Rodenticides in Raptors Project

We began finding dead Cooper’s Hawks in Seattle that had no evidence of trauma, and we wondered whether rodenticides (rat poison) could be a factor. Rodenticides also poison non-target species (reviews in Quarles 2011, Elliott et al. 2016). A non-target species is any animal for which the rodenticide is not intended, such as birds and other mammals (including children and pets).

Anticoagulant rodenticides (ARs) are the main class of pesticide used worldwide to control rodent damage. In North America, everywhere that the prevalence of ARs in non-target wildlife has been studied, widespread contamination has been found. Raptors are a population of particular concern, because they prey heavily on rodents in human-dominated areas, where ARs are used.

The prevalence of ARs in raptors in Washington State has not been documented. Effects of AR poisoning are often not obvious without necropsy and testing of liver tissue. Toxicological testing is costly, so this question has not been widely studied. Urban Raptor Conservancy, in collaboration with wildlife veterinarians at PAWS Wildlife Center in Lynnwood, Washington, will document baseline levels of exposure to first- and second-generation rodenticides in raptors in Washington State. We will compare our results to reports of anticoagulant poisoning in other regions of North America.

Anticoagulant Rodenticides: A Problem, Not a Solution

Anticoagulant rodenticides prevent blood clotting and cause death from excessive bleeding. ARs such as warfarin were introduced in 1950, but rodents rapidly developed resistance to these first-generation anticoagulants. In the 1970s, far more toxic second-generation anticoagulant rodenticides, or SGARs, were developed. SGARs (brodifacoum, bromadiolone, difenacoum, and difethialone) are now the most commonly used rodenticides in the U.S.

SGARs are toxic not only to rodents but also to predators (e.g., mountain lions, raptors), scavengers (e.g., crows, pets), and the curious (children). SGARs present three major problems to non-target populations (Stone et al. 2003).

• Although only one dose is enough to kill a rodent, the victim dies slowly, making it easy prey for hunt or scavenge rodents.
• A poisoned rodent can continue to feed on bait before it dies, resulting in a super-lethal dose that can be passed on to predators and scavengers.
• SGARs, especially brodifacoum, have very long half-lives (100-350 days) and break down very slowly in the livers of exposed animals. This means that non-target predators and scavengers can bio-accumulate a lethal dose from multiple sublethal exposures to a poisoned animal (Stone et al. 2003).

Anticoagulant poisoning in non-target wildlife is difficult to detect. Many animals exposed to anticoagulants do not show obvious signs of poisoning, so effects of ARs are inconclusive without necropsy. Worldwide, all studies that have tested dead animals for exposure to anticoagulant rodenticides in liver tissue show strong evidence of poisoning in birds and mammals (Quarles 2011).

Secondary Wildlife Poisoning in Raptors

Raptors are particularly vulnerable to ARs because they are the natural agents for controlling rodent populations. In human-dominated landscapes, raptors are drawn by the abundance of prey, and rodents are a large percentage of this prey.

Growing evidence suggests that AR poisoning in raptors is common and widespread in North America. Raptor poisoning by SGARs in North America has been documented in four states (New York, Massachusetts, New Jersey, California) and in Canada, with anecdotal reports from many other regions.

New York

• Nearly half (49%) of 265 dead raptors from 19 species tested positive for at least one AR (Stone et al. 2003)

Massachusetts

• 86% of 161 dead raptors from four species (Red-tailed Hawk, Barred Owl, Eastern Screech Owl, Great Horned Owl) tested positive for ARs. 99% of these positive tests contained the SGAR brodifacoum (Murray 2011)
• In a 2012-2016 study, AR levels in raptors were documented after EPA restrictions on SGARs prohibited their retail sale. AR toxicosis was found in 17% of 94 raptors, nearly all (96%) from SGARs (Murray 2017).

New Jersey

• 58% (45 of 78) of Red-tailed Hawks and 81% (43 of 53) of Great Horned Owls tested positive for at least one SGAR, most commonly brodifacoum (Stansley et al. 2014).

