Often seen hovering high over open water, ospreys (Pandion haliaetus) are large, slender-bodied hawks with long, narrow wings and long legs. They feed almost exclusively on live fish, diving feet first to grab them. Barbed pads on the soles of their feet help ospreys grip the slippery fish. Ospreys build large, bulky nests made up of sticks and lined with bark, sod, grasses, vines, and algae. Their nests are found in an open area on top of dead trees, poles, cliffs, or human-built platforms.

The osprey's dependence on fish make it highly susceptible to water contamination. The Clark Fork River, which flows through Grant-Kohrs Ranch National Historic Site, contains heavy metals due to contamination from copper mining waste a century ago. Although efforts to remove and clean up contaminated sediments all along the Clark Fork River are underway, species like ospreys are being monitored to determine the effectiveness of the Clark Fork River Superfund clean-up.

Research conducted on osprey chicks compares levels of contaminants found in the river, such as arsenic, copper, lead, and mercury with contaminants found in the chicks' blood. Currently, osprey chicks seem to be regulating all of these metals well except mercury. Continued monitoring is necessary to determine the effect mercury will have on the Clark Fork River's osprey population.