Seasonal rainfall in subtropical montane cloud forests drives demographic fluctuations in a Green-backed Tit population

Montane birds are vulnerable to climate change. However, the mechanisms by
which weather drives demographic processes in montane birds have seldom
been investigated. We conducted a long-term study (2009–2019) on the
Green-backed Tit (Parus monticolus), an insectivorous passerine, in the
montane cloud forest of subtropical Taiwan. We explored the effects of
weather variability on the productivity and survival of adult Green-backed
Tits. Nest survival was negatively associated with seasonal rainfall
during the breeding season (April–July) and was lower in early clutches
than in late clutches. Higher typhoon-induced precipitation during the
postbreeding period (July–September) was related to reduced adult
survival, but neither summer temperature nor winter weather conditions
were found to be related to adult bird survival. We developed a stochastic
simulation model for Green-backed Tit population dynamics based on
empirical data. We compared the simulated time-series and observed
population growth rates (λ) and found that 80% (8/10 yr) of the observed λ
fell within the 5th and 95th percentiles of the simulated data over the
10-yr period. Moreover, the simulated average (± standard deviation) of
the geometric mean of λ over 10 yr (1.05 ± 0.07) was close to that
observed from 2009–2019 (0.99), which provided confidence that the model
effectively simulated the population growth rate of the Green-backed Tit.
We conducted a sensitivity analysis for λ, and found that juvenile and
adult survival influenced by typhoon-induced rainfall were the greatest
contributors to the variance in the growth rate of the Green-backed Tit
population. With the onset of intensified seasonal precipitation
associated with global warming, the population growth and density of
Green-backed Tits will decline substantially. Our results suggest that
under scenarios of high emissions of greenhouse gas, this local population
of Green-backed Tits will not persist in the near future.

山地鸟类易受气候变化影响，但天气驱动山地鸟类种群统计过程的机制却鲜有研究。我们在亚热带台湾的山地云雾林中，对食虫性雀形目鸟类绿背山雀（Parus monticolus）开展了一项长期研究（2009–2019年）。我们探究了天气变异性对绿背山雀成鸟繁殖力和存活率的影响。巢存活率与繁殖季节（4月至7月）的季节性降雨呈负相关，且早期窝卵的巢存活率低于晚期窝卵。繁殖后期（7月至9月）台风引发的降水增多与成鸟存活率降低相关，但夏季温度和冬季天气条件均未发现与成鸟存活率存在关联。我们基于实证数据开发了绿背山雀种群动态的随机模拟模型。通过比较模拟时间序列与观测到的种群增长率（λ），我们发现10年间观测到的λ中有80%（8/10年）落在模拟数据的第5至95百分位区间内。此外，10年间λ几何均值的模拟平均值（±标准差）为1.05±0.07，与2009–2019年观测到的几何均值（0.99）相近，这表明该模型能有效模拟绿背山雀的种群增长率。我们对λ进行了敏感性分析，发现受台风引发降雨影响的幼鸟和成鸟存活率是绿背山雀种群增长率变异的最大贡献因素。随着全球变暖相关的季节性降水加剧，绿背山雀的种群增长和密度将大幅下降。我们的结果表明，在高温室气体排放情景下，这一绿背山雀本地种群在近期将无法存续。