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 intraspecific variation and much overlap in shape space. Additional interspecific differences include a species which does not move its wings at all, two quantitatively 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.

Methods
We induced aggressive displays using song playback, filmed the displays using high-speed video, and adapted methods from geometric morphometrics to ask the extent to which the form of the display is conserved or varies across species. Using these videos, we also collected information on wing flick rate. The data was processed using QuickTime player to measure rates, and all further analyses conducted in R. We also measured habitat illuminance in the primary breeding habitats across the elevational gradient using Onset light and temperature loggers. Data from the loggers were downloaded to the HOBOconnect app via Bluetooth at the time of collection and analysed using scripts written for R.