Outdoor house cats spend 97 per cent of their time sleeping or resting, a two-year study has found.

But that other 3 per cent could kill them.

For his Master’s thesis at the University of Illinois, Jeff Horn strapped radio transmitters on to 42 owned and unowned cats and tracked them for two years.

The strays covered a vast territory, with one male ranging over 547 hectares, in their hunt for food and shelter.

House cats stuck much closer to home but still managed to cover a range of two hectares as their roaming territory.

Outdoor house cats were also prime targets for disease and injury from feral cats when their territories overlapped.

Cats can infect other cats with rabies, cat scratch fever, feline leukemia and feline immunodeficiency virus. The toxoplasma gondii parasite spread by cats can infect humans as well as other cats.

“I don’t know why anyone lets their cats outside,” said study author Jeff Horn, who grew up on a farm with cats and now works as a land steward specialist for the Jo Daviess Conservation Foundation in Illinois..

The painstaking tracking revealed unexpected patterns in feral cats.

“They’re efficient killers and very good hunters,” he told the Star. “Some of the males are only active for a few hours at night, but they find enough prey to get by.”

One far-ranging feral male managed to navigate coyotes, foxes, streets and parking lots to bed down under a softball field, Horn said.

“I was surprised he survived the whole study.”

Despite their range and much more active days, feral cats stayed within 300 metres of buildings during their hunts. Feral cats were active 14 per cent of the time.

Even though the range of house cats was smaller, it still took them “over several backyards,” said Horn. “I don’t think the owners realize.”

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Unlike house cats, feral cats tailored their habits to the seasons, sticking closer to cities and towns in the winter.

The study has been published in the Journal of Wildlife Management.