In some environments songbirds exhibit inconsistency in their songs which may be caused by non-lethal levels of contaminants that persist in the sediments of the Hudson River region. Biologists studied songbirds that nest along the Hudson River valley, a region with legacy levels of PCBs as a result of decades of electronics manufacturing upriver. Polychlorinated biphenyls (PCBs) are synthetic chemical pollutants with demonstrated detrimental toxic and developmental effects on humans and wildlife.