If domestic cats wore bell collars in urban areas the numbers of native birds caught and killed could be reduced by as much as half, a new University of Otago study shows.

Dr Yolanda van Heezik and Dr Christoph Matthaei from the Department of Zoology - with Zoology Honours student Jo Gordon - studied cats known by their owners to be prolific hunters.

They asked owners of 37 Dunedin cats to record the number of prey caught and brought back home during a six-week period while wearing a belled collar, and during another six week period without a belled collar.

"We found that cats caught fewer birds while wearing the belled collar," Dr van Heezik said.

"This study shows it is worthwhile for domestic cats to wear bells and would go some way towards reducing the huge numbers of native birds cats catch. It won't eliminate the problem completely, but it's a start."

Previous studies have shown that domestic cats kill tens of thousands of native birds each year in New Zealand.

"People might consider cats as a nuisance because they dig in their gardens, or they may be concerned about cats roaming for welfare issues, but people hardly ever think about the impacts cats are having on our native birds.

"Most cats don't catch a lot of prey and they may bring back perhaps only one bellbird a year. But if you consider the large number of domestic cats in towns and cities, then the cumulative impact is likely to be huge," Dr van Heezik said.

In New Zealand urban environments, there are on average 220 domestic cats per square kilometre, and each cat has an average travelling range of about 2.2ha.

In total during the six-week study, the cats not wearing the collar caught 378 animals, including 82 birds. When the cats wore bells, they caught only 41 birds by comparison.

This study has been published in the Australian journal Wildlife Research.

New Zealand Bird Rescue Charitable Trust wildlife rehabilitator Lyn MacDonald said the research's findings were "interesting".

"People find [bell collars] don't work a lot of the time," she said. "The little bells don't tinkle. An open-type cow bell does work a lot better."

"It is less likely for a cat to catch a bird with a bell."

Ms MacDonald said Bird Rescue do have a lot of birds come in who have been caught by cats, but often the birds are already injured or sick.

There were other ways to ensure domestic cats do not catch birds, she said, such as collars which emit a sonic warning to birds which are more effective than conventional bell collars.

"You can teach cats not to catch birds, but it is not easy. Some cats you cannot teach," she said.

"The best thing people can do is keep their cats in at night. That's when they catch the most birds.