California

• Positive exposures to ARs were found in:
  • 16 species of raptors, including 82% of Cooper’s Hawks, 72% of Red-tailed Hawks, and 59% of Barn Owls
  • most of the 14 species of mammals tested (n=298), including all 28 mountain lions, 82% of 44 coyotes, and 69% of 110 endangered San Joaquin kit foxes (California Department of Pesticide Regulation, 2013)
• In studies of sublethal effects of ARs on wild mammals, SGAR exposure was linked to severe and sometimes fatal mange in mountain lions and bobcats, as well as to immune system suppression in bobcats (Fraser et al. 2018). Sublethal effects of SGARs on non-mammals are poorly documented, but it is reasonable to be similarly concerned.

Canada

• In British Columbia and the Yukon Territory, most (70%) of 164 owls (Barred Owl, Barn Owl, Great-horned Owl) found dead from 1988 to 2003 had evidence of at least one rodenticide Albert et al. (2010).
• A statistical analysis of AR toxicosis in Great-horned Owls projected that at least 11% of this species is at risk of dying from SGARs (Thomas et al. 2011).

Laboratory Studies

Evidence from laboratory studies is consistent with these results in wild raptors. In a summary of EPA laboratory experiments with the SGAR brodifacoum in birds, 42% of 149 raptors or scavengers died of secondary poisoning after eating brodifacoum-poisoned prey (Erickson & Urban 2004).

Regulation of Anticoagulant Rodenticides

Nationwide, first-generation ARs continue to be available in retail stores. In 2008, the EPA attempted to ban retail sales of SGARs while allowing their continued use by commercial agriculture and pest-control professionals. These regulations, delayed for years by litigation by three rodenticide manufacturers, were finally implemented in 2015 (EPA 2015). Despite EPA’s partial ban, the SGARs brodifacoum and bromadiolone were still available for individual purchase from Amazon and other online retailers, at least through March 2018. SGARs and other rodenticides are also widely used commercially by pest-control professionals.

Anticoagulant Rodenticides in Raptors and Scavengers in Washington State

Although studies of AR poisoning in non-target wildlife have so far been done in only four states, the weight of their results suggests that they are representative of nationwide levels of AR exposure in non-target wildlife (US EPA, 2008).

No studies have yet examined the prevalence of anticoagulant rodenticides in non-target wildlife in Washington. However, wildlife professionals in Washington State have long suspected rodenticide poisoning in wildlife, and several unpublished reports on raptors suggest that more information is needed:

• Northern Spotted Owls from central Washington tested positive for brodifacoum (two records; National Wildlife Health Center, cited in US EPA, 2004).
• Bald Eagles in western Washington tested positive for brodifacoum (two records; Washington Department of Fish and Wildlife).
• Veterniarians at PAWS Wildlife Center in western Washington have reported raptor deaths from symptoms suspicious for AR poisoning (J. Huckabee and N. Rosenhagen, personal communication).

Our Study

To document exposure to rodenticides in urban raptors in Washington, we are fortunate to have partnered with PAWS Wildlife Center to test dead raptors at their clinic for exposure to rodenticides.

We will test for rodenticides in 120 raptors (hawks, falcons, eagles, and owls). PAWS staff estimates that they receive 60-70 dead raptors each year. URC and PAWs have the necessary federal and state permits for handling these birds and their tissue.

SGARs do not remain long in blood but instead are routed quickly to the liver. Thus, liver tissue from recently dead raptors will be sampled for toxicological analysis. Tissue samples will be sent to the Louisiana Animal Disease Diagnostic Laboratory at Louisiana State University.

URC will analyze the results of toxicology testing in Washington raptors. Results will be compared to those from similar studies done elsewhere.

Cost

The cost of toxicology testing is about $100 per sample. Our fundraising goal for this study is $20,000, with funds from grants and donors. In addition to funding, this independent study would not be possible without the expertise and hours of time donated by the professionals at PAWS.