Origins of lead in populations of raptors

Although poisoning from anthropogenically derived lead threatens wildlife of many species, routes of lead exposure are unclear and rarely empirically tested. We used blood lead concentration and isotope ratio (Pb-207/Pb-206) data from populations of four species of raptors from across North America to test hypotheses associated with lead exposure via inhalation versus ingestion. Mean variation in blood lead concentration among cohort siblings was non-zero at nests of ferruginous hawks Buteo regalis and osprey Pandion haliaetus (P<0.001 and P<0.001), indicating exposure via episodic ingestion. However, within-nest variation in blood lead concentration was not significantly different from zero among cohort siblings at nests of bald eagles Haliaeetus leucocephalus and golden eagles Aquila chrysaetos (P=0.014 and P=0.023), consistent with exposure via continuous inhalation. Isotope ratio data corroborated the lead concentration data and within-nest average and variance of blood lead concentrations were positively correlated (r=0.70 to 0.94), indicating episodic ingestion. This study provides some of the first empirical population-level data to evaluate mechanisms of lead exposure and demonstrates the importance of lead ingestion to avian predators and scavengers.