The Owl Caper

Death on the sawdust trail

For curators at London's Regent's Park zoo, the deaths were troubling. Between March 1974 and September 1976, 55 owls in the zoo's celebrated collection died. For a while, zookeepers thought that the birds, which usually live to ages of 15 years or more, were simply succumbing to old age. But when younger birds began dying too, sometimes after repeated convulsions, the zoo's chief veterinarian, David Jones, decided it was time for some serious sleuthing.

After performing post-mortems on the carcasses (which had carefully been preserved for further study), Jones and his colleagues learned that 20 of the birds had excessively high concentrations of dieldrin, a chemical kin of DDT, in their livers and brains. But use of the lethal insecticide is sharply restricted in Britain, as it is in the U.S. and other countries. So how did the owls pick up the poison?

Jones began checking into their diet, mainly laboratory mice provided by a scientific supply house. The mice too turned out to be contaminated by dieldrin, although there was nothing in their standard pellets of food that could account for the poison's presence. Jones, however, did find that there was a potentially damaging concentration of dieldrin in the sawdust used as bedding in the rodents' cages. With all the fervor of a Baker Street Irregular, he then traced the suspect sawdust to a maker of wooden window frames. There, Jones found, the manufacturer had, quite legally, sprayed a wood preservative containing dieldrin on his lumber to protect it against infestations of woodworms. The mice took in a little sawdust each time they ate the food pellets. While the amount of dieldrin was not great enough to kill short-lived mice, Jones reports in Nature, it was certainly enough, over time, to do in the owls, which fed almost exclusively on what to them were tasty but, alas, tainted rodents.

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