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.