Mercury is a persistent contaminant that biomagnifies up the food web, causing mortality, reproductive failure, and other health effects in predatory wildlife and humans. From 1930 to 1950, industrial mercuric sulfate entered the South River, a tributary of the Shenandoah River in Virginia (United States). To determine whether this mercury concentration had moved into the adjacent terrestrial food web, the researchers anlayzed total mercury concentrations in blood from adults of 13 terrestrial feeding bird species breeding within 50 m of the river.