Fairy Wren Passwords
Mother fairy wrens teach their chicks passwords while they’re still in their eggs to tell them from cuckoo impostors:
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.
It’s worth noting that this is primarily of use to the chicks’ parents, so they know not to expend time and energy on the impostor cuckoo chick. Cuckoo chicks, as part of their evolutionary adaptation, kick the real chicks out of the nest, so they’re lost in any case. It’s the fact that the signal allows the parents to identify impostors and start a new brood that’s of evolutionary advantage.