Subspecies discrimination based on song structure by Willow Flycatchers

Animals use acoustic signals to repel competitors and attract mates, and signal divergence among populations can promote reproductive isolation. Empidonax flycatchers are insectivorous songbirds distributed across North and Central America that are conservative in plumage, but often exhibit differences in songs both among and within species. Four subspecies of Willow Flycatchers (Empidonax traillii) have been recognized and previous analyses have revealed differences in song structure among a subset of these subspecies. Using reciprocal playback experiments in the field, we tested for subspecific song discrimination among these four putative subspecies of Willow Flycatchers. We found that three subspecies (E. t. adastus, E. t. brewsteri, and E. t. traillii) responded similarly to their own songs and those of each other, but all three subspecies responded significantly less aggressively to songs of the southwestern subspecies (E. t. extimus). In contrast, the southwestern subspecies (E. t. extimus) responded significantly more aggressively to its own song than to those of the other three subspecies. Our results indicate that behavioral responses reflect differences in song structure among subspecies; subspecies responded more strongly to songs of subspecies with similar structures, less strongly to songs most different in structure, and the subspecies with the most distinctive song (E. t. extimus) responded less to songs of the other three subspecies. If responses of males to songs reflect relative reproductive compatibility within and among subspecies, songs may contribute to reproductive isolation of the four subspecies of Willow Flycatchers.