Building variation in visual displays through discrete modifications of motion

Interactions between conspecifics are often composed of one or more discrete behavioural displays. Here, we evaluate a behaviour used in aggressive interactions between conspecific males of 10 species of leaf warblers (Phylloscopus). Using high-speed videography and methods derived from geometric morphometrics, we find that the form of a primary visual display differs significantly among species, but with large intra-specific variation and much overlap in shape space. Additional interspecific differences include a species that does not move its wings at all, two qualitatively different displays in the behavioural repertoire and the loss or gain of a pale patch on the wing. We conclude that display evolution proceeds largely by adding or subtracting discrete components from an established repertoire, accompanied by slight modifications of the core display. In these ways, more complex displays evolve on the background of the ancestral signal, thereby enabling modifications to appear without loss of ancestral efficacy.