Caspian terns ("Hydroprogne caspia"), North America's largest tern, return to the Bay every spring to nest, migrating from as far away as Colombia. According to scientists from the U.S. Geological Survey, Caspian tern populations in the South Bay are declining at the same time that high levels of mercury are being found in their eggs. The highest mercury levels found in animals from the Bay were in the eggs of Caspian and Forster's ("Sterna forsteri") terns that nest near the Cargill salt ponds at the mouth of the Guadalupe River. A study conducted by the U.S. Fish and Wildlife Service has found that nearly three-quarters of the eggs examined from black-crowned night heron ("Nycticorax nycticorax") nests in the Guadalupe watershed contained mercury exceeding thresholds known to kill the embryos of other bird species.