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 the 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 the 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 similar to 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. Parents are constrained by time and resources, and parental investment in offspring can vary, even among siblings in the same family. Birds may lay multiple eggs in a single family, but may not allocate each egg the same resources, perhaps to influence sibling competition once the eggs hatch. While past research has assessed some egg components and their relationship to the order eggs were laid in a single family, little is known about how each component relates to each other and to parental strategies. We studied the resources invested in Turdus migratorius (American Robin) eggs, asking which resources varied among eggs and time of year. The first egg robins laid in each clutch were smaller than the rest but had higher concentrations of carotenoids, vitamins, and steroid hormones. When artificially incubated in a laboratory, the eggs of different laying orders did not produce embryos of different sizes despite differences in egg resources. La inversi & oacute;n parental puede moldear el crecimiento de la descendencia, con consecuencias para la competencia entre hermanos y la supervivencia. Las aves altriciales a menudo enfrentan cuellos de botella severos de mortalidad antes de alcanzar la independencia; sus huevos pueden variar en atributos que incluyen tama & ntilde;o, cantidades de componentes internos (yema, alb & uacute;mina), y concentraciones de micronutrientes (e.g., carotenoides, vitaminas) y hormonas. La composici & oacute;n del huevo puede variar seg & uacute;n la disponibilidad de recursos maternos y el orden de puesta. Si los huevos dentro de la puesta eclosionan de manera asincr & oacute;nica, la variaci & oacute;n en el contenido seg & uacute;n el orden de puesta podr & iacute;a exacerbar las jerarqu & iacute;as de tama & ntilde;o entre polluelos para promover una reducci & oacute;n adaptativa de la nidada, o bien mitigar el costo de una asincron & iacute;a no adaptativa al mejorar el crecimiento embrionario en los huevos puestos m & aacute;s tarde. Revisamos la literatura sobre el orden de puesta en aves para identificar vac & iacute;os de conocimiento existentes y cuantificamos la masa de m & uacute;ltiples componentes del huevo, la coloraci & oacute;n de la c & aacute;scara, y las concentraciones de 12 esteroides de la yema y 8 vitaminas y carotenoides de la yema en huevos recolectados reci & eacute;n puestos (n = 100) por individuos silvestres de Turdus migratorius en Illinois, EUA, durante la primera y la segunda mitad de la temporada reproductiva. El orden de puesta influy & oacute; significativamente en el color de la c & aacute;scara, la masa del huevo y de la yema, y en las concentraciones de 4 esteroides de la yema y 7 vitaminas y carotenoides de la yema. Los primeros huevos puestos, independientemente del momento de la temporada en que se produjo la puesta, fueron los m & aacute;s livianos y presentaron las mayores concentraciones de andr & oacute;genos y progest & aacute;genos en la yema. Las concentraciones de carotenoides en la yema disminuyeron con el orden de puesta en la primera parte de la temporada reproductiva, y la coloraci & oacute;n de la c & aacute;scara covari & oacute; con las concentraciones de carotenoides. Para evaluar los efectos del orden de puesta sobre el desarrollo embrionario, tambi & eacute;n incubamos huevos con orden de puesta conocido (n = 64) y evaluamos el desarrollo metab & oacute;lico embrionario, los cambios en las concentraciones de hormonas y carotenoides de la yema, y el tama & ntilde;o aproximadamente 4 d & iacute;as antes de la eclosi & oacute;n. En los huevos incubados en laboratorio, la frecuencia card & iacute;aca embrionaria, los cambios en las concentraciones de carotenoides y hormonas de la yema, y el tama & ntilde;o previo a la eclosi & oacute;n no estuvieron relacionados con el orden de puesta. Si los primeros huevos puestos reciben una inversi & oacute;n materna reducida para promover la supervivencia de la nidada en esta poblaci & oacute;n, los efectos de esta estrategia de inversi & oacute;n no se manifiestan en rasgos embrionarios.
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