The association Huiskat Thuiskat, which advocates keeping cats indoors, is taking the Noord Holland provincial authorities to court on Wednesday for failing to keep protected bird species and other wildlife safe from free-roaming domestic cats.

There are more than three million cats in the Netherlands, which, together with an unknown number of feral cats, are said to kill millions of birds, rodents and small invertebrates each year. Campaigners say keeping the cats inside is the best way of reducing their impact.

The case centres around the question of whether or not owners who allow their cats outside are doing so with the intention of harming nature.

The province argues they don’t, but Huiskat Thuiskat claims owners have a choice, including to keep their cats indoors. That makes them responsible for what their cat is up to, the campaign group says.

Supporting evidence includes reports of birds and other wildlife brought home by cats to their owners, accompanied by photographs.

Campaign leader Roel van Dijk, himself the owner of a cat he keeps indoors, does not agree confining cats is cruel as has been claimed, as long as the animal gets enough exercise and attention.

“The provincial authorities have a duty of care for protected animal species and must intervene if they are harmed. Free-roaming cats are causing damage to nature but nothing is being done about it. Not even a single fine has been issued,” Van Dijk told Het Parool.

Tilburg University researchers said in 2019 that cats are a risk to 370 protected species and that this means their owners are breaking the law by allowing their pets to go outside. Others have called for cats to be kept in at night, particularly during the breeding season.

Nevertheless, Van Dijk is not optimistic about the outcome of the legal challenge. “The most we have achieved are campaigns to raise awareness,” he said. Amsterdam became the first city to call on owners to fit them with a collar with a bell so that birds and rodents are warned off by the sound of an approaching cat.

Van Dijk cited Canberra in Australia as an example of good practice. In 2021, the city decided that from 2022, all new cat owners should keep their pet indoors. “If they can do it there, why not here?” he said.