Laying order predicts avian maternal investment in egg yolk hormones and nutrients but not embryo size in the asynchronously hatching <i>Turdus migratorius</i> (American robin)

Abstract Parental investment can shape offspring growth, with consequences for sibling–sibling competition and survival. Altricial birds often face severe mortality bottlenecks before they reach independence; their eggs can vary in attributes including size, amounts of internal components (yolk, albumen), and concentrations of micronutrients (e.g., carotenoids, vitamins) and hormones. Egg composition may vary with maternal resource availability and egg-laying order. If eggs within the clutch hatch asynchronously, variation in contents with laying order could exacerbate nestling size hierarchies to promote adaptive brood reduction or could mitigate the cost of maladaptive asynchrony by enhancing embryonic growth in later-laid eggs. We reviewed the avian egg-laying order literature to identify existing knowledge gaps and quantified mass of multiple egg components, eggshell coloration, and concentrations of 12 yolk steroids and 8 yolk vitamins and carotenoids in freshly laid eggs (n = 100) of wild Turdus migratorius (American Robins) collected in Illinois, USA, in the early and late halves of their breeding season. Laying order significantly influenced eggshell color, egg and yolk mass, and the concentrations of 4 yolk steroids and 7 yolk vitamins and carotenoids. First-laid eggs, regardless of seasonal timing of lay, were lightest and had highest concentrations of yolk androgens and progestogens. Yolk carotenoid concentrations decreased with laying order in the early breeding season, and eggshell coloration covaried with carotenoid concentrations. To test for laying order effects on embryonic development, we also incubated eggs of known laying order (n = 64) and assessed embryonic metabolic development, shifts in yolk hormone and carotenoid concentrations, and size ∼4 days before hatch. In lab-incubated eggs, embryonic heart rate, shifts in yolk carotenoid and hormone concentrations, and pre-hatch size were not related to laying order. If first-laid eggs receive reduced maternal investment to promote brood survival in this population, the effects of this investment strategy are not realized in embryonic traits.