique note — which the nestlings must later incorporate into their begging calls if they want to get fed. The trick allows fairy-wren parents to distinguish between their own offspring and those of the two cuckoo species that frequently invade their nests. The female birds also teach their mates the password.
Via The QI Elves, Dr. Stefan Gruenwald