- Out of 10 fledgling red kites that left
their nests last year in the Midlands, seven died. In an ironic twist,
rats, mice and other rodents may be indirectly responsible for killing
the predators that hunt them, according to an article in the Jan. 23 issue
of New Scientist magazine.
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- Predatory birds and mammals in Britain
are dying from ingesting rodents that have high levels of pesticides in
their bodies. Because rats and mice have become increasingly resistant
to the poisons designed to kill them, they have become a threat to their
predators.
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- Researchers at the Institute of Terrestrial
Ecology in Monk's Wood, Huntingdon, Britain, found that the proportion
of barn owls found with anticoagulant rodenticides in their livers increased
from five percent in 1983-84 to 36 percent in 1995-96. The figures are
based on autopsies of 717 dead barn owls found all over Britain over the
period. About half of them were killed by cars. In the same study, Richard
Shore of the ITE found that nine out of 29 dead polecats collected between
1992 and 1994 contained rat poison. The findings will be published later
this year in Advances in Vertebrate Pest Management.
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- The rodenticides had killed only five
percent of the poisoned owls, and none of the polecats. However, authors
Ian Newton and colleagues stress that the true mortality may be higher.
They say that when animals succumb to the poisons they become lethargic,
and tend to settle and die in woodland or some other concealed place where
they are unlikely to be found.
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- Ken Wildey of the agriculture ministry's
Central Science Laboratory in York warns that rodenticide-resistant rats
and mice are a threat to many species, from domestic dogs and cats to foxes,
weasels, stoats and birds of prey. It is a problem in many countries. "Resistance
is rising worldwide," says Newton.
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- The first anticoagulant poisons, which
work by knocking out the blood's clotting mechanism, were introduced in
the 1950s. They accumulate in a rodent's body until a lethal dose is reached
and the animal dies of internal bleeding. But most rats are now resistant
to first-generation anticoagulants such as warfarin. Many are also resistant
to the second-generation poisons such as difenacoum and bromadialone, which
are more toxic. These chemicals also stay in the body longer, making it
more likely that rodents will accumulate enough of these chemicals to poison
predators.
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- The threat to rare birds of prey is especially
worrying. Out of 10 fledgling red kites that left their nests last year
in the Midlands, seven died. An autopsy on the only corpse that was recovered
showed that it had been poisoned by bromadialone after eating a rodent.
In May, an adult red kite in the Chiltern Hills, northwest of London, was
killed by brodifacoum, an anticoagulant so toxic that it is only licensed
in Britain for indoor use. Another adult kite found dead last November
in the Chilterns is also thought to have been poisoned.
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- "We think this is potentially a
huge problem," says Ian Carter, who leads the red kite recovery program
run by English Nature, the government's conservation agency. "Kites
are scavengers, so they will be selecting dead or dying prey and they are
more likely to eat rats with high rodenticide loadings than barn owls,
which take live prey."
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- Adrian Meyer, a rodent control consultant
based in Newbury, Berkshire, sees no easy way out. "In some areas,
rodenticide resistance has got to the point where the poisons licensed
for outdoor use no longer work, and it is only human nature to use something
else that does work," he says. Meyer claims that the Ministry of Agriculture,
Fisheries and Food appears to have no coherent strategy to deal with the
resistance problem.
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- For more information, contact Claire
Bowles, New Scientist Magazine, email: <mailto:claire.bowles@rbi.co.ukclaire.bowles@rbi.co.uk.
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