Anja Murray: Furry, cute, whiskery... bird-killing predators

Domestic cats are stealthy predators, able to lay in wait, pounce, chase, catch, and kill a plenitude of birds, small mammals, reptiles, and amphibians. