Data from: Experimental manipulation of brood size affects several levels of phenotypic variance in offspring and parent pied flycatchers

Parental provisioning of offspring should reflect selection on life history aspects of parenting and on foraging behavior. Life history and foraging theory generally make predictions about mean behavior, but some circumstances might favor changes in the variance of parent and offspring behaviors. We analyzed data on free-living pied flycatchers (Ficedula hypoleuca) experiencing a brood size manipulation. We used double hierarchical generalized linear models to investigate patterns in means and variances of provisioning, brood begging, and parental mass. As predicted by life history theory, parents with enlarged broods of intensely begging nestlings fed at higher rates and delivered more food per unit of time. They also delivered food at a more consistent rate. This contradicts the prediction from variance-sensitive foraging theory that parents facing higher brood demand should choose more variable foraging options. Indirect evidence suggests that reduced variance in trip time arose from shifts in parental time budgets. Exploratory analyses revealed patterns in residual variance of both nestling begging and parental mass changes, with enlarged broods begging less consistently and female body mass changes being more variable after longer foraging trips. We show that parent pied flycatchers simultaneously adjust means and variances in multiple aspects of their provisioning effort to changes in brood demand and that these responses might be linked with nestling begging and changes in parental body mass. Our study highlights both the importance of adopting sophisticated statistical approaches and the potential intersection of two bodies of theory that may affect strategic adjustments of individuals engaged in central place provisioning.

亲鸟对后代的育雏投入，理应反映针对育儿生活史特征与觅食行为的选择压力。生活史理论与觅食理论通常仅对行为均值做出预测，但部分情境下，亲代与子代行为的方差变化可能更受选择青睐。我们针对接受育雏规模操控实验的野生斑姬鹟（Ficedula hypoleuca）的野外数据开展分析，采用双分层广义线性模型（double hierarchical generalized linear models）探究育雏喂食、雏鸟乞食以及亲鸟体重的均值与方差特征。正如生活史理论所预测的那样，育雏群规模被扩大且雏鸟乞食强度更高的亲鸟，其喂食频率更高、单位时间内投递的食物总量更多，且喂食速率的一致性更强。这一结果与方差敏感型觅食理论（variance-sensitive foraging theory）的预测相悖——该理论认为，面对更高育雏需求的亲鸟应选择觅食策略波动性更强的方案。间接证据表明，觅食往返时间的方差降低，源于亲鸟时间分配格局的调整。探索性分析还揭示了雏鸟乞食与亲鸟体重变化的残差方差模式：育雏规模扩大组的雏鸟乞食一致性更低，且雌鸟在经历更长觅食行程后，其体重变化的波动性更高。本研究证实，野生斑姬鹟亲鸟会同时调整育雏投入多维度的均值与方差，以响应育雏需求的变化，且这类响应或与雏鸟乞食行为以及亲鸟体重变化存在关联。本研究既凸显了采用精密统计方法的重要性，也揭示了两类理论的潜在交叉点——这类交叉或可影响从事中心地育雏的个体的策略性调整行为。