The nestlings of fairy-wrens don’t have much of a choice when it comes to listening to their mothers: The birds must reproduce a particular sound learned from their moms to be fed, according to a study published Thursday in the journal Current Biology.

The researchers, led by Sonia Kleindorfer of Flinders University in Australia, call the sound the bird’s “learned password.”

The team discovered the remarkable mother-chick ritual by chance when they noticed that the mothers were aiming calls at their eggs well before they hatched. So they set out to determine why. First, the researchers realized that the call made by the nestlings in order to be fed was different in each nest, suggesting it might be learned.