Daily and seasonal changes of vocal activity of the Common Crane Grus grus: implications for conservation and monitoring efforts

The vocal activity of Common Cranes Grus grus varies with time of day, season, population abundance, and nesting habitat area, and consequently affects the accuracy and precision of single survey estimates of breeding population size. To examine (1) how the vocal activity of Common Cranes varies during the day and seasonally, (2) how these changes affect the probability of observer detection of a pair during a single survey, and (3) which environmental variables influence vocal activity. Autonomous sound recorders recorded soundscapes (24 hours per day) in 24 occupied territories of Common Cranes, three times during the breeding season. Recordings were analysed by manual spectrogram scanning. The probability of detection was calculated for four survey durations: 10 and 30 min, one and two hours. Environmental characteristics were measured from satellite images. Common Crane vocal activity during a single survey significantly varied during the day, decreased with the progress of the breeding season, negatively correlated with the area of nesting habitat, and positively correlated with the number of neighbouring pairs within 2 km. 24-hour surveys recorded at least one vocalization in 96% of occupied territories in March, 67% in April, and 46% in May. The highest probability of detection during a single, one-hour survey ranged from 0.46 in March, 0.25 in April to 0.17 in May. Reducing survey duration from 1 h to 10 min caused a decrease of detection to 0.29 in March and 0.13 in April and May. We recommend surveying Common Cranes at the beginning of the breeding season, from one hour before sunrise to four hours after sunrise and from sunset to one hour after sunset, to ensure the highest (0.33–0.46) probability of detection during a single, one-hour survey. Alternatively, autonomous sound recorders may ensure detection of almost 100% of breeding pairs during a single, 24-hour survey at the beginning of the breeding season.灰鹤（Common Crane，学名Grus grus）的鸣唱活动随每日时段、季节、种群数量及筑巢栖息地面积而变化，进而影响单次调查对繁殖种群数量估算的准确性与精度。本研究旨在探讨三个核心问题：(1) 灰鹤的鸣唱活动在日间及季节尺度上的变化规律；(2) 这些变化如何影响单次调查中观测者发现鹤对的概率；(3) 哪些环境变量会对鸣唱活动产生影响。研究人员在繁殖季内分三次，于24个已被灰鹤占据的领地内布设自主声学记录仪，全天候（每日24小时）记录声景数据。通过手动声谱扫描对录音数据进行分析，并针对10分钟、30分钟、1小时及2小时四种调查时长计算发现概率。环境特征参数通过卫星影像提取获取。研究结果显示，单次调查期间的灰鹤鸣唱活动存在显著日间差异，且随繁殖季推进逐渐降低；其与筑巢栖息地面积呈负相关，与2公里范围内的邻近鹤对数量呈正相关。3月、4月、5月的24小时调查中，分别有96%、67%、46%的已占据领地被记录到至少一次鸣唱。单次1小时调查的最高发现概率在3月为0.46、4月为0.25、5月为0.17；若将调查时长从1小时缩短至10分钟，3月的发现概率降至0.29，4月和5月则降至0.13。本研究建议在繁殖季初期开展灰鹤调查，最佳时段为日出前1小时至日出后4小时，以及日落至日落1小时后，以确保单次1小时调查的发现概率达到最高（0.33~0.46）。此外，在繁殖季初期开展单次24小时调查时，使用自主声学记录仪可实现对几乎100%繁殖鹤对的检测。