Rainfall regime, the amount and timing of annual precipitation, can influence the breeding phenology, individual fitness, and population dynamics of tropical birds. In Neotropical regions with rainfall seasonality (i.e. wet and dry seasons), the warm phase of the El Niño Southern Oscillation (ENSO) can exacerbate seasonal drought and negatively impact avian survival and reproduction. However, the mechanisms underlying associations between seasonal drought conditions and avian demography are largely unexplored. One hypothesis is that nutritional condition mediates demographic responses to seasonal drought: individuals in poor condition may be less capable of balancing their energy budgets and consequently suffer reduced survival, lower reproductive output, or both. We estimated nutritional condition (i.e. scaled mass index, percent hematocrit, plasma lipid metabolites) as a proxy of energy balance in understory forest birds with contrasting population-level responses to dry season length. This study took place across two dry seasons of differing intensity in central Panama: an El Niño dry season (2016, severe drought) and a more typical dry season (2017). Scaled mass index remained relatively constant throughout both dry seasons and across years for 5 common focal species and among 4 foraging guilds (22 additional species, 27 species total). Three of 5 focal species did exhibit reduced nutritional condition (i.e. lower hematocrit and/or higher β-hydroxybutyrate) during the El Niño dry season but not during the more typical dry season. However, foraging guilds did not show consistent nutritional responses to seasonal drought and we found little evidence of the reduced nutritional condition at the guild level, suggesting that many Neotropical forest bird species are capable of tolerating seasonal drought.

中文翻译：

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旱季强度对新热带森林林下鸟类的营养状况有模棱两可的影响

降雨情况、年降水量和时间，会影响热带鸟类的繁​​殖物候、个体适应性和种群动态。在具有降雨季节性（即雨季和旱季）的新热带地区，厄尔尼诺南方涛动（ENSO）的暖期会加剧季节性干旱，并对鸟类的生存和繁殖产生负面影响。然而，季节性干旱条件与鸟类人口学之间的关联机制在很大程度上尚未探索。一种假设是，营养状况介导了人口对季节性干旱的反应：状况不佳的个体可能无法平衡其能量预算，从而导致生存率降低、生殖产出降低，或两者兼而有之。我们估计了营养状况（即比例质量指数、血细胞比容百分比、血浆脂质代谢物）作为林下森林鸟类能量平衡的代表，具有对比的种群水平对旱季长度的反应。这项研究发生在巴拿马中部两个强度不同的旱季：厄尔尼诺旱季（2016 年，严重干旱）和更典型的旱季（2017 年）。对于 5 个常见的焦点物种和 4 个觅食行会（另外 22 个物种，总共 27 个物种），比例质量指数在整个旱季和多年间保持相对恒定。5 个重点物种中的 3 个在厄尔尼诺旱季确实表现出营养状况降低（即血细胞比容降低和/或 β-羟基丁酸盐升高），但在更典型的旱季则没有。然而，