Superb Fairy-wrens teach their embryonic chicks a secret code. This incubation call contains a special note that will later serve as a password. When the chicks have hatched, this password enables the adult birds to identify their babies in the darkness of their domed nest. A species of Australian cuckoo lays its eggs in the wren’s nest, hoping to pawn off the task of parenting. But wren chicks learn their mother’s song and incorporate the password note into their begging calls.

More info and transcript at BirdNote.org. 

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