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Dry season intensity has equivocal effects on the nutritional condition of understory birds in a Neotropical forest

ABSTRACT 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. LAY SUMMARY The amount and timing of rainfall influences the ecology, phenology, and population dynamics of tropical organisms. Seasonal drought can negatively impact the survival, reproduction, and breeding phenology of tropical forest birds, although the mechanisms underlying these impacts remain unclear. We tested the hypothesis that seasonal drought negatively impacts the nutritional condition of resident forest birds in Central Panama. We estimated body condition, hematocrit, and plasma lipid metabolite concentrations in 27 bird species across two dry seasons of differing intensity—an El Niño year (2016, very dry) and a more typical year (2017). We found some evidence of reduced nutritional condition (i.e. lower hematocrit and greater concentrations of β-hydroxybutyrate) during the El Niño dry season in certain focal species, but little evidence of nutritional stress among foraging guilds. Our findings suggest that many tropical forest species can tolerate seasonal drought without exhibiting evidence of energetic stress.