Can Nocturnal Flight Calls of the Migrating Songbird, American Redstart, Encode Sexual Dimorphism and Individual Identity?

Bird species often use flight calls to engage in social behavior, for instance maintain group cohesion and to signal individual identity, kin or social associations, or breeding status of the caller. Additional uses also exist, in particular among migrating songbirds for communication during nocturnal migration. However, our understanding of the information that these vocalizations convey is incomplete, especially in nocturnal scenarios. To examine whether information about signaler traits could be encoded in flight calls we quantified several acoustic characteristics from calls of a nocturnally migrating songbird, the American Redstart. We recorded calls from temporarily captured wild specimens during mist-netting at the Powdermill Avian Research Center in Rector, PA. We measured call similarity among and within individuals, genders, and age groups. Calls from the same individual were significantly more similar to one another than to the calls of other individuals, and calls were significantly more similar among individuals of the same sex than between sexes. Flight calls from hatching-year and after hatching-year individuals were not significantly different. Our results suggest that American Redstart flight calls may carry identifiers of gender and individual identity. To our knowledge, this is the first evidence of individuality or sexual dimorphism in the flight calls of a migratory songbird. Furthermore, our results suggest that flight calls may have more explicit functions beyond simple group contact and cohesion. Nocturnal migration may require coordination among numerous individuals, and the use of flight calls to transmit information among intra- and conspecifics could be advantageous. Applying approaches that account for such individual and gender information may enable more advanced research using acoustic monitoring.鸟类通常借助飞行叫声（flight call）开展社交行为，例如维持种群凝聚力、传递个体身份、亲缘关系或社交关联信息，以及信号发出者的繁殖状态。此类叫声还存在其他应用场景，尤其在迁徙鸣禽中，可用于夜间迁徙过程中的通信。然而目前学界对这类鸣唱所传递的信息仍缺乏完整认知，在夜间迁徙场景下这一局限尤为显著。 为探究飞行叫声是否可编码信号发出者的性状信息，我们对夜间迁徙鸣禽美洲红尾鸲（American Redstart）的叫声开展了多项声学特征量化分析。研究样本采集自宾夕法尼亚州雷克托市鲍德米尔鸟类研究中心，通过雾网捕捉（mist-netting）作业捕获的暂留野生个体的叫声。我们量化评估了个体间与个体内、不同性别组间与组内，以及不同年龄组间与组内的叫声相似度。 同一个体的叫声彼此间相似度显著高于其与其他个体叫声的相似度，且同性个体间的叫声相似度显著高于异性个体间。当年孵化个体与非当年孵化个体的飞行叫声并无显著差异。 本研究结果表明，美洲红尾鸲的飞行叫声或可携带性别与个体身份标识信息。据我们所知，这是首次在迁徙鸣禽的飞行叫声中发现个体特异性与性别二态性的相关证据。此外，本研究结果提示，飞行叫声的功能或不止于简单的种群联络与凝聚力维持。夜间迁徙需要众多个体间的协同配合，借助飞行叫声在种内及同种个体间传递信息或具备演化优势。引入兼顾此类个体与性别信息的研究方法，或可推动基于声学监测的更高阶研究开展。