Investigating spatial patterns of mercury and rodenticide residues in raptors collected near the Charlotte, NC, USA, metropolitan area

Raptor population growth is dynamic and trends vary across species and by location in the United States. For those species that are declining, it is important to identify potential causes including chemical contaminants. Sampling wild raptors is problematic due to their small population sizes and role as a top predator. Therefore, we obtained liver samples (n = 56) from carcasses of several raptor species, including common species like red-tailed hawks, red-shouldered hawks, barred owls, great horned owls, and osprey that arrived dead or were euthanized from a non-profit rehabilitation center in Charlotte, North Carolina. Raptors were found or collected in South Carolina, North Carolina, and Virginia, but most samples were located near the metropolitan region of Charlotte, NC. We analyzed livers for total mercury residue (mg/kg, dry weight) and five anti-coagulant rodenticides (μg/kg wet weight). Mercury was analyzed using a direct mercury analyzer approach and rodenticides were quantified by LC-MS. Mercury residues were high in piscivorous birds (15.09 mg/kg for osprey and 6.93 mg/kg for great blue herons, dry weight) and relatively high in red-shouldered hawks and one eastern screech owl tested. Six of our samples exceeded a health threshold of 1 mg/kg (wet weight) including three osprey and one each of great blue heron, red-shouldered hawk, and eastern screech owl. Brodifacoum was the only rodenticide consistently detected in our samples. Brodifacoum detections exceeded 75% in barred owls, great horned owls, and red-shouldered hawks. Sixty-nine percent of owl samples were within (or exceeded) a threshold of brodifacoum residue associated with a 10–20% risk of acute toxicity. Correlations between residues and human population density were not significant for either mercury or brodifacoum. Our data suggest that mercury residues for most raptors were not of significant concern with the exception of osprey and possibly red-shouldered hawks. Rodenticide exposures associated with a risk of acute toxicity appear to be common and warrant further investigation.