A contender for favorite new bird: the fairy wren. It has developed an ingenious trick for weeding out imposters from the nest, reports Discover (via Kottke.org). After laying her eggs, mom sings them a special song over and over with a note "that acts like a familial password," writes Ed Yong. By the time they hatch, the chicks know it well and can sing it back. If they don't learn it, they won't get fed.
Why is this necessary? Because cuckoos like to lay their eggs in the nests of fairy wrens. If one hatches, the cuckoo chick ejects the fairy wren hatchlings to get all the attention from its foster parents. At some point, however, fairy wrens caught on and developed the song trick. It works because the cuckoo eggs come along late, and "the cuckoo embryo appears to have insufficient time to correctly learn the password note,” says the Australian researcher who did the study. If the fairy wren parents discover that their only remaining chick can't pass the test, they abandon the nest.