Depending on the listener, song is a lure or a threat. By imitating the song of established seniors with whom they would have to compete, young breeders presumably gained an advantage in courtship and territorial defence.

The vocal imitation expressed by adults, however, is a complex behaviour requiring sophisticated underlying brain circuits, Nottebohm said.

How would birds with only innate, genetically foreordained vocal repertoires have evolved the ability? One part of a plausible explanation is that vocal learning emerged initially as a vehicle for creating variability in juveniles before territory and mate are an issue, according to Nottebohm.

Such a development would require a simpler beginning brain circuit, which could later become part of the complex brain architecture required for imitation.

The new research is compatible with the idea that vocal learning first emerged outside the context of reproductive pressures, said a Rockefeller release.

Liu found that while the food begging calls of young males vary considerably from moment to moment and between individuals, those of young females are very stereotyped and all alike.

These findings were published in PLoS ONE.