Eggshell thickness was measured at the egg equator and poles (sharp pole and blunt pole) for 12 avian species. Eggshell thickness was related to egg morphometrics (egg length and width), embryonic development, egg status, and mercury contamination. Individual eggshells each have up to 3 thickness measurements, linked by the eggshell identification. Each measurement has the corresponding egg length, egg width, egg status, mercury concentration, and nest identification. Additionally, eggshell thickness was measured at 5 equally spaced positions along the longitude of the egg (sharp pole, quarter egg, half egg, three quarter egg, and blunt pole) in a subset of Forster's tern (Sterna forsteri) eggs. These data support the following publication: Peterson, Sarah H., Ackerman, J.T., Herzog, M.P., Toney, M.S., Cooney, B., and Hartman, C.A. 2020. Avian eggshell thickness in relation to egg morphometrics, embryonic development, and mercury contamination. Ecology and Evolution