As predators at the top of the food chain, great egrets are good barometers for the herons, ibises, storks and spoonbills that also live in the Everglades, Frederick said. Chicks' parents, which bring them food, hunt within about 15 miles of nests, so chicks are ideal study subjects because the mercury they accumulate comes from wide areas. When birds ingest mercury, they release some of it in their growing feathers. The results surprised and pleased the researchers. Between 1994 and this year, average levels in the chicks' feathers dropped 73 percent, Frederick said.
Some natural processes can affect mercury uptake in wading birds.
Mercury levels also are predicted to rise in years immediately following droughts because of a phenomenon known as the reservoir effect. During dry years, submerged areas become exposed, drying out and releasing mercury stored in sediments. Upon reflooding, this mercury becomes available to the aquatic food web and, eventually, the birds. The last two years were dry years, which would suggest mercury levels would increase. But levels have declined both years, continuing a steady decline that began in 1997.