Thick-billed grasswren (Amytornis modestus) songs differ across subspecies and elicit different subspecific behavioural responses

Passerine song has many functions including mate attraction and territory defence. When songs across populations diverge, this can lead to changes in conspecific recognition and barriers to gene flow, which affect evolutionary processes that could lead to speciation. Two subspecies of thick-billed grasswren (Amytornis modestus) have a parapatric distribution characterised as a narrow region of high genetic admixture where the two subspecies meet. Outside the region of parapatry, the subspecies are genetically and morphologically diverged and weak inter-subspecific gene flow is asymmetric from A. m. indulkanna to A. m. raglessi. We examined the differences between song of A. m. indulkanna and A. m. raglessi and experimentally broadcast each subspecies song to compare territory-holder response in relation to intruder subspecies type. Our aim was to determine if territory owners have a different response to intruders based on the subspecific song type. The song of each subspecies contained unique vocal elements that were absent in the other subspecies. A. m. raglessi responded similarly to con-subspecific and hetero-subspecific intruder song, and A. m. indulkanna responded more often and with greater intensity to hetero- compared to con-subspecific intruder song. The stronger response by A. m. indulkanna towards hetero-subspecific intruders provides a plausible behavioural explanation for the observed patterns of asymmetrical gene flow.