Although DDE caused reproductive failures in a wide variety of bird species during the 1950s–1970s, the mechanism(s) by which it acted have still not been fully elucidated. This experimental study was designed to test an hypothesis that the primary mechanism by which DDE caused hatching failure in Common Terns (Sterna hirundo) was by disrupting the structure of the eggshell, leading to reduced exchange of O₂ and CO₂ through the shell to and from the embryo. In the study, gas exchange through two groups of Common Tern eggshells was reduced by 12% or 28% by sealing parts of the surface with acrylic resin. Contrary to predictions from the hypothesis, all treated eggs hatched and there were no other changes in breeding success relative to untreated controls. The result suggests that DDE caused embryonic deaths in Common Terns by mechanisms acting within the egg rather than by modification of the structure or respiratory properties of the shell.