How common is secondary poisoning in owls that eat poisoned mice and rats?—Aaminah Townsend, via email
That’s tough to determine. “Usually, the only way to know for sure is post-mortem,” says Rob Hope, the director of the Orphaned Wildlife Rehabilitation Society in Delta, B.C., which specializes in helping raptors. But it’s common enough in B.C. that the government permanently banned second-generation anticoagulant rodenticides (SGARs) for general use in 2022. First, some background: all anticoagulant rodent poison works by disrupting the animal’s normal blood clotting process so they bleed uncontrollably. The poisons are classified as either first-generation (the rodent must eat several helpings before the effects are lethal) or second-generation (these are stronger and kill the rodent after one feeding). There are also non-anticoagulant poisons that work in various ways—as a neurotoxin that destroys a mouse’s nervous system, for example.
The risk of secondary poisoning from first-generation rodenticides—the one most familiar to cottagers is probably warfarin—or the non-anticoagulant varieties is lower for birds and mammals compared to the risk from SGARs. And obviously, eating only one poisoned mouse, compared to, say, five of them in quick succession, wouldn’t have as severe an effect on a predator. But the impact of anticoagulants is cumulative. Compounds from the poison build up in the liver and can last for months, so repeated exposures can eventually result in the animal hemorrhaging to death.
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Because fast-acting SGARs are the poisons that are used the most frequently, the ban did help. “The suspected cases of secondary poisoning have dropped,” says Hope. But it’s still happening. For one thing, commercial pest control companies can use it, at least in certain areas—restaurants and hospitals, for example. (And unfortunately, people still seem to be able to get it and use it illegally, he says.)
There’s mounting evidence that secondary poisoning is a real problem in birds of prey all over the world. Their diet is heavily rodent-based, after all. One 2022 study that looked at secondary poisoning in southern Ontario raptors found rodenticide in 62 per cent of the dead birds tested (through liver samples). The researchers concluded that exposure to these poisons is common in birds in that area. In Canada, there are some country-wide restrictions—they were introduced by Health Canada in 2013—on the use of poison. For example, some products are prohibited from use in certain outdoor areas. But the goal of those restrictions was to protect children, pets, and non-target wildlife from directly consuming the poison. The results of the 2022 study indicate “that legislative changes in Canada may not be protecting non-target wildlife as intended,” according to the researchers.
It’s especially hard to know how many owls (and other birds) become sick or die from secondary poisoning in cottage country or rural areas, says Brian Salt, the founder of Salthaven Wildlife Rehabilitation and Education Centre in Strathroy, Ont. “They fly away and hide,” he says. “If it’s on someone’s 100-acre property…nobody ever finds them.”
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Salthaven sees about a dozen cases of secondary poisoning, on average, in a year. Most of the cases are birds of prey. But that in no way reflects actual numbers, of course; Salthaven is only brought birds that are still alive. Staff give the birds Vitamin K to counteract the effects of the poison. Sometimes it works. Mostly it’s too late. And the irony? “We’re poisoning the allies we have in keeping the rodent population under control,” says Salt.
If there’s growing evidence showing how bad rodent poison—any rodent poison—is for owls and other raptors, there’s an equal amount of evidence showing how good these birds are for rodent control. There are studies (mostly in the U.S.) that have demonstrated how making habitat raptor-friendly (by installing nesting sites, for example) is more effective and cheaper as a form of rodent control. Even some pest control companies encourage this.
So, if you want to help the owls and decrease the mice population at your cottage, consider installing an owl nesting box. You can buy one, or build one yourself. (Visit cottagelife.com/project/owl-house for plans.) Then let nature do nature: the predator population goes up, the prey population goes down.
Got a question for Cottage Q&A? Send it to answers@cottagelife.com.
This article was originally published in the June/July 2024 issue of Cottage Life.
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