Data from: Characterising demographic contributions to observed population change in a declining migrant bird

Populations of Afro-Palearctic migrant birds have shown severe declines in recent decades. To identify the causes of these declines, accurate measures of both demographic rates (seasonal productivity, apparent survival, immigration) and environmental parameters will allow conservation and research actions to be targeted effectively. We used detailed observations of marked breeding birds from a ‘stronghold’ population of whinchats Saxicola rubetra in England (stable against the declining European trend) to reveal both on-site and external mechanisms that contribute to population change. From field data, a population model was developed based on demographic rates from 2011 to 2014. Observed population trends were compared to the predicted population trends to assess model-accuracy and the influence of outside factors, such as immigration. The sensitivity of the projected population growth rate to relative change in each demographic rate was also explored. Against expectations of high productivity, we identified low seasonal breeding success due to nocturnal predation and low apparent first-year survival, which led to a projected population growth rate (?) of 0.818, indicating a declining trend. However, this trend was not reflected in the census counts, suggesting that high immigration was probably responsible for buffering against this decline. Elasticity analysis indicated ? was most sensitive to changes in adult survival but with covariance between demographic rates accounted for, ? was most sensitive to changes in productivity. Our study demonstrates that high quality breeding habitat can buffer against population decline but high immigration and low productivity will expose even such stronghold populations to potential decline or abandonment if either factor is unsustainable. First-year survival also appeared low, however this result is potentially confounded by high natal dispersal. First-year survival and/or dispersal remains a significant knowledge gap that potentially undermines local solutions aimed at counteracting low productivity.

近几十年来，非洲-古北界迁徙鸟类（Afro-Palearctic migrant birds）的种群数量出现了严重下降。为明确此类种群下降的成因，精准测定种群统计参数（季节性繁殖生产力、表观存活率、迁入率）与环境参数，可使保护与研究行动得以精准施策。
本研究针对英格兰境内的穗䳭（Saxicola rubetra）核心稳定种群——其种群未随欧洲整体下降趋势出现波动——的佩戴环志繁殖个体开展详细观测，以揭示影响种群动态的本地与外部机制。基于2011至2014年的野外种群统计数据，研究人员构建了种群模型。将观测得到的种群趋势与模型预测趋势进行对比，以评估模型准确性及迁入等外部因素的影响。此外，本研究还探究了预测种群增长率对各项种群统计参数相对变化的敏感性。
尽管预期繁殖生产力较高，但研究发现夜行性捕食导致季节性繁殖成功率偏低，且当年幼体表观存活率较低，据此预测的种群增长率为0.818，表明种群呈下降趋势。但该预测趋势并未在普查计数中体现，这表明高迁入率可能缓冲了种群的下降态势。
弹性分析显示，种群增长率对成体存活率的变化最为敏感；但在考虑种群统计参数间的协方差后，种群增长率对繁殖生产力的变化最为敏感。本研究表明，优质繁殖栖息地可缓冲种群下降趋势，但如果高迁入率与低繁殖生产力任意一方不可持续，即便此类核心稳定种群也可能面临种群下降乃至局部消亡的风险。
当年幼体存活率同样偏低，但该结果可能因高出生扩散（natal dispersal）而存在混淆偏差。当年幼体存活率和/或扩散行为仍是当前的重要知识空白，这可能削弱旨在改善低繁殖生产力的本地修复方案的有效性。