Blame it on a headache or stomachache caused by eating some bad fish. A University of Florida study found increased mercury exposure is turning egrets off from sex. The study also noted the full effects of mercury exposure among wading birds may be “systematically underestimated." Certain microorganisms can change mercury into methylmercury, a highly toxic form that builds up in fish, shellfish and animals that eat fish, in a process known as bioaccumulation. Most human exposure to mercury is from eating fish and shellfish contaminated with methylmercury.