CUCKOOS are pragmatic parents. They lay eggs in the nests of other birds, fooling the unwitting adopters into expending resources on brood that is not their own. Their eggs mimic those of the host. And their chicks, which grow rapidly and have quite an appetite, sometimes push the legitimate nestlings out. In an evolutionary arms race, however, the Australian superb fairywren has developed a wily trick of its own. It tells its fledgling warblers from intruders by their begging calls. If its trill isn’t right, the hatchling goes hungry. Ornithologists have, however, long wondered whether the songs are genetically imprinted or learned.

Sonia Kleindorfer from Flinders University in Australia and her team stumbled on the answer while studying how fairy wrens use their call-signs to protect themselves from predators. As they report in Current Biology, the songbirds teach a signature tune before the offspring hatch.

The researchers arrived at their conclusion after studying the wren's 15-day incubation cycle. They noticed that the mother would typically start singing to its unhatched brood on day nine or ten and stop whenever the eggs hatch (cuckoos' incubation cycle is a few days shorter than wrens' is). A tiny microphone tucked inside 22 nests picked up the incubation calls. When these were later compared to the warbler's chirping the two notes turned out to be almost identical. The female uses the same note to solicit food from its pair-male, in effect teaching it too the unique password.

Dr Kleindorfer then concealed loudspeakers inside a few nests to play back the nestlings' begging calls. Parents would swoop down to the nests with food only if the replayed calls matched those of their brood. Otherwise, they abandon the nest altogether.