Data from: Quantifying population size of migrant birds at stopover sites: combining count data with stopover length estimated from stable isotope analysis

1. Regular counts of migrating animals at stopover sites have been used as a measure of site importance at the global scale as well as for monitoring long-term population changes. However, migratory passage can last for several weeks and the turnover rate of individuals is often high, preventing the use of peak counts to estimate the total number of migrants. This estimate can be achieved, however, by combining count data with information on stopover length. 2. Here, we developed a new method to quantify the total number of migrant birds using stopover areas hosting overlapping populations of local (breeding, wintering or resident) and passage individuals of a given species. We illustrate the application of this method by estimating the number of spring migrant dunlins Calidris alpina stopping over at the Tagus estuary (Portugal). We used carbon and nitrogen isotopic signatures in toenails to identify migrants and to estimate their proportion in total counts. We then employed isotopic clock-models to determine time since arrival (TSA) of individuals from signatures in red blood cells and plasma and, using a simulation approach, we derived the relationship between TSA and stopover length. Finally, by dividing the number of migrants.day (obtained from counts) by the stopover length, we assessed the size of the migrant population. 3. We demonstrated that stopover length can be directly predicted from mean TSA values of birds sampled during migration, regardless of variations in (1) migratory rate, i.e., the phenology of arrivals throughout the migratory period and (2) the number of birds involved in the migratory event. Migrant dunlins stayed on average 7.5 days at the Tagus estuary during spring migration, which combined with count data resulted in an estimate of ca. 30000 passage dunlins using the study site. 4. Our novel approach is not strict in its assumptions, and therefore can be customized to the specificities of different taxa and study areas. Estimates of total number of migrants along migratory flyways are critical to identify priority sites for conservation action. In fact, single stopover sites may host a very significant proportion of a global population albeit for a short period.

1. 在全球尺度上，研究者常通过统计中途停歇地（stopover sites）的迁徙动物数量，来评估该停歇地的重要性并监测种群长期动态变化。然而，迁徙过境过程往往持续数周，个体更替速率通常较高，这使得无法通过峰值计数来估算迁徙动物的总数量。不过，若将计数数据与停歇时长信息相结合，便可实现这一估算目标。
2. 本研究开发了一种全新方法，可用于量化停歇地内的迁徙鸟类总数量——此类停歇地同时容纳某一物种的本地种群（繁殖种群、越冬种群或留居种群）与过境个体的重叠种群。我们以葡萄牙塔霍河河口（Tagus estuary）停歇的春季过境黑腹滨鹬（Calidris alpina）为例，展示了该方法的应用流程：首先通过趾甲中的碳、氮同位素特征（isotopic signatures）识别过境个体，并估算其在总计数中的占比；随后借助同位素时钟模型（isotopic clock-models），基于红细胞（red blood cells）与血浆（plasma）中的同位素特征测定个体的到达后时间（Time Since Arrival, TSA），并通过模拟方法推导TSA与停歇时长之间的关系；最终通过将总计数得到的迁徙个体日数（migrants·day）除以停歇时长，估算得到迁徙种群的总规模。
3. 本研究证实，无需考虑（1）迁徙速率——即迁徙期间到达的物候变化，以及（2）迁徙事件涉及的鸟类数量，均可通过迁徙期间采样个体的平均TSA值直接预测停歇时长。春季迁徙期间，塔霍河河口的过境黑腹滨鹬平均停歇时长为7.5天，结合计数数据后，本研究估算得到该研究区域内的过境黑腹滨鹬总数量约为30000只。
4. 该全新方法的假设条件并不严苛，因此可针对不同类群与研究区域的特异性进行定制化调整。沿迁徙飞行通道（migratory flyways）估算迁徙鸟类总数量，对于识别保护行动优先区域至关重要。事实上，单个中途停歇地虽仅能短暂容纳种群，但往往承载着全球种群中相当可观的一部分个体。