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

Abstract: 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. Significance statement: Central place foragers, such as many parent birds provisioning offspring, are subject to selective forces from both life history and foraging ecology. Most effects of selection are expected on mean behaviors, but adaptive life histories or foraging decisions can also influence variances. We analyzed both means and variances in parent pied flycatchers experiencing either increases or decreases in brood size. Parents with enhanced broods had shorter foraging trips and offspring that begged more, matching predictions from life history theory. They also exhibited less variation in trip time, contrary to predictions from variance sensitive foraging theory. However, patterns in the residual variance in offspring begging and parental mass are less easily explained and raise new questions. Statistical models of means and variances illuminate the intersection between two bodies of theory and reveal new processes affecting provisioning behavior.