Incubating females give a specific call to the clutch many times a day. After hatching, the nestlings' begging calls include a unique note element matching their own mother's incubation call, and the match is specific to the individual female rather than to the species. Cross-fostering experiments, in which eggs are moved between nests, show that chicks learn the element they were exposed to rather than inheriting it.
The function appears to relate to brood parasitism. Superb fairywrens are hosts to Horsfield's bronze-cuckoo, whose chick evicts the wrens' own young. A cuckoo laid late in the incubation period has less exposure to the call, and adult wrens have been observed reducing provisioning or abandoning nests where the begging call does not match.
How decisive the mechanism is in practice remains debated. Rejection is not absolute, cuckoo chicks do sometimes acquire an approximate match, and some researchers argue the calls serve broader functions in parent-offspring recognition rather than acting as a strict password.