Talk about "singing for your supper": New research shows that fairy-wren chicks have to sing a specific song to their mother or she will refuse to feed them. It's not that the mama birds are music critics; the song actually stops cuckoo birds from taking advantage of the fairy-wrens' maternal instincts.
Because cuckoo birds frequently lay their eggs in the nests of fairy-wrens—and feeding these invaders is a costly waste of resources—the mother teaches her unborn chicks (that's right, while they're still in their eggs) a particular song to sing when feeding time arrives. If the birds don't sing the tune, they won't get any food. If none of the chicks sing the song, she will assume the nest is overrun with cuckoo chicks and abandon it.
Each mother has her own particular song to teach her chicks, and if mates or other birds help her to feed them, she will teach those birds the password as well—out of earshot of the nest—to further ensure no intruding chicks receive valuable nutrients.
According to io9, which has more information on the study, this strategy is pretty remarkable; that this trait evolved "shows just how versatile and exploratory nature can be when confronted with a problem. But given how detrimental brood parasitism is to the host bird's reproductive fitness, it shouldn't come as a surprise that evolution found a way."