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Over four study years we have investigated hatching success and failure, the presence of an embryo and other egg biometrics, and eggshell thickness of the Eurasian Reed Warbler Acrocephalus scirpaceus. Of 1354 eggs from 350 nests, 940 (69.4 %) survived until hatching, of which 159 (16.9 %) were incubated but failed to hatch, while 273 (20.2 % of all) were in nests destroyed during incubation. Of 161 intact unhatched eggs from 97 nests, 64 (39.8 %) had no visible embryo and 88 (54.7 %) had a visible embryo. Eggs without visible embryos were present in 42.7 % of normally incubated clutches; among deserted clutches over 50 % had no visible embryo. Eggshell thickness at the egg equator (the widest part) was negatively correlated with clutch size and the presence of an embryo. A similar relationship was found for the blunt end of the eggs but was not statistically significant. Shells at the equator of eggs with visible embryos were significantly thinner (on average 8.0 % less) than the shells of eggs without embryos. Our study clearly implies that shell thickness decreases in the course of embryonic development. It also indicates that comparison of eggshell thickness data without controlling for the presence of an embryo in eggs can produce biased results, and may, therefore, fail to identify the actual causes of eggshell thinning. Our findings also suggest that shell thickness at the blunt end is a reliable estimator of shell thickness, irrespective of embryonic development.