If a bird calls, will we detect it? Factors that can influence the detectability of calls on automated recording units in field conditions

Automated recording units (ARUs) are an integral instrument in a broad range of applications such as invasive species management, behavioural studies, and environmental impact assessments. They have become particularly important in conservation programmes for a range of cryptic threatened bird species. While the fundamental issues of signal attenuation and degradation are well known, understanding how environmental factors influence detectability of calls on ARU recordings in particular circumstances is essential for the accurate interpretation of their results. Using five transects spaced at equal angles running from a central playback source we tested the detectability of Pezoporus flaviventris (Western Ground Parrot) and Atrichornis clamosus (Noisy Scrub-bird) calls on Wildlife Acoustics SM2 and SM4 devices. Both species call from on or near the ground in moderately dense heathlands. Detectability of calls was affected by wind direction: fewer calls were detected on an ARU that was up-wind of the broadcast source compared with another on a down-wind unit. More calls were detected on ARUs deployed at higher relative elevation, and detectability decreased with increased distance from the playback source. Calls were detected at a greater distance from the playback source when using SM4 ARUs, compared to SM2 ARUs. Inconsistency in the detection of bird calls through space and time could lead to errors in metrics calculated from acoustic data. Further investigation is required to determine how environmental factors such as wind direction can be accounted for in data analysis.

自动录音设备（Automated Recording Units, ARUs）是一类应用场景广泛的核心仪器，覆盖入侵物种管理、行为学研究与环境影响评价等诸多领域。在针对多种隐秘濒危鸟类的保护项目中，其重要性愈发凸显。尽管信号衰减与劣化的基本原理已为人熟知，但明确特定场景下环境因素如何影响自动录音设备录制的鸟鸣声可检测性，仍是准确解读其监测结果的核心前提。本研究以中央播放声源为原点，设置5条等角度分布的样带，测试了西地鹦鹉（Pezoporus flaviventris）与噪薮鸟（Atrichornis clamosus）的鸣叫声在Wildlife Acoustics SM2及SM4型自动录音设备上的可检测性。两种鸟类均在中等密度的灌丛生境中的地面或近地面处鸣叫。鸟鸣声的可检测性受风向影响：相较于位于播放源下风侧的设备，位于上风侧的自动录音设备检测到的鸣声声数更少。部署于相对海拔更高处的自动录音设备可检测到更多鸣叫声，且可检测性随与播放源距离的增加而降低。与SM2型自动录音设备相比，使用SM4型设备时，可在距离播放源更远的位置检测到鸣叫声。鸟鸣声检测在空间与时间维度上的不一致性，可能会导致基于声学数据计算的监测指标出现误差。后续仍需开展深入研究，以明确如何在数据分析中纳入风向这类环境因素的影响。