San Antonio Express-News (Sunday)
Study finds wide exposure of owls to deadly rat poison
Owls in Texas are facing widespread exposure to rat poisons, some at lethal doses, according to new research by scholars from the University of Texas at San Antonio.
More than half of the 53 owls tested by researchers between June 2021 and June 2023 at two wildlife rehabilitation centers had been exposed to what are known as anticoagulant rodenticides. The exposure appears to have been going undetected for years, in large part because tests for the poisons were not routinely done.
Researchers reviewed a decade’s worth of data on the reasons owls entered the centers, looking at 1,620 cases, and did not find a single case of rodenticide poisoning reported. Instead, the largest number of owls were admitted with “no apparent injury” and, when diagnoses were clear, “entrapment in human infrastructure” and “collision with vehicles” led the
pack.
“Whilst we know a little bit about rodenticides in owls in other parts of the country, in Texas we knew pretty much nothing,” said Jennifer Smith, a UTSA wildlife ecologist who supervised the research. It can be difficult to detect the poison without expensive testing, and since owls can have large home
ranges, it is hard to know where they were exposed.
“I think this is a wake-up call,” Smith said. “Not a lot of people think about it, or know that these rodenticides have an indirect effect.”
The sale of brodifacoum, the rodenticide found most frequently in the owls tested by the researchers, was banned for the general public in 2011 by the U.S. Environmental Protection Agency “to better protect children, pets and wildlife.” It is still permitted for use by pest control firms and in the agricultural sector.
Eres Gomez, who came up with the study as a student of Smith’s, said there are effective alternatives to the rodenticides that have secondary effects on species like owls. Simple sanitation to ensure that an indoor space is not appealing for rats can have an important impact, as can both lethal and nonlethal traps that avoid the use of poison such as electric zappers.
Gomez said those owls that do die from eating rats poisoned with rodenticide are not necessarily ingesting the lethal dose all at once.
“They’re eating a little bit of rat poison here and there, here and there,” Gomez said. “Eventually, it builds up in their system to where it starts to cause other effects. So it might not kill them right away, but they’re getting weak, it’s harder to fly, then it’s harder to hunt and find food, it makes them more prone to disease and predation, and then other anthropogenic stresses that we see a lot.”
She said rodents that develop bait shyness, eating only a little bit of the poisoned bait at a time, become “little time bombs” for the owls trying to swoop down and eat them.
The lag in wildlife poisoning research can be attributed to its expense. In this case, the two San Antonio rehabilitation centers — Last Chance Forever the Bird of Prey Conservancy and Wildlife Rescue & Rehabilitation — worked with researchers to provide samples, but the authors turned to community members to crowdfund the medical testing itself, which costs nearly $200 per sample.
With rapid urbanization across the state, Smith and Gomez said it will be important to have a baseline understanding of how human pest control interacts with owl habitats, while researchers should keep a close eye on shifting threats.