Keeping cats in at night will keep more young birds safe, particularly now the breeding season is in full swing, a city ecologist has said.

The curfew could start at 8pm, Delft city ecologist Diny Tubbing told the AD. ‘Around that time the cats have eaten and then it is best they stay inside. I don’t want to force anything on people but in Australia, for example, it is normal for cats to be kept inside at night. I hope this will start a public discussion and people will think about what they’re doing.’

By leaving their cats to roam at night, owners are harming the bird population, which, in cities in particular, are having difficulty enough to breed, according to bird protection organisation Vogelbescherming.

The Netherlands is home to between two and three million cats and some 10,000 feral cats and strays. Between them they kill some 18 million birds a year, posing a threat to some 370 species.

‘It wouldn’t be a curfew in the sense of checks and fines but it is important that people realise that cats go after birds and many other small animals, such as rabbits, mice and bats. Birds are particularly vulnerable. They can hear the cat during the day and sound the alarm. But at night they are asleep and the cat can take them unawares,’ Tubbing said.

Keeping the cats inside shouldn’t be a problem, Tubbing said. ‘They will cry a little at first and keep their owners awake at night but if you think the birds are worth it you’ll find a solution. Cats are hunters and that won’t change. But by giving them structure they’ll soon get used to staying indoors at night, preferably all year long.’

Garden owners can help by planting bushes for birds to flee in and where they can’t be easily pursued by cats, Tubbing said. Nesting boxes should be made cat proof by making sure cats don’t find a way of getting on top of them.

Tubbing’s is not the only attempt to curb killer cats. A paper by Tilburg university environmental law professors Arie Trouwborst and Han Somsen in the Journal of Environmental Law argued that under the European Bird and Habitat Directives countries have a legal obligation to protect wildlife, so allowing cats to roam and kill is illegal.

So far, however, neither the Dutch government nor the European Commission has  taken action on the subject.