Selenium, mercury, and other toxic elements in agricultural runoff and drainage from mines and power plants have caused an "epidemic of deformed birds in the U.S."
Scientists at the University of Glasgow have developed a technique to measure the mercury levels in birds by testing 3 or 4 feathers, which can be taken from living birds wihout causing harm. These scientists also obtain skin and feather samples from museums and historical collections to compare the mercury levels from the past with today. Puffins from the 1800s , for instance, show half the level of mercury as puffins today. While the mercury levels in the birds tested were still not enough to be harmful, the increase gives scientists insights into changes in our environment.