How well do bird atlas reporting rates reflect bird densities? Correlates of detection from the Fynbos biome, South Africa, with applications for population estimation

Relationships between true population densities and reporting indices from atlas data are important for the calculation of population sizes, though these relationships are remarkably little-known and likely confounded by issues of detection. We examine issues of detection for a single-observer point-count survey across the Fynbos biome in South Africa. We created an index of relative abundance comparable to atlas reporting rate and calculated detection coefficients for each species. We explore various models that explain relative abundance as a function of detection covariates, e.g. mass, colour, group size, vocal behaviour and density. Density was consistently included across models and the best predictor when used alone in a validation exercise. We then calculated mean reporting rates for the citizen science South African Bird Atlas Project2 (SABAP2). A model of species’ reporting rates as determined by density for this set of birds suggested the correlation between atlas reporting rates and density estimates was weak, with species capable of using modified habitats exhibiting higher reporting rates than expected from the density estimates. There was a positive correlation between bird density and reporting rates corrected for by species mass. We used this relationship to calculate densities for the Fynbos bird species and compare results to published data.

真实种群密度与图谱数据中的报告指数之间的关联，对于种群规模计算至关重要，尽管这类关联迄今仍鲜为人知，且很可能因各类检测问题而产生混淆。我们针对南非凡波斯（Fynbos）生物群系内的单观察者点计数调查，开展了检测相关问题的研究。我们构建了可与图谱报告率相媲美的相对多度指数，并为每个物种计算了检测系数。我们探索了多种模型，这些模型将相对多度作为检测协变量的函数进行阐释，协变量包括体型质量、体色、集群规模、鸣唱行为以及种群密度。在验证实验中，种群密度始终被纳入各类模型，且单独作为预测因子时表现最优。随后我们计算了公民科学项目南非鸟类图谱项目2（South African Bird Atlas Project2, SABAP2）的平均报告率。针对该鸟类类群构建的以密度为自变量的物种报告率模型显示，图谱报告率与密度估计值之间的相关性较弱；相较于基于密度估计的预期值，能够利用人为改造生境的物种拥有更高的报告率。经物种体型质量校正后的鸟类密度与报告率之间呈现正相关关系。我们借助这一关联计算了凡波斯鸟类的种群密度，并将计算结果与已发表的相关数据进行了对比。