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Claremont Raptors Killed by Rodent Poison

During October and November, 2000, a Cooper's hawk and two great horned owls were found to have died from poisoning by the anti-coagulant rodenticide brodifacoum (broh-dif'-a-coom) near the vicinity of the Bernard Field Station. Within the last four months a barn owl, another owl, and four coyotes were also found dead or sick within Claremont with symptoms or appearance consistent with poisoning; however, it was not possible to have autopsies performed on these animals to determine the cause of death.

The poisoning was most likely accidental secondary poisoning; that is, the birds consumed rodents that had been poisoned before the rodents died.

BRODIFACOUMD-Con mouse & rat poison packages

Brodifacoum is one of the "second-generation" anti-coagulant rodenticides that were developed in response to rodent resistance to warfarin. They are more toxic than warfarin and persist much longer in the body before they are degraded. Both of these properties make them more likely to cause secondary poisoning than the original warfarin. Brodifacoum in particular has an extremely high potential for secondary wildlife poisoning. In studies in both California and New York, brodifacoum was found to account for 80% of the secondary poisonings by rodenticides, even though it accounted for only 20% of sales. Similar problems with brodificoum have also occurred in Australia and New Zealand.

Brodifacoum is currently registered in California for the control of Norway rats, roof rats, and house mice in residential, industrial, commercial, agricultural, and public buildings. It is the active ingredient in D-Con mouse and rat poison and may be purchased over the counter in grocery and hardware stores. As I understand the EPA regulations, brodifacoum purchased by consumers may only be used inside buildings or placed outside against the foundation of buildings. The labels, however, are not very clear about the restrictions. Brodifacoum may be used outside in certain agricultural situations, but only by a licensed pest control operator.

The California Department of Pesticide Regulation is reevaluating brodifacoum at the request of the California Department of Fish & Game, who are concerned that the currently registered uses expose California's wildlife to brodifacoum with adverse effects. The review is currently underway. It seems likely that some change will be made in the allowed uses and labeling, but that change will take a year or more.


These deaths are obviously a serious problem. Not only are they tragic and disruptive to natural communities, but killing raptors is also under some circumstances a federal offense. Furthermore, secondary poisoning poses a risk for domestic dogs and cats as well as wildlife.

The poisoning is almost certainly accidental, not deliberate, but we still want to stop it. We have already been able to effect change. After finding out about the problem of secondary poisoning, Pomona College discontinued its use brodifacoum. They had had 185 brodifacoum bait stations on their campus to control rats. We have not identified any other institutional sources of brodifacoum, but any number of households could be using brodifacoum-containing products.


  • Help us track information on secondary wildlife poisoning to forward to Fish & Game:

    If you find a dead predator in the Claremont area for which there is no obvious cause of death (such as being hit by a car), please call one of the following peopole:

    • Nancy Hamlett, Chair of Biology, Harvey Mudd College (909) 607-3811
    • Bill Wirtz, Chair of Biology, Pomona College (909) 621-8606
    • Gene Fowler, Director of the Bernard Biological Field Station (909) 621-8970

    These people will notify the authorities and contact appropriately trained biologists to collect the body and arrange for an autopsy. If no one answers, please leave a message that includes:

    1. the type of animal,
    2. exact location,
    3. your name and phone number, and
    4. anything else that seems relevant.

    Unless you are trained, do not try to handle the animal yourself because of the risk of disease.

  • Identify sources of brodifacoum:

    Be on the lookout for rodent bait stations. These are plastic containers that are typically green or black, about 3 inches high and about a foot across, as shown below. They will be labeled "POISON". They should have the name of the poison, emergency first-aid information, and a phone number to contact for more information. Unfortunately, most of the stations I have found are not so labeled.

    Bait stations

    If you see bait stations in Claremont, please copy the label information if there is any, and phone (909/260-4403) or e-mail (friends@fbbfs.org) the Friends. In your message please include:

    1. location of the bait station,
    2. any label information, and
    3. your name and phone number.

    We would appreciate hearing about any bait stations even if they don't contain brodifacoum, so we can cross those potential sources off our list.

  • Educate yourself and others about the danger of secondary poisoning of wildlife

    Here are some references available on-line:

  • Adopt rodent control methods that minimize the risk of non-target wildlife poisoning

    • Try non-chemical methods of rodent control first. The key aspects of non-chemical control for rats and mice are:

      • Reduce harborage or shelter. Remove piles of yard debris, trash, construction waste, etc. where rats or mice could make homes.

      • Eliminate food sources. Don't leave pet food outside. Keep wild bird seed and other materials rats or mice may eat (such as some organic fertilizers) in rodent-proof containers. Collect and remove fallen fruit from fruit trees in the yard.

      • Exclude rodents from your home. Rodents can squeeze through amazingly small holes - 1/4 inch for mice and 1/2 inch for rats. Go around the outside of your house looking for openings and seal them with metal, hardware cloth, mortar, concrete, etc.

      • Use old-fashioned spring traps, live traps, or glue boards. Of course, you have to deal with the rodents if you use live traps. House mice, Norway rats, and roof rats are not native rodents and should not be released into the environment. Drowning, freezing, and whacking the back of the head are recommended methods for dispatching these animals. You may prefer to contact a professional pest control company. Tell the company representative you are concerned about secondary poisoning and would prefer to try non-chemical methods.

      If you would like more detailed information about rodent control, here are some sources:

    • Sometimes chemical rodenticides may be the only way to eliminate a severe rat or mouse infestation. If you do use chemical rodenticides, consider contacting a licensed pest control operator and asking them to use a rodenticide that has a low potential for secondary poisoning. Also make sure that you and others you know and comply with current use restrictions and safety precautions for any rodenticide that you use.

Last updated 10 October 2003 by Nancy Hamlett.

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