Humpback song is a hierarchical sequence of units, phrases and themes, sung by males on and near the breeding grounds. At any moment the males within a population converge on a shared version, which drifts gradually as small changes are copied. Individual whales adopt the current version rather than keeping their own.
Occasionally the gradual drift is interrupted. In 1996 a song type from the west Australian population appeared off eastern Australia and within two seasons had displaced the resident song entirely. Later work followed successive song types eastward through New Caledonia, Tonga, the Cook Islands and on to French Polynesia, each population adopting a version a year or two after its western neighbour.
What is not established is why the imported song wins. Novelty, the movement of a few individuals between populations, and shared feeding grounds have all been proposed. The songs are also often described as mating calls, though direct evidence linking a particular song to reproductive success remains thin.