Cumulative cultural evolution and mechanisms for cultural selection in wild bird songs

Cumulative cultural evolution, the accumulation of sequential changes
within a single socially learned behaviour that results in improved
function, is prominent in humans and has been documented in experimental
studies of captive animals and managed wild populations. Here, we provide
evidence that cumulative cultural evolution has occurred in the learned
songs of Savannah sparrows. In a first step, “click trains” replaced “high
note clusters” over a period of three decades. We use mathematical
modeling to show that this replacement is consistent with the action of
selection, rather than drift or frequency-dependent learning biases.
Generations later, young birds elaborated the “click train” song form by
adding more clicks. We show that the new songs with more clicks elicit
stronger behavioural responses from both males and females. Therefore, we
suggest that a combination of social learning, innovation, and sexual
selection favoring a specific discrete trait was followed by directional
sexual selection that resulted in naturally occurring cumulative cultural
evolution in the songs of this wild animal population.