Aleutian Tern Onychoprion aleuticus numbers have been in steep decline at known Alaskan breeding colonies in recent decades (IUCN recently uplisted to ‘Vulnerable’). Available data suggest that most of the species may currently breed in Russia. Efforts to document global abundance and trends have been hampered by remoteness of colonies, lack of a formal monitoring programme, and the absence of reproducible population estimates with quantifiable errors, especially for large colonies. We surveyed four historically large colonies in Russia (2018) and Alaska (2019), which together may comprise 30–50% of the global breeding population. At each colony we obtained high resolution aerial photographs using a small Unmanned Aircraft System (sUAS). The large size of the colonies and the minimum altitude required to identify terns made it impractical to collect imagery of the entire colony. Instead, we employed a sampling approach, with sample locations selected based on spatially balanced acceptance sampling. Statistically sampled, low altitude sUAS images provided a fast, reproducible, and rigorous count of abundance for geographically large colonies, with low disturbance, and were generally consistent with concurrent ground-based observations. Concurrence among observers in photo counts indicated high precision in counts of attending birds and unattended nests, although species attribution in mixed tern colonies remains a source of significant uncertainty. Our results indicate that the four colonies surveyed here together supported <2,500 pairs of Aleutian Terns in the survey years. None of the colonies approached their peak size reported previously, likely due to recent predation, long-term decline, cold early season weather, or other factors. If these reduced colony sizes are representative of the current conditions, the implications for the global population would be dire.

阿留申燕鸥Onychoprion aleuticus近几十年来，已知的阿拉斯加繁殖地的数量急剧下降（国际自然保护联盟最近将其列为“脆弱”）。现有数据表明，大多数物种目前可能在俄罗斯繁殖。记录全球丰度和趋势的努力受到了殖民地偏远、缺乏正式监测计划以及缺乏具有可量化误差的可重复种群估计（尤其是对于大型殖民地）的阻碍。我们调查了俄罗斯（2018 年）和阿拉斯加（2019 年）的四个历史上大型群体，它们合计可能占全球繁殖种群的 30-50%。在每个殖民地，我们使用小型无人机系统（sUAS）获得了高分辨率航空照片。由于栖息地面积较大，而且识别燕鸥所需的最低海拔高度，使得收集整个栖息地的图像变得不切实际。相反，我们采用了抽样方法，根据空间平衡验收抽样选择样本位置。统计采样的低空无人机图像为地理上较大的菌落提供了快速、可重复且严格的丰度计数，干扰较低，并且与同时进行的地面观测结果基本一致。观察者对照片计数的一致表明，尽管混合燕鸥群落的物种归属仍然是一个重大不确定性的来源，但参与鸟类和无人看管巢穴的计数具有很高的精确度。我们的结果表明，在调查年份，这里调查的四个群体总共养活了不到 2,500 对阿留申燕鸥。没有一个群体达到之前报道的峰值大小，可能是由于最近的捕食、长期的衰退、寒冷的早季天气、或其他因素。如果这些减少的蜂群规模代表了当前的情况，那么对全球人口的影响将是可怕的。