Relative amplitude modulation in woodpecker drums shows species-specific patterns

Abstract

Amplitude is one of several components that comprise multicomponent acoustic signals. We aimed to study relative amplitude modulation over the duration of an acoustic signal that woodpeckers produce to defend their territories, the drum. Woodpecker drumming is a tractable system for this type of analysis because drum recordings are readily available through public databases and the physical mechanism for amplitude generation is easily observable and modelled. Accordingly, we characterized relative amplitude modulation of drums from eight woodpecker species by fitting models to the amplitude modulation pattern and found that species' modulation patterns (1) showed consistency within species and (2) differed between species. Functionally, relative amplitude modulation could showcase motor skill when individuals reach peak amplitude quickly or maintain a high amplitude. Alternatively, amplitude modulation could be used to evaluate individual quality through consistently repeating drums without mistakes. Our findings highlight the importance of studying both the mechanical production and functional role of multicomponent signals to understand how a signalling system works.