Calcium and metals are not evenly distributed in avian eggshells over their longitudinal section

Whether Ca and other micronutrients are equally distributed in an avian eggshell over its longitudinal section and what portion of these local resources are utilized by developing embryos are unanswered questions in avian reproductive physiology. Here, we measured the thickness and concentrations of Ca and 16 other chemical elements (Al, As, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Na, Ni, Pb, Se, and Sr) in 4 shell regions (sharp pole, equator, shoulder, blunt pole) of White Stork (Ciconia ciconia) eggs representing different stages of embryonic development, from unresorbed eggshells to almost fully resorbed ones (with near-to-hatch embryos). We found that unresorbed eggshells displayed several significant differences in the concentrations of 15 elements (Al, As, Ca, Cd, Co, Cr, Fe, K, Hg, Mg, Mn, Na, Ni, Se, and Sr) between various regions of the same shell. Only 2 metals (As and Ca) showed a cross-sectional decrease in concentrations from the sharp pole to the blunt one. In particular, we observed that unresorbed eggshells at the blunt pole were less calcified (with 2.4% less Ca) compared to the sharp pole. In contrast, the concentrations of 6 other metals (Co, Fe, Mg, Mn, Na, and Se) peaked in the relatively less calcified region of the blunt pole, where concentrations (such as Se) were up to 10 times as high as in other regions of unresorbed eggshells. Our findings highlight that eggshells over their longitudinal sections are not chemically homogeneous. Surprisingly, however, and contrary to our expectations, we found that unresorbed eggshells were thinner (2.1-5.9% less depending on the region) and at the same time more strongly calcified than resorbed eggshells. This suggests that some modification has occurred in the shell structure and raises the question of thin-shelled eggs in populations of wild birds.