Glen Park owl died from secondary rodenticide poisoning


A female great horned owl and chicks in San Francisco’s Golden Gate Park on April 8, 2016. Photo by Anne Roughton.

By Murray Schneider

The death of one of Glen Canyon’s great horned owls throws into stark relief the conundrum faced by areas working to both protect wildlife while also minimizing rodent infestations in homes and retail areas.

Each spring hundreds of Glen Park residents go to see an owl pair and their yearly broods of chicks in their nest in a eucalyptus tree near the Glen Canyon recreation center.

However in March, a female owl believed to be one of that pair was found dead in the park. A necropsy showed that the owl died after ingesting rodenticide, likely from eating a rat or mouse that had consumed rodenticide.

It’s a reminder of the difficulties wildlife face sharing a crowded urban area with some 10,000 humans, their retail stores and the inevitable trash that result.


A great horned owl nesting in a eucalyptus tree in Glen Canyon Park in 2012. Photo by Michael Waldstein.

Discovery and testing

The owl was discovered March 16 near the Bosworth Street entrance to Glen Canyon park by a preschool teacher on an outing with her students. She notified a Recreation and Park Department gardener working nearby with several Friends of Glen Canyon Park volunteers.

The gardener conferred with her Natural Areas Program supervisor and arrangements were made to transfer the bird’s body to California Department of Fish and Wildlife.

By March 23 Fish and Wildlife had issued a preliminary report. A gross necropsy showed the death was consistent with anticoagulant rodenticide toxicosis “with subcutaneous bleeding in the wing and bruising of the chest and abdomen.”

On April 19, with final confirming laboratory tests still pending, Stella McMillin, a senior Fish and Wildlife environmental scientist added details.

“The rodenticides that were detected in the owl, brodifacoum and difethialone, are second generation anticoagulants,” she emailed the Glen Park News.

Weise-2012-04-17-2 fledgling

A young great horned owl in Glen Canyon park on April 17, 2012. Photo by Michael Waldstein.

Rodent control is necessary

Mice and rats pose a significant economic and health risk to people, according to the California Department of Pesticide Regulation. They directly spread 11 diseases and indirectly 15, as well as damaging homes, contaminating food, stores and restaurants and doing significant harm to crops. Because of that, California considers controlling them a priority.

That said, protecting wildlife is also a priority and the state has attempted to balance the needs of our state’s animal as well as human populations.

Rodenticides come in three types, first generation, second generation and non-coagulant. First generation rodenticides are available for sale to individuals but can require multiple feedings to kill mice and rats. Another issue was that in the 1970s Norway rats, roof rats, and mice began to develop resistance to these poisons.

That led to the development of second generation rodenticides. These are more effective because they require only a single feeding and no resistance has been reported. However the poisons have longer half-lives and can thus be more toxic to birds and mammals.

Because of that, in 2014 the state of California designated them restricted and they can only be used by licensed exterminators. Nationally, the Environmental Protection Agency placed similar restrictions on their use in 2015.

Without being controlled, rodents can run rampant in urban settings such as San Francisco and become a serious public health issue, said Dr. Richard Geller, medical director of California Poison Control System.

Compared to rat poisons used in the past such as strychnine and arsenic, the second generation rodenticide that killed the owl “does more good than harm,” said Geller. “Rat populations can explode, multiplying incredibly fast.”

Legions of rats left unchecked, in other words, would presumably overrun the capacity of San Francisco’s Department of Public Health to thwart them, and the few owls and hawks populating natural areas such as Glen Canyon might mount a good fight but lose the battle.

“It’s a trade-off between human safety and modern environmentalism,” continued Geller, who has practiced medicine for 29 years and was not unsympathetic to the plight of the canyon owl when he learned of it. ‘

Still, “strychnine was hideously toxic and brodifacoum is remarkably safer.”

