Structural and elemental changes of the eggshell of Mimus saturninus (Birds: Paseriformes) during embryonic development

Structural and elemental changes in avian eggshells during embryonic development remain poorly characterized, particularly in altricial species. This study describes the morphological and elemental organization of the eggshell of Mimus saturninus (chalk-browed mockingbird) throughout embryogenesis. Eggshell structure and embryonic development were analyzed using optical microscopy and scanning electron microscopy (SEM) on 90 sections (transversal, dorsal, and internal surface) from an ontogenetic series of 30 eggs at different developmental stages, including hatching. SEM analysis identified an external calcareous layer, an internal fibrous organic layer, and an internal limiting layer in contact with the yolk. During embryonic development, localized calcium reabsorption was observed, reflected in a progressive decrease in shell thickness. Increased porosity was detected in the external calcareous layer, suggesting its involvement in calcium mobilization. Embryonic developmental stages were described alongside changes in eggshell structure, including thickness reduction, increased porosity, and compaction of shell layers. These observations suggest a temporal association between eggshell structural modifications and embryonic development. Interpreting these changes contributes to understanding the redistribution of essential elements during embryogenesis.