Willow Flycatcher subspecies song discrimination experiment data

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

动物可借助声学信号（acoustic signals）驱赶竞争者并吸引配偶，种群间的信号分化可促进生殖隔离（reproductive isolation）的形成。纹霸鹟属（Empidonax）霸鹟为分布于北美与中美洲的食虫雀形目鸟类，其羽色较为保守，但种间与种内常表现出鸣曲差异。目前柳纹霸鹟（Empidonax traillii）已被划分为4个亚种，既往分析已揭示其中部分亚种间的鸣曲结构差异。本研究通过野外双向回放实验（reciprocal playback experiments），针对这4个推定亚种开展鸣曲识别能力测试。研究结果显示：2个西部亚种（E. t. adastus与E. t. brewsteri）对自身鸣曲、彼此鸣曲以及东部亚种（E. t. traillii）的鸣曲展现出相近的攻击应答，但对西南亚种（E. t. extimus）的鸣曲应答强度显著更低。东部亚种（E. t. traillii）同样对自身鸣曲的攻击应答显著强于西南亚种（E. t. extimus），但对E. t. adastus与E. t. brewsteri的鸣曲应答存在波动。与之相对，西南亚种（E. t. extimus）对自身鸣曲的攻击应答强度高于其余所有亚种。本研究结果表明，行为应答模式可反映亚种间鸣曲结构的差异：鸣曲结构相似的亚种会对同类鸣曲产生更强应答，而对鸣曲结构差异最显著的类群应答更弱；鸣曲最具特异性的亚种，则对其余三个亚种的鸣曲应答较弱。若雄性个体对鸣曲的应答可反映亚种内外的相对生殖兼容性，则鸣曲差异可能推动柳纹霸鹟亚种间的生殖隔离形成。