A known hazard

Birds of prey such as owls, hawks and eagles that feed on rodents are at risk from secondary poisoning due to rodenticides when they eat animals that have consumed poisoned bait. The Department of Fish and Wildlife attributes hundreds of such deaths to rodenticides each year.

“Secondary poisoning of predators is a world-wide problem,” said Krysta Rogers,” a senior environmental scientist and avian specialist employed by Fish and Wildlife at its Wildlife Investigations Laboratory. “Anytime we can get the word out to the public it is important.”

This is the second owl death due to rodenticide ingestion in Glen Park in four years. In November 2012 officials performed a necropsy on another owl, whose death was also attributed to rodenticide.

Still, such occurrences are infrequent in San Francisco, said Joey Kahn, Rec and Park spokesperson. “Anytime something like this happens we investigate the situation in connection with San Francisco Animal Care & Control and California Department of Fish and Wildlife.”

The public can help

It’s likely the owl in question ate a mouse or rat that had eaten rodenticide relatively close by, as the species tends to stay close to its nests.

“Great horned owls are territorial and reside and return to certain areas,” said Rogers. “They don’t migrate like other owls. When chicks get old enough to fledge they eventually move away.”

Glen Park residents can make choices that will minimize the potential threat to local wildlife, say experts.

Fish and Wildlife’s Stella McMillin suggests non-lethal alternatives to the tamper-proof bait boxes often used near commercial properties.

“If residents would like to safeguard owls and other predatory and scavenging wildlife such as red-tailed hawks and coyotes, they should insure that any pest control business they hire is not using second-generation anticoagulants,” she said.

“Sanitation and exclusion are the best ways to control rodents. Take away anything that might attract them, pet food, seeds accumulating around bird feeders, fallen fruit, and then make sure all entry points to homes are sealed to prevent rodents from entering,” she said.

San Francisco’s Department of the Environment encourages residents to use alternatives to rid neighborhoods of rodents.

In the department’s ‘Don’t Take the Bait’ campaign in 2011, it asked 130 stores to remove the less lethal first generation products from their shelves. “One hundred stores pledged to do so,” the department’s head of safe pest management, Chris Gieger, told the Glen Park News in an email.

Others encourage residents to make safe habitats for raptors, which help keep down rodent population through their hunting.

Berkeley writer Lisa Owens Viani founded an organization called Raptors are the Solution that is dedicated to educating the public about secondary poisoning of owls, hawks and falcons.

“It is my belief that people should be given choices and there are other solutions to the rodent problem,” she said. “Raptors can’t do it alone, but if most people knew what secondary rodenticides such as brodifacoum do to raptors, they’d think twice and make the correct decisions.”

Keeping trash down and areas clear are key to reducing rodent infestations.

“We’re a messy species,” Owens Viani said. “Garbage and food on the street result in rodents, so proper sanitation is one of only several strategies.”

“But if we have to rid ourselves of rats,” she added emphatically, “then we should use snap traps.”

However she cautioned that such traps must be set carefully.

“They must be put in places, usually inside something, where other animals such as songbirds cannot be caught in them,” she said.

One Glen Park resident is doing his part to encourage birds of prey to take up residents here for a little localized rodent control.

Richard Craib has lived on Turquoise Way for 52 years. His Diamond Heights backyard abuts Glen Canyon and some years ago he took matters into his own hands. He built a number of barn owl boxes and placed them on the eastern slopes of Glen Canyon adjacent to Crags Court and Berkeley Way.

“I’d hear them hooting at night,” said Craib, a member of Friends of Glen Canyon Park. The smaller owl species shares the canyon with great horned owl. “Barn owls can eat four or five mice a night,” said Craib.

In fact, the barn owl has a much more prodigious appetite. A single barn owl, smaller than the great horned owl and sometimes prey for it, can consume 3,000 rodents in a single four-month period, nature’s efficacious alternative to indiscriminate killing by second generation rodenticides.

