Examining Sublethal Effects of Anticoagulant Rodenticides on Haemosporidian Parasitemia and Body Condition in Migratory Red-Tailed Hawks

Abstract

Anticoagulant rodenticides (ARs) are a source of mortality to nontarget wildlife, including raptors. Raptors that consume a large quantity of rodents, such as Red-tailed Hawks (Buteo jamaicensis), are especially at risk of secondary exposure due to bioaccumulation and biomagnification of ARs. However, few studies have focused on the sublethal effects of ARs and how they can lead to potentially lethal outcomes. We investigated the effects of ARs on both parasitemia and body condition in juvenile Red-tailed Hawks from the Marin Headlands—the site of a migratory bottleneck for raptors on the west coast of the United States—north of San Francisco, California. We investigated the presence of several ARs in the plasma of 116 Red-tailed Hawks. We used the ratio of heterophils to lymphocytes (H/L ratio), levels of haemosporidian parasitemia, and polychromatic erythrocytes in blood smears as indices of body condition. Based on studies of other carnivores, we predicted higher levels of parasitemia and increased H/L ratios in raptors with ARs in their blood serum, but we found no significant difference in parasitemia or H/L ratio between raptors with and raptors without ARs in their blood. In contrast, individuals exposed to ARs had increased levels of polychromasia, or immature red blood cells, possibly as a result of AR exposure, though levels of polychromasia were within the typical range. Because ARs in blood have a short half-life, we do not know each individual's exposure history, which may play a role in subsequent parasitemia. Future studies should focus on individuals with known exposure history or obtain a longer temporal resolution using liver samples.