They Are What They Eat: An Investigation of Secondary Poisoning in Birds of Prey

By Maureen Murray, D.V.M., and Florina Tseng, D.V.M.

Anticoagulant rodenticides (ARs) are used to control rodent populations in urban and suburban areas. These toxins kill target species by interfering with an animal’s blood-clotting system, causing the animal to bleed to death. However, secondary poisoning of non-target species who ingest the dead or dying rodent has been documented in a variety of wild birds and mammals, and puts birds of prey who feed on rodents at a particularly high risk of poisoning and death from severe blood loss.

At Tufts Wildlife Clinic, we have confirmed the presence of the AR, brodifacoum, in cases of suspected poisoning of red-tailed hawks. These clinical cases, along with a report describing widespread anticoagulant rodenticide exposure in birds of prey in New York State,1 led to an ongoing project to survey the extent of exposure to these compounds that birds of prey experience in urban and suburban areas of Massachusetts.

The purpose of this study, funded by the Animal Welfare Institute’s (AWI) Christine Stevens Wildlife Award, was to screen birds of prey presented to the wildlife clinic for exposure to ARs and to investigate potential effects of these compounds when they are present below the level at which signs of poisoning occur. We hypothesized that, consistent with the findings in New York State, a significant proportion of tested birds (less than or equal to 50 percent) would be positive for AR exposure below a level that causes signs of poisoning. We further hypothesized that these low levels might cause damage to the liver, where these compounds accumulate. Birds included in the study were red-tailed hawks, barred owls, eastern screech-owls and great horned owls.

A total of 92 birds were tested. Consistent with our hypothesis, 88% of those birds tested positive for the presence of an AR, most commonly brodifacoum. Of the positive birds, only two showed signs consistent with AR poisoning. Our second hypothesis was not supported, however, as birds with low levels of an AR stored in their livers did not show evidence of liver damage.

The data from this study has been used to obtain further funding to continue surveying this population of birds for AR exposure, and to attempt to identify the levels at which signs of poisoning occur. As the Environmental Protection Agency recently took steps to restrict the use of certain ARs, effective in 2011, due in part to the risk they present to wildlife, continuation of this project may help assess the effectiveness of new regulations. Once data collection for the second phase of the project is complete, we intend to combine these data with those collected during the AWI-funded project for publication.

Maureen Murray, D.V.M., is a Clinical Assistant Professor and Staff Veterinarian of the Wildlife Clinic at the Cummings School of Veterinary Medicine at Tufts University in North Grafton, MA. She began investigating anticoagulant rodenticide exposure in birds of prey in 2006 and is currently continuing this study.

Florina Tseng, D.V.M., is an Assistant Professor and Director of the Wildlife Clinic at the Cummings School of Veterinary Medicine at Tufts University in North Grafton, MA. She has been a wildlife veterinarian for the past 19 years and has particular expertise with seabirds and petroleum spills.

1 Stone WB, Okoniewski JC, Stedelin JR. 2003 Anticoagulant rodenticides and raptors: recent findings from New York, 1998-2001. Bulletin of Environmental Contamination and Toxicology 70: 34-40