Multimodal begging signals reflect independent indices of nestling condition in European starlings

Parental food allocation decisions are based on information about nestling condition conveyed by begging signals. It is expected that parents should have evolved the ability to obtain detailed information about their nestlings' condition through multiple independent signals, thus allowing them to use optimal food allocation strategies depending on resource availability. In this study, we explore 1) which components of acoustic and visual begging signals produced by European starling nestlings (Sturnus vulgaris) vary independently from each other and consequently have evolved as separate signals and 2) whether these multimodal signals convey multiple or redundant information about nestling condition. We measured nestling stature and an estimate of lipid reserves, 2 independent indices of nestling phenotypic quality that have been suggested to be used in parental food allocation decisions. In multivariate analyses, we show that 1) acoustic and visual begging signals consist of several components that vary independently from each other and that 2) begging components correlated with nestling lipid reserves were different from those correlated with stature. Our results show that nestling begging signals include independent components in multimodal sensory channels that can provide parents with information about 2 independent indices of nestling condition that could be used by parents for flexible allocation strategies when facing changing environmental conditions.