Craib barn owl box

Glen Park resident Richard Craib inspecting a barn owl box he placed near the park to encourage the rodent-eating birds of prey to take up residence. Photo by Murray Schneider


For additional information please see

Hungry Owl

Raptors are the solution





















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7 responses to “Glen Park owl died from secondary rodenticide poisoning

  1. Carolyn Plakias

    Thank you for the update – I’ve been wondering about the owls. This is sad and upsetting but even more so when I read what we can do to control rodent problems. I walk in Glen Canyon Park every day and am disgusted by the overflowing trash bins. I’ve been looking at the same bins with trash spilling out, too full to close, for over two weeks now. Why can’t the Parks Department take more responsibility and empty their trash. We residents would receive a fine if we let this situation exist in front of our homes.

  2. Heather World

    Fabulous reporting, Murray–thank you!

  3. Denise Louie

    Hi Murray,This is sad news, indeed.  Thanks for letting us know, though. I will write about alternatives to rodenticides in the Sept or Oct issue of Miraloma Life.  Here’s a preview:Instead of using poison to killrodents, make your property less attractive to them. “Keep your home and yardneat and clean. Don’t give rats places to hide. Remove objects and plants thatrodents can hide under, such as wood piles, debris, construction waste, densevegetation and ground-covering vines like ivy. Pick up fruit that has fallen from trees as soon as possible.

    Secure your garbage in a tightlysealed can.  Seal water leaks and removestanding water that can attract unwelcome animals, breed mosquitoes and wastewater.

     “To remove unwelcome rodents, settraps in secluded areas where they’ve been seen or are likely to travel: closeto walls, behind objects, in dark corners, on ledges, shelves, fences, pipesand garage rafters. In areas where children, pets or birds might go, put thetrap inside a box or use some kind of barrier for their safety. Check trapsdaily and wear disposable gloves when removing rodents from traps. Place themin a sealed plastic bag then into your garbage bin for weekly collection.”  []

    If you like, use this information any way you can. Thanks,Denise

  4. Gloria

    Safeway is the biggest offender with their uncontrolled garbage. They have not done enough to improve their sanitation. The area behind the store still smells and swarms with rats day and night. As a result they have rat bait boxes all around. Let’s put pressure on the store to do better.

  5. Pingback: Dangers of Rodenticides | tamaratamburro

  6. I appreciate the attempt at balanced reporting here, but I am deeply disappointed that the DPR representative and Dr Geller cannot give us more intelligent details regarding the rodents impacts than rats “run rampant” – and “populations explode.” That sounds like 1953 cold war language; we are better than that in 2016; we know lots more about how the world works.

    If these are scientists, let’s get real here: what are the numbers? How many humans are made ill by rats in California annually? What exactly are the economic and health risks if rats in 2016? Language like: “Rats left unchecked might overrun San Francisco?!!” Ridiculous! And “Great Horned Owls might lose the battle?” It’s not their battle; it’s their food. It is their survival.

    The big point is that we humans alone lose the battle If we don’t use every tool at our disposal. And to cut urban raptors and terrestrial predators out of the equation is a high form of stupid. A Barn Owl can kill 500 mice a year. In trying to present a balanced view, the author has left the pro-rat poison people speaking in ancient, useless, fear-based generalities.

    Which is fine really because we have a job to do. We need: (1) to toss all rat poisons into the incinerator and get them off the table as an option; (2) to apply IPM standards to Bay Area neighborhoods; (3) to intentionally promote opportunities for predators to be active in our midst; and (4) to educate people about these tasks and their importance.

  7. Actually, the “modern environmentalism” referred to by Mr. Geller is not “modern” at all. As far back as the 1950s and ’60s, Rachel Carson was warning about the effects of DDT in the food web. Today, rat poison is infiltrating food webs and causing serious health issues in wildlife, if not outright death. Sadly, we don’t seem to have learned the lesson that poison–in this case rat poison–can have unpredictable, unintended, and unacceptable impacts on wildlife, pets, and even children.

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