Coloration of spotless starling nestlings shows genetic and environmentally determined characteristics while begging for food

Abstract

Similar to other signals, coloration of traits expressed by nestlings while they are begging for food are assumed to reflect their phenotypic conditions, and, therefore, they should be environmentally determined. However, these colorations are also species specifics and, thus, a genetic component explaining the evolution of nestling coloration should exist; a possibility rarely explored in the literature. Cross‐fostering experiments allowed us to quantify genetic and environmental components of coloration of mouth, flanges and skin in a spotless starling Sturnus unicolor population. In addition, by supplementing some nestlings within each brood with non‐pigmented antioxidants (Vitamin E or Vitamin complex), we explored the effects of food supplementation on coloration of begging‐related traits and plasma carotenoid concentration. We detected significant genetic components of colorations of skin and mouth, but not of flanges or plasma carotenoid concentration. Moreover, experimental supplementation with Vitamin E affected UV coloration of mouth, flanges and skin, while vitamin‐complex supplementation exclusively affected flange coloration. Furthermore, coloration of flanges, but not that of other traits, predicted plasma carotenoid concentration suggesting a direct link between these two traits. All these results suggest that coloration of begging‐related traits is a multifaceted signal informing parents about genetic characteristics, and of short‐term needs of nestlings, including antioxidative capacity, that parents could use to adjust feeding efforts, and deciding which chick to feed. Future research should focus on the parents' perspective, taking into account their feeding effort and feeding preferences in relation to genetically and/or environmentally determined coloration of begging‐related traits. A free Plain Language Summary can be found within the Supporting Information of this article

Made-up mouths with preen oil reveal genetic and phenotypic conditions of starling nestlings

“…Twelve days after laying, we visited nest boxes again and then daily until hatching, when nests were randomly assigned to one of the two performed experiments (VitE or cross-fostering). This approach has previously been used to estimate genetic and environmental components of coloration of begging-related traits in spotless starlings ( Martínez-Renau et al 2021 ), and we use it here to estimate those components of the uropygial secretion coloration.…”

“…The food supplementation experiment also started the day after hatching and consisted in oral administration of an age-dependent dose of VitE (DL-α-tocopherol acetate (Sigma-Aldrich T 3376-256)) diluted in corn oil, or of only corn oil, to experimental and control nestlings respectively (for further details see Supplementary Material S2 , and for a similar experimental approach see de Ayala et al (2006) ). For dose estimates we also followed Martínez-Renau et al (2021) . Ten days later, we ringed birds and collected biometrical measurements (tarsus length with a digital caliper (precision 0.01 mm), wing maximum-length with a metal ruler (precision 0.1 mm) following procedures described in Svensson (1992) , and body mass with a digital balance (Ascher CS, precision 0.01 g)) of all nestlings in the nest.…”

“…In addition to nestling coloration, in order to statistically control for possible maternal effects on nestlings coloration (see below), we also took three measures of reflectance of the eggshells at the pointed and blunt ends, and at the center of the eggshell. In nestlings, we measured colorations of mouth, flanges, and breast skin following protocols described elsewhere ( Soler and Avilés 2010 ; Martínez-Renau et al 2021 ). Color of uropygial secretion was estimated on a piece of blotting paper after gently smearing approximately 4 μl of the collected secretion on a 1-cm-diameter circle ( Soler et al 2014 ).…”

“…Spectrum of each measured trait is shown in the Supplementary Material S1 . For details on considered color variables and spectra characteristics of each measured trait, see Martínez-Renau et al (2021) .…”

“…Finally, (iii) by swabbing gapes and inspecting swabs color, we explore the possibility that nestlings cosmetically use their uropygial secretion to stain signaling traits directed to parents. Moreover, (iv) we compared the coloration of secretions with those of begging-related traits such as mouth, flanges, and skin of nestlings, which in this species are known to reflect phenotypic and genetic quality of nestlings ( Soler et al 2007 ; Martínez-Renau et al 2021 ). Finally, (v) we video-recorded parental and nestling behaviors to detect directly preening behavior of nestlings at an age far before feathers are developed, and quantify parental food allocation depending on nestling colorations.…”