Effects of dietary methyl mercury on red-tailed hawks.

Six groups of 1-year-old red-tailed hawks (Butco jamaiccusis) of three birds each were fed chicks contaminated with methyl mercury at three different levels (3.9, 7.2, and 10.0 ppm of mercury were found in their livers, respectively). The experimental feeding periods were 4 or 12 weeks. Mortality occurred in the groups on the most contaminated diets after an exposure period of 1 month or more, the signs of poisoning prior to death essentially neurological. The mercury levels in the livers of hawks that succumbed to mercury poisoning were about 20 ppm. These hawks, and those that survived on the two most heavily contaminated diets, showed pathological changes. The most consistent changes were swelling of axons of myelinated nerves in the spinal cord, with dilatation of myelin sheaths and loss of myelin.