Data from: Repertoire-based individual acoustic monitoring of a migratory passerine bird with complex song as an efficient tool for tracking territorial dynamics and annual return rates

In field ecological and behavioural studies, it is often necessary to identify specific individuals. In birds, colour rings are often used to mark individuals; however, rings are often difficult to observe, especially in small species and dense habitats. Acoustic-based monitoring detecting individuals by their characteristic vocalization is a potentially suitable alternative, but this approach is challenging in species with complex songs. On the example of the Tree Pipit (Anthus trivialis), a small migratory passerine often singing in flight or from perches obscured by foliage, we demonstrate that acoustic monitoring based on the syllable repertoire can be very efficient tool for individual recognition. During a three-year study, we obtained over 500 recordings from males from one study population (a number of them returning after winter). Males banded with colour rings were repeatedly recorded throughout the seasons, and syllable repertoires were determined from spectrograms for each recording. The repertoire of each unambiguously identified male was distinct and stable within as well as between seasons; and males with similar syllable repertoires differed in syntax. Based on the congruence between identification based solely on spectrogram assessment, and that based on observation of colour rings, we inferred that reliable identification of singing males (including non-ringed ones) was possible in the studied population from assessing a repertoire and song syntax of less than 5 minute recording (containing 20–30 songs). The acoustic-based data: 1) increased the overall estimated number of territorial males at the study locality (from 49 ringed to 61), 2) revealed dynamic within-season changes in territory occupancy that would otherwise be missed; 3) allowed identification of returning birds (including non-ringed ones and those actively avoiding approaching humans); 4) improved estimates of the total number of males appearing at the locality during the breeding season, and the period of their presence. Our results suggest that some commonly used methods may substantially underestimate return rates of migratory bird species. Individual acoustic monitoring should be applicable on various bird species with complex song and stable repertoires, and may be particularly useful for those living in dense habitat or sensitive to handling.

在野外生态学与行为学研究中，通常需要对特定个体进行识别。对于鸟类而言，个体标记常采用彩色环志（colour rings），但该类环志往往难以被观测到，尤其是针对小型鸟类种群以及生境郁闭度较高的场景。基于鸣唱特征的声学监测是一种颇具潜力的替代方案，但对于鸣唱结构复杂的鸟类类群而言，该方法仍存在较大挑战。本研究以林鹨（Anthus trivialis）——一种常于飞行间或被枝叶遮蔽的栖木上鸣唱的小型迁徙鸣禽——为研究对象，证实基于鸣唱音节库（syllable repertoire）的声学监测可作为一种高效的个体识别手段。本研究为期三年，从某研究种群的雄性个体中累计获取了超过500条鸣唱录音（其中部分个体为越冬后回迁的个体）。研究人员对佩戴彩色环志的雄性个体进行了多季重复录音，并通过每条录音的声谱图（spectrograms）提取其鸣唱音节库。经明确识别的每只雄性林鹨，其鸣唱音节库均具有独特性，且在单季内与跨季间均保持稳定；即便音节库相似度较高的个体，其鸣唱句法（syntax）也存在显著差异。基于仅通过声谱图分析得到的个体识别结果与彩色环志观测结果之间的一致性，本研究推断：在该研究种群中，仅需分析时长不足5分钟、包含20~30首鸣唱的录音中的音节库与鸣唱句法，即可对鸣唱的雄性个体（包括未佩戴环志的个体）实现可靠识别。本研究的声学监测数据具有多重价值：1）提升了研究区域内领域雄性个体的总估算数量（从49只佩戴环志的个体增至61只）；2）揭示了单季内领域占用的动态变化，而这类变化若采用传统方法则会被遗漏；3）可实现对回迁个体的识别（包括未佩戴环志的个体，以及主动躲避人类靠近的个体）；4）优化了繁殖季内出现在研究区域的雄性个体总数量及其停留时长的估算结果。本研究结果表明，部分常规使用的调查方法可能大幅低估了迁徙鸟类的回迁率。基于个体鸣唱特征的声学监测方法，可推广应用于各类拥有复杂鸣唱与稳定音节库的鸟类类群，尤其适用于栖息于郁闭生境或对捕捉操作较为敏感的物种。