Cats are the ultimate predators

IN September, the Outdoors page carried a piece on domestic cats and the damage they do to wildlife.

Kerrie Anne Loyd and Sonya Fernandez of the University of Georgia had fitted ‘kittycams’ to cats, hoping to unlock the secrets of their lives. The camcorders revealed that only 30% of the pussies ever caught anything. The number of victims was 2.1 per cat each week, 13% of them birds. This suggested that house cats kill about 500m birds every year in the United States. Feral, or ‘stray’, cats were not included in the study.

We don’t know how many pet cats we have in Ireland but, assuming ownership patterns are similar to those in the UK, there may be about 700,000 here. The ISPCA claims that there are a further 200,000 living rough. If predation rates for American and Irish cats are broadly similar, we would have about four million bird casualties annually.

Now, scientists at the Smithsonian Conservation Biology Institute and the US Fish and Wildlife Service have come up with new estimates; they argue that cats do much more damage than was previously thought.

Scott Loss, Tom Will and Peter Marra reviewed all of the published studies and combined the results from the most reliable ones. Their conclusions appear in the journal Nature Communications.

There are, they claim, about 114m cats in the United States, 84m of them with owners. Between 40% and 70% of the house cats are allowed to roam freely outside the home. Half to 80% of these kill mammals and birds, a much larger percentage than found in the Georgia study. Stray cats, living wild, do much more damage than domestic ones, according to the new report. They account for 69% of all bird deaths due to cats and 89% of mammal deaths. Between 1.4 and 3.7bn birds are killed annually by cats in the US and the number of mammal victims may be as high as 20.7bn. Nor is the problem confined to the developed world. Cats, introduced to islands by people, were responsible for 14% of the documented extinctions of birds, reptiles and mammals worldwide.

These figures far exceed previous estimates. More birds and mammals die as a result of cats than are poisoned, collide with windows or are struck by vehicles and wind-turbines.

Free-ranging cats are probably the greatest source of wildlife mortality for which humans are responsible. The authors complain that ‘policies for management of free-ranging cat populations and the regulation of pet ownership are dictated by animal welfare issues rather than ecological impacts’. ‘This study should finally put to rest the notion that outdoor cats represent some harmless new component to the natural environment’ declared Dr George Fenwick, president of the American Bird Conservancy.

If the predatory behaviour of US and Irish cats is similar, we have between nine and 25m bird casualties here annually. However, with so many unknown factors in the calculations, this estimate must be taken with a ‘grain of salt’. There are huge environmental differences between the US and Ireland. The bird species in North America, apart from a few introduced from Europe, are not the same as ours and the role of cats in the eco-system may be different.

The domestic cat is more of an alien at the far side of the Atlantic than it is here. Its ancestors lived in the Middle East and North Africa but they belonged to the same species as the European wildcat. Although it was never in Ireland, this tabby-like predator still survives in Scotland where it breeds with the local house cats. On our side of the Atlantic birds have ‘co-evolved’ with wildcats over millennia. They have learned to cope with them. In America, the house cat was introduced by Europeans comparatively recently so the native birds may be less well equipped to deal with it. It takes time to establish equilibrium between species.

According to the RSPCA, fitting a bell collar to your cat will reduce the number of birds it kills by about a third.