Fairy wrens have a cuckoo problem. Specifically, cuckoos lay their eggs in the nest of the fairy wrens and, if undetected, they would end up raising the baby cuckoos to the potential detriment of their own children. But what the fairy wren mother does is after laying her eggs, she sings a unique song to the eggs until they hatch. Having learned the song while in-egg, the hatched baby wrens sing back part of the song to get fed.
She kept 15 nests under constant audio surveillance, and discovered that fairy-wrens call to their unhatched chicks, using a two-second trill with 19 separate elements to it. They call once every four minutes while sitting on their eggs, starting on the 9th day of incubation and carrying on for a week until the eggs hatch.
When Colombelli-Negrel recorded the chicks after they hatched, she heard that their begging call included a single unique note lifted from mum’s incubation call. This note varies a lot between different fairy-wren broods. It’s their version of a surname, a signature of identity that unites a family. The females even teach these calls to their partners, by using them in their own begging calls when the males return to the nest with food.
These signature calls aren’t innate. The chicks’ calls more precisely matched those of their mother if she sang more frequently while she was incubating. And when Colombelli-Negrel swapped some eggs between different clutches, she found that the chicks made signature calls that matches those of their foster parents rather than those of their biological ones. It’s something they learn while still in their eggs.
(via bruce schneier)