The great cat flap

by Dave Hansford / 17 November, 2007
Kiwi cats are more likely to feast on mice and rats than to decimate our native fauna.

Lolling on the couch, your moggy might not look like "the ultimate predator", but it's been branded thus by regional councils and features in the World Conservation Union's top 100 most unwanted.

Certainly overseas studies make astounding reading: the British Mammal Society concluded that cats kill at least 20 million British birds annually. When rodents and other victims were added, the toll leapt to around 70 million casualties each year.

But Wellington ecologist John Flux says such findings are of little use here. "Cats are important predators of small mammals," he says, "but in Britain and Australia, small mammals are invariably native animals. In New Zealand, all our small mammals (except bats) are vermin."

Craig Gillies, a predator ecologist with the Department of Conservation, has the unenviable task of trying to tease out the complex interrelationship between cats, rats, mice, stoats, ferrets and weasels, and native wildlife.

"The simple answer," he says, "is that there's no simple answer." Any discussion of cat predation can only be relevant to each particular site. "If you live somewhere with no native wildlife, then clearly cats aren't having any impact on it. But if you live, say, next to a forest reserve, or on the coast next to a shorebird breeding site, then yes, your cat is a liability."

In Lower Hutt, Flux has just completed a 17-year study of his own cat's toll. It caught 223 birds over the study period, but just 54 of them were natives - 43 were silvereyes, a self-introduced Australian bird at no risk of extinction.

And in any case, says Flux, those depredations need to be balanced against the effect on pest numbers. His cat also brought in 221 mice, 63 rats, 35 rabbits, four hares and two weasels.

Cats kill a great many lizards and insects too, but Flux argues that a single pair of rats can produce 1700 progeny a year; without cats to keep them in check, native creatures would be a great deal worse off. "Cats are a good tool, and in the end, they're quite easy to manage."

In Australia, some local authorities have imposed "cat curfews" to protect local wildlife; if a ranger catches a cat abroad at night, its owner cops a hefty fine. But Gillies and Flux agree that such measures are only partly useful in suburbia, because of the huge number of stray and wild cats (although Gillies still sees a good case for cat bans, or at least owner education around critical native wildlife habitat such as braided rivers).

Cat-free subdivisions, popular of late, can never ensure a cat-free environment; cats will wander in a 5km radius in a single night.

Cat collars with bells - that ostensibly warn prey of the cat's approach - have been marketed as another solution, but they're dubious deterrents. Cats are stealthy enough to keep the bell silent until the final spring and, in one study, collared cats actually caught more birds than uncollared cats did.

"There's evidence that collars work," says Gillies, "and there's evidence that they don't."

A new approach is the "pounce protector" or cat bib, a neoprene panel that attaches to the cat's collar and hangs down over the chest. The idea is to fit the bib every time your cat goes outside. Believe it or not, cats so encumbered still caught birds and lizards, but their success rate plummeted by 75 percent.

Keeping your cat indoors after dark won't stop it killing birds by day, but it will prevent it killing rats and mice at night. It might, however, save some -lizards and insects.

Stuffing it with gravy beef won't work either; cats hunt by instinct, hungry or not. But do get your cat desexed, don't leave it unattended over holidays and never abandon it if you can't look after it; take it to the SPCA for rehousing or humane destruction. Always feed it indoors, and never leave food out for strays.

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