Data from: Hierarchical fear: parental behaviour and corticosterone release mediate nestling growth in response to predation risk

Nestling development, a critical life-stage for altricial songbirds, is highly vulnerable to predation, particularly for open-cup nesting species. Since nest predation risk increases cumulatively with time, rapid growth may be an adaptive response that promotes early fledging. However, greater predation risk can reduce parental provisioning rate as a risk aversion strategy and subsequently constrain nestling growth, or directly elicit a physiological response in nestlings with adaptive or detrimental effects on development rate. Despite extensive theory, evidence for the relative strength of these effects on nestling development in response to prevailing predation risk and the underlying mechanisms remain unclear. For an alpine population of horned lark (<i>Eremophila alpestris</i>), we elevated perceived predation risk (decoys/playback) during the nestling stage to assess the influence of predator cues and parental care on nestling wing growth and the glucocorticoid hormone corticosterone. We used piecewise path analysis to test a hypothesized causal response structure composed of direct and indirect pathways. Nestlings under greater perceived predation risk reduced corticosterone and increased wing growth, resulting in an earlier age at fledge. This represented both a direct response that was predator-specific, and an indirect response dependent on parental provisioning rate. Parents that reduced provisioning rate most severely in response to predator cues had smaller nestlings with greater corticosterone. Model comparisons indicated the strongest support for a directed, causal influence of corticosterone on nestling wing growth, highlighting corticosterone as a potential physiological mediator of the nestling growth response to predation risk. Finally, cold temperatures prior to the experiment constrained wing growth closer to fledge, illustrating the importance of considering the combined influence of weather and predation risk across developmental stages. We present the first study to separate the direct and indirect effects of predation risk on nestling development in a causal, hierarchical framework that incorporates corticosterone as an underlying mechanism and provides experimental evidence for an adaptive developmental response to predation risk in ground-nesting songbirds.

晚成鸣禽（altricial songbirds）的雏鸟发育是其生命周期的关键阶段，极易遭受捕食者侵袭，对于采用开放式杯状巢的物种而言尤为如此。由于鸟巢被捕食的风险随时间推移呈累积性上升，快速生长或许是一种适应性策略，可帮助雏鸟尽早离巢。然而，更高的捕食风险会促使亲鸟采取风险规避策略，降低育雏投喂率（parental provisioning rate），进而限制雏鸟生长；或是直接引发雏鸟的生理响应，对其发育速率产生适应性或有害影响。尽管已有大量相关理论推演，但目前仍不清楚，针对当前捕食风险，这些效应对雏鸟发育的相对强度以及其背后的潜在作用机制。本研究以高山种群的角百灵（*Eremophila alpestris*）为研究对象，在雏鸟发育阶段通过放置天敌模型与播放捕食鸣叫声，提升雏鸟感知到的捕食风险，以此探究捕食者信号与亲鸟抚育行为对雏鸟翼部生长以及糖皮质激素皮质酮（glucocorticoid hormone corticosterone）水平的影响。我们采用分段路径分析（piecewise path analysis），对包含直接通路与间接通路的假设性因果响应结构进行检验。实验结果显示，感知捕食风险更高的雏鸟，其皮质酮水平更低，翼部生长速度更快，最终离巢年龄更早。这一结果既体现了天敌特异性的直接响应，也依赖于亲鸟育雏投喂率的间接响应：那些在感知到捕食信号后大幅降低投喂率的亲鸟，其雏鸟体型更小且皮质酮水平更高。模型比对结果显示，皮质酮对雏鸟翼部生长具有最强的定向因果影响，这表明皮质酮或是介导雏鸟针对捕食风险产生生长响应的潜在生理因子。最后，实验前的低温环境会在雏鸟临近离巢时限制其翼部生长，这说明在雏鸟发育的不同阶段，综合考量天气与捕食风险的共同影响至关重要。本研究首次在整合皮质酮作为潜在作用机制的因果层级框架中，分离出捕食风险对雏鸟发育的直接与间接效应，并为地面筑巢鸣禽针对捕食风险产生适应性发育响应提供了实验证据。