Whose Kids?

Bringing up the children is an expensive business, whether you're a human or a bird. Unknowingly bringing up someone else's offspring instead of your own is even more costly, especially when that offspring ousts your own.

Superb Fairywren nests are regularly parasitised by Horsfield's Bronze-Cuckoos. The cuckoo lays one egg in a Superb Fairywren nest and this egg hatches earlier than the fairywren eggs. The cuckoo nestling then ejects all other eggs from the nest so it will receive all the food that is brought by the parent birds.

A male Superb Fairywren at its nest.
But Superb Fairywrens have developed a way of detecting intruders.
Researchers at Flinders University found that female Superb Fairywrens sing to their eggs, with each female having her own song. When the eggs hatch, the nestlings use elements of the mother's song as part of their begging calls. The male parent bird also learns the mother's song and responds to the nestlings' begging calls as well.

Because the cuckoo egg is laid later than the host eggs, that embryo does not have the same opportunity to learn the host's song. Then, when the cuckoo nestling doesn't reproduce the 'password', the Superb Fairywrens are very likely to abandon that nest and start again. This is especially so when the fairywrens have detected cuckoos in the area beforehand.

The researchers checked whether the call elements used by nestlings were learned or innate by swapping eggs between a number of nests. Later, the researchers used call playback to see if mothers responded to their genetic offspring in the same way as they did to their fostered nestlings. The results showed clearly that call elements were learned, not innate.

Horsfield's Bronze-cuckoo
The full article is: Embryonic learning of vocal passwords in Superb Fairy-Wrens reveals intruder cuckoo nestlings, by Dianne Colombelli-Ne.grel and others, in Current Biology (2012). Find it at: