How are rodenticides affecting raptors?

A brownish Red-tailed hawk perches on green evergreen branches with a blue sky background.

Secondary anticoagulant rodenticide exposure in migrating juvenile Red-tailed Hawks (Buteo jamaicensis) in relationship to body condition

The following story highlights a raptor research project that tells us something new about the incredible birds of prey we monitor at the Golden Gate Raptor Observatory (GGRO). Thank you to our GGRO volunteers for preparing this summary. Read more raptor science stories here >>

Authors: Emily V. Abernathy, Elisha Hull, Allen M. Fish, Christopher W. Briggs

Year: 2018

Journal: Journal of Raptor Research

Anticoagulant rodenticides (ARs) are a common method for controlling rodent pest populations in agricultural, suburban, and urban environments worldwide. ARs inhibit normal blood clotting and lead to hemorrhaging. Secondary exposure through the consumption of small mammal prey exposed to ARs has been documented in a number of non-target species, including raptors.

Red-tailed Hawks (Buteo jamaicensis) are one of the most common raptors found throughout North America. Red-tails feed primarily on small mammals and frequent open suburban and agricultural areas where ARs may be used. This makes Red-tails particularly susceptible to secondary AR exposure. AR poisoning in resident Red-tailed Hawks have been documented in wildlife rehabilitation facilities throughout the United States.

Emily Abernathy, a UC Davis researcher and GGRO volunteer, led this study documenting AR exposure in juvenile Red-tails migrating through the Marin Headlands and investigating the relationship between AR exposure and body condition. With help from her co-authors and GGRO volunteers, Abernathy collected blood samples and body measurement data from 97 juvenile Red-tails captured during fall banding operations in the Marin Headlands. The collection of blood from live birds is a novel approach to evaluating the presence of ARs in migrating raptors.

Eight Red-tailed hawks (8.2%) tested positive for some amount of ARs, and there was no relationship between AR exposure and Red-tail body condition. These results differ from other AR studies which have shown higher exposure rates, ranging from 19% to 100%. The variation in Red-tail AR exposure at GGRO compared to other studies may be due to differences in sampling techniques. Most studies of AR exposure to date have analyzed liver samples from dead raptors. ARs are deposited in the liver over the lifespan of an individual raptor. Therefore, sampling ARs in the bloodstream may only represent short-term exposure to ARs.

Additionally, migrating Red-tails may utilize a larger variety of prey compared to resident or wintering raptors that may spend a longer time in high-risk areas with high AR use. Further studies should be conducted over time on raptor populations near agricultural areas where there is a high likelihood of AR application to better evaluate this technique.

Read the full paper here >>