Data from: Estimating national population sizes: methodological challenges and applications illustrated in the common nightingale, a declining songbird in the UK

1. Estimation of national population size can be important for setting conservation priorities but its methodology has received little critical attention. Sites for highly aggregated species are often prioritised if they contain 1% of national or biogeographical populations but the utility of this approach for other species is unclear. 2. To make recommendations for study design, we present methods used to estimate the UK population size of the common nightingale Luscinia megarhynchos. We assess the sensitivity of the population estimate to the analytical method used and identify sites of national importance for this territorial songbird. 3. Survey effort was directed by prior knowledge of the species’ distribution and the survey design maximised detectability by focussing on the period of greatest song output. We used three different statistical methods to account for detectability, estimating that 55–65% of the national population was detected during surveys. 4. Birds in areas not known to contain the species accounted for 13–23% of the population estimate. Methods to account for these individuals contributed the greatest uncertainty to the results, due to the difficulty of surveying a very large sample of random sites and consequent need to stratify the sample. 5. The 12 derived estimates ranged between 5094 and 5938 territorial males, with the confidence limits ranging from 4764 to 6534. Site delimitation, using clustering based on nearest-neighbour distances, identified one site clearly of national importance and several others potentially nationally important, depending on the population threshold and clustering distance used. 6. Synthesis and applications. National population estimation is difficult and requires that species-specific variability in detectability and individuals present outside surveyed areas are accurately accounted for through survey design and statistical analysis. Accounting for these sources of error will not always be possible and will hamper efforts to assess true population size and consequently to determine whether sites, however defined, exceed critical thresholds of importance. Resources may be better invested in other activities, for example in generating population trends based on relative indices. The latter are generally easier to produce, potentially more robust and arguably more suitable for many conservation applications.

1. 国家种群规模估算对于制定保护优先策略至关重要，但其相关方法却鲜有得到严谨的批判性审视。对于高度聚集的物种，若其栖息点包含本国或生物地理分区种群的1%，通常会被列为优先保护地，但该方法对其他物种的适用性尚不明确。
2. 为给研究设计提供建议，本文阐述了估算普通夜莺（common nightingale，学名：Luscinia megarhynchos）英国种群规模的方法。我们评估了种群估算结果对所采用分析方法的敏感性，并确定了这种领域性鸣禽的国家级重要栖息点。
3. 本研究依托该物种分布的先验知识规划调查工作，并将调查时段聚焦于鸣唱量最大的时期，以最大化物种检测率。我们采用三种不同的统计方法校正检测偏差，估算得出本次调查共检测到该国种群55%~65%的个体。
4. 未被记录分布有该物种的区域内的个体，占种群估算总规模的13%~23%。由于难以对大量随机样点开展全面调查，因此需对样本进行分层处理，而针对这类未调查区域个体的校正方法，是导致本研究结果不确定性最高的来源。
5. 本次得到的12项种群估算结果显示，领域雄鸟数量介于5094至5938只之间，置信区间为4764至6534。采用基于最近邻距离的聚类分析开展栖息点划定，结果识别出1处明确具有国家级重要性的栖息点，以及若干潜在具备国家级重要性的栖息点，其判定结果取决于所采用的种群阈值与聚类距离参数。
6. **综合与应用**：国家种群规模估算极具挑战性，需通过调查设计与统计分析，精准校正物种特异性的检测率差异，以及调查区域外个体的遗漏偏差。但此类误差的校正并非总能实现，这将阻碍对真实种群规模的准确评估，进而无法判定无论采用何种定义的栖息点是否达到关键保护重要性阈值。因此，科研资源或许更宜投入其他研究方向，例如基于相对指数构建种群动态趋势。这类趋势分析通常更易生成，结果潜在更具稳健性，且可认为更适配多数保护应用场景。