A call in the dark: Nocturnal flight calls and their potential to advance the study of avian migration

Migratory birds have experienced widespread declines in abundance and face numerous threats. The conservation of migratory species relies in part on an improved knowledge of active migration behavior, but this behavior is difficult to study as most birds migrate at night. Flight calls, which are species-specific calls produced by many nocturnal migrants during flight, offer an opportunity to improve our understanding of migration behavior and serve as a tool to monitor populations. Although nocturnal flight call monitoring has been historically limited to small spatial and temporal scales, recent technological advancements have allowed researchers to largely shed these constraints. Despite this expansion, there are many unanswered questions regarding the function of flight calls and the proximate drivers of calling behavior. There are also unaddressed concerns that the methods used to record nocturnal flight calls, as well as other organismal, environmental, and social factors, may bias data in ways that impede (or prohibit) comparisons across time and space. Research that addresses these limitations and potential sources of bias will advance the use of nocturnal flight call monitoring for migratory bird research.  

Methods
Data for Figure 1 (The number of peer-reviewed publications using nocturnal flight call monitoring to address ecological, behavioral, or methodological questions by year from (a) 1899 to 2024 and, (b) 1999 to 2024), were generated using the Web of Science online database with the search term “nocturnal flight call AND bird”. The search was conducted on June 25th, 2024. All titles and abstracts were screened to ensure the relevance of included articles. Articles cited within relevant articles produced by the search were screened for inclusion as well. All articles can be found in the table "Screened_WebofScienceSearch_6.24.24.xls" associated with this dataset.

Code to generate Figure 1 can be found int he "Figures1and3.RMD" document associated with this manuscript.

Data for Figure 3 (An example of how weather conditions (in this case, strong headwinds aloft) may lower the flight height of migrants, causing an increase in call rates (nocturnal flight call per hour, NFC/hr) detected at ground-based autonomous recording units that is not attributable to changes in the density of birds aloft (as measured by radar). Panel (a) depicts avian migrant density at various heights above ground level (agl) as measured by the KHDX Next Generation Weather Radar in southern New Mexico on the night of September 30th, 2022, showing migrants clustering at low flight heights, most of which are below the horizontal dotted line representing the maximum sampling height of ground-based autonomous recording units (although this height has not been well defined). Panel (b) depicts headwinds aloft (assuming southbound migration) within the area sampled by the KHDX radar on the same night, indicating strong headwinds aloft and weaker headwinds at ground level. Panels (c) and (d) depict the same data as (a) and (b) but for a night (September 8th, 2022) in which headwinds were more evenly distributed, resulting in more migrants flying above recorder sampling range), were generated using the bioRad package in r (Dokter et al. 2019), wind data were accessed from the North American Regional Reanalysis dataset (Mesinger et al. 2006). Nocturnal flight call data were extracted from the dataset- https://doi.org/10.5061/dryad.8w9ghx3x3.

Code to create figure 3 can be found in the "Figures1and3.RMD" document associated with this dataset. This code file also includes all necessary information for processing radar and wind data.

候鸟的种群丰度已普遍下降，且面临诸多生存威胁。候鸟保护工作在一定程度上有赖于对其活跃迁徙行为的认知深化，但由于多数候鸟于夜间迁徙，该行为的研究难度极大。飞行鸣叫声是诸多夜间迁徙鸟类在飞行过程中发出的物种特异性鸣叫声，这为我们深化对迁徙行为的认知提供了契机，同时也可作为种群监测的有效工具。尽管长期以来夜间飞行鸣叫声（nocturnal flight call）监测的应用仅局限于较小的空间与时间尺度，但近年来的技术进步已大幅突破了这些限制。即便如此，关于飞行鸣叫声的功能以及引发鸣叫行为的近因驱动因素，仍有诸多未解答的问题。此外，尚有尚未解决的顾虑：当前用于记录夜间飞行鸣叫声的方法，以及其他生物体、环境与社会因素，可能会以妨碍（甚至完全阻碍）跨时空数据对比的方式引入数据偏差。针对这些局限与潜在偏差来源开展的研究，将推动夜间飞行鸣叫声监测在候鸟研究中的应用。

## 方法
图1的数据（图1为1899年至2024年（a）、1999年至2024年（b）间，采用夜间飞行鸣叫声监测手段以解答生态、行为或方法学问题的同行评议论文数量年度变化），通过Web of Science在线数据库，以检索式“nocturnal flight call AND bird”生成。检索于2024年6月25日完成。研究人员对所有检索到文献的标题与摘要进行了筛选，以确保纳入文献的相关性。同时，还对此次检索所得相关文献的参考文献进行了筛选，以纳入符合要求的文献。所有相关文献均可在本数据集附带的"Screened_WebofScienceSearch_6.24.24.xls"表格中获取。

生成图1的代码可在本论文附属的"Figures1and3.RMD"文档中找到。

图3的数据（图3示例展示了高空强逆风如何降低迁徙鸟类的飞行高度，进而导致地面自主录音设备检测到的鸣叫声率（每小时夜间飞行鸣叫声数，NFC/hr）上升，而这一变化并非由高空鸟类密度变化所引起——该密度通过雷达（radar）测得。（a）面板展示了2022年9月30日夜间，新墨西哥州南部KHDX下一代天气雷达所测得的不同离地高度（above ground level, agl）的候鸟密度，结果显示迁徙鸟类聚集于低飞行高度，其中多数低于代表地面自主录音设备最大采样高度的水平虚线（尽管该高度尚未明确界定）。（b）面板展示了同一夜间KHDX雷达采样区域内的高空逆风（假设为南向迁徙）情况，结果显示高空存在强逆风，而地面逆风较弱。（c）与（d）面板分别对应（a）与（b）的数据集，但对应日期为2022年9月8日，当日逆风分布更为均匀，因此更多迁徙鸟类飞行于录音设备采样范围之上），通过R语言的bioRad包生成（Dokter等人，2019），风场数据来源于北美区域再分析数据集（North American Regional Reanalysis dataset），该数据集由Mesinger等人于2006年发布。夜间飞行鸣叫声数据提取自数据集https://doi.org/10.5061/dryad.8w9ghx3x3。

生成图3的代码可在本数据集附属的"Figures1and3.RMD"文档中找到，该代码文件同时包含了处理雷达与风场数据所需的全部信息。