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Published information relating to changes in the chemical element content of avian eggs caused by embryonic development is extremely scarce, although it may be crucial for understanding both the presence of anthropogenic pollutants as well as physiological levels of micronutrients. We assessed the variation in concentrations of calcium (Ca) and magnesium (Mg) and nine trace elements: seven essential (chromium (Cr), copper (Cu), nickel (Ni), manganese (Mn), iron (Fe), cobalt (Co) and zinc (Zn)) and two non-essential (lead (Pb) and cadmium (Cd)) in shells and contents (both egg yolk and egg white) of embryonated and non-embryonated eggs. We investigated the eggs of the Eurasian Reed Warbler Acrocephalus scirpaceus, a large proportion of whose eggs are infertile in our study population (almost 43% of clutches contain unhatched eggs) as well as significant embryo-induced eggshell thinning at the equator of embryonated eggs. We found significantly higher concentrations (ὅ 22.7%) of all the focal elements in the contents of embryonated eggs in comparison with non-embryonated eggs, and a very pronounced one for Ca (nearly twice as high). The shells of embryonated eggs contained significantly higher concentrations of Zn (104.1%), Fe (56.5%), Pb (32.8%) and Cu (28.0%) but significantly lower ones of Co (8.9%) and Ca (9.3%) than the shells of non-embryonated eggs. The simultaneous higher concentrations of all elements in the content of thinnershelled embryonated eggs suggest the parallel transfer of these elements along with Ca resorption from the shell into the egg interior during embryo formation. The higher concentration of most elements in the thinner shells of embryonated eggs may be indicative of the maternal deposition of some of these elements in a shell layer not subject to embryonic depletion, or in the eggshell membrane. Our results highlight the need for the careful selection of egg samples, which should differentiate between embryonated and non-embryonated eggs in the analytical treatment of eggs and eggshells. [ABSTRACT FROM AUTHOR]