Scientists Provide an Understanding of Anticoagulant Rodenticide Exposure in Non-Target Bird Species

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U.S. Geological Survey scientists and their partners utilize laboratory and field studies and existing information to improve understanding of anticoagulant rodenticide exposure and effects to wild birds.

Anticoagulant rodenticides (ARs) have a long history of successful use for rodent pest management and their use likely will continue into the foreseeable future. Consumption of an AR bait by target pest species inhibits an enzyme responsible for the cycling of vitamin K, which in turn impairs activation of blood clotting factors, leading to hemorrhage and death.

raven with rodent

Raven with Rodent

(Credit: Dan Varland. Public domain.)

Unfortunately, unintentional consumption of bait by non-rodent pests occurs. Poisoned rodents and other exposed prey in agricultural, urban, and suburban settings can be consumed by birds and mammals. Predatory and scavenging birds, with their high trophic position in terrestrial food webs, are particularly susceptible to exposure and bioaccumulation of environmental contaminants, including ARs. In general, granivorous, invertivorous, carnivorous, and omnivorous bird species that typically or opportunistically feed on the ground are inquisitive and readily accept novel foods. These birds are also accustomed to humans; forage over large areas; feed on the target pest and (or) exploit ephemeral food resources; and, therefore, are at risk of rodenticide exposure and poisoning.

The U.S. Geological Survey (USGS), in collaboration with stakeholders, has been addressing critical knowledge gaps by reviewing and analyzing existing data on exposure, toxicity, and toxicokinetics in combination with field and laboratory studies to understand exposure and effects.

In a recent report, the scientists describe the challenges in extrapolating data derived from controlled studies of exposure to free-ranging birds, including gaps and steps to advance and prioritize research. Captive and wild birds have unique life histories and nutritional status that can influence the effects of AR exposure, which is one of the shortcomings of extrapolating findings from laboratory and field studies. For example, chronic malnutrition in wild birds can decrease vitamin K levels, potentially affecting AR toxicity, and could be a significant factor affecting AR sensitivity of captive versus free-ranging birds.

The scientists also highlight that healthy free-ranging birds with a history of AR exposure may be more likely to develop and exhibit overt toxicosis from repeated AR exposure. The scientists highlight that a better understanding of toxicokinetics and potency are critical steps forward to better interpret the extent to which AR exposure could cause subtle and prolonged effects on tolerance, physiological function, and overall fitness of nontarget wildlife.

barred owl sitting on a tree branch

Barred owls (Strix varia) have expanded their geographic range from eastern to western North America. Their newly expanded range now completely overlaps that of the federally threatened northern spotted owl (S. occidentalis caurina).

(Public domain.)

To gain a better understanding of wild bird exposure, a related field study was conducted on 40 barred owls collected from areas in central Washington and western Oregon. The livers of these barred owls indicated that 48 percent of the owls were exposed to more than one AR. The scientists determined that there was no relation between AR exposure and proximity to agriculture or urban areas, indicating contaminant sources in the forested landscape. Additionally, field studies in agricultural areas have highlighted the breadth of species exposed to ARs including raptors that feed on the prey targeted by AR baits.

These efforts to synthesize the state of knowledge regarding AR risk to wildlife and to characterize data gaps can serve as a framework for future research and improve some of the uncertainties associated with assessing risks of ARs to wildlife.  The USGS Environmental Health Program (Contaminant Biology and Toxic Substances Hydrology) of the Ecosystems Mission Area funded this research, as well as the U.S. Fish and Wildlife Service (Western Golden Eagle Management Team), the U.S. Department of Agriculture, Environment and Climate Change Canada, and the University of Nevada, Reno.