The superb fairy-wren is a small Australian bird whose nests are often invaded by cuckoo birds. The cuckoos lay their eggs in the wrens' nests, leaving their young to be taken care of by the wren family. This is bad for the actual wren chicks, because it limits the amount of resources, like food, that they receive. The fairy-wren has developed an astonishing system to protect their young and identify the invading cuckoo chicks.
Researchers from Flinders University in Adelaide found that mother wrens sing a unique tune to their eggs as the chicks grow inside. Once the eggs hatch, only those chicks that can repeat the "password" tune are fed. Prior to this, researchers had no idea that the embryo (egg) stages of development had any ability to learn.
To test if the tune was inherited, researchers switched around the eggs of different fairy-wren eggs before the teaching period began. When they hatched, the chicks had adopted the tune of the nest they were moved to, and not that of their real parents. This shows the tune is learned, and not something they inherited from their genes.
The wren starts teaching the tune about 10 days after the eggs are laid, and continues for the remaining 5 days until they hatch. The cuckoo eggs hatch after only 2 days and are unable to learn the whole tune. This allows the parent wrens to escape the nest and build another before they waste their time and energy on the young cuckoo birds.
The system is not perfect though. One cuckoo species is caught by the fairy-wrens every time. The young cuckoos of the other species, however, have learned to try and guess the food password, with a success rate of about 60%.
The possibility that eggs can be taught by mother birds is an exciting one with numerous branches. It might allow identification of family members, a system for selecting certain traits in young and be expanded to research the learning abilities of other animals' embryonic stages.