Eggs or empty shells of the American woodcock were collected from 10 states in 1971 and shell thickness (mean of clutch means) was compared with that of eggs collected from 16 states during the years 1859–1939. The 1971 shells (n=91) from hatched eggs or those containing fully developed embryos were about 10 percent thinner (P<0.001) than both unembryonated shells (n=26) from the same year and the 1859–1939 shells (n=169) from essentially unembryonated eggs. The difference is attributed to the transfer of calcium from the shells to the embryos and not to environmental pollutants.