Pesticides that kill rodents also poison owls.
Albert, CA, LK Wilson, P Mineau, S Trudeau and JE Elliott. 2009. Anticoagulant rodenticides in three owl species from Western Canada, 1988–2003. Archives of Environmental Contamination and Toxicology doi:10.1007/s00244-009-9402-z.
The chemicals used to control mice and other rodents are being found in the livers of wild owls and causing some to die.
Rodenticides are pesticides designed to kill pest rodents such as voles, gophers, squirrels, rats and mice. The newer types of rodenticides are anticoagulants. They are used worldwide to control rodent populations by causing them to bleed to death. In some regions, sales of certain rodenticides have skyrocketed. In the areas in Canada that were part of this study, sales of one of the most common chemicals increased by up to 100 percent between 1991 and 2003.
Bait laced with the chemicals is eaten by the rodents. However the mice, rats and other rodents may live for several days after eating the bait. The poisoned animals often have different behaviors after they are poisoned and may spend more time out in the open, making them vulnerable to predators, including owls and other raptors.
Birds of prey, like owls, are beneficial because they eat rodents and help keep their populations in check. The birds can be exposed to the rodenticides through their diets, and sometimes this has led to larger-scale poisonings.
In this study, 164 owls were collected between 1988 and 2003 from several parts of British Columbia and the Yukon Territory, Canada. The birds included 61 great horned owls (Bubo virginianus), 25 barred owls (Strix varia), and 78 barn owls (Tyto alba); these owls feed mainly on rodents but also on birds, insects, amphibians, reptiles and fish. Most had been in some sort of accident and had died just prior to or just after arriving at the center.
Livers from these owls were analyzed for 7 different rodenticides that are commonly used in the region. Seventy percent of all owls had at least one of these chemicals in their body; 41 percent of the owls had more than one of the pesticides. Also, levels of the chemicals in 15 percent of the owls were above those believed to be lethal (0.2 parts per million). Six owls were confirmed to have died from rodenticide poisoning, because they had hemorrhaging in their tissues.
Owls collected in recent years tended to have more rodenticides in their livers than ones from earlier in the study. This may be because of higher use of the chemicals or because the newer rodenticides are more persistent in the body (up to two years).
The researchers also found differences between the owl species. Barred owls had more rodenticides in their bodies than the other two species, which may be because barred owls tend to live closer to populated areas.
The researchers speculate that owls exposed to these rodenticides may change their behavior and become more lethargic. In turn, this may cause higher rates of accidents and deaths. Given the higher sales of some types of rodenticides in recent years, it is likely that effects on owls will also increase.
