It's happened to the best of us.
A recent study published in the journal Current Biology has discovered that fairy wren mothers will start singing a particular song to their eggs about a week before they hatch, which will later act as a "learned password." Once they are born, the wren chicks then have to sing the tune back to their mom in order to be fed, and if they fail, the mother will abandon the nest and leave the birds to die. They do that because fairy wrens often fall victim to the bronze cuckoo, which lays eggs in their nests and tricks them into raising the home-invading brood as their own, who, other than being moochers, also have this silly habit of pushing their step-siblings out of the nest.
They're the jobless futon crashers of the animal kingdom.
Scientists concluded that the fairy wrens evolved this system of password voice recognition to tell their real offspring from cuckoo interlopers, even if the latter are massive, gray monstrosities that look absolutely nothing like wrens. Yeah, for all their complex nest security, birds are kind of stupid like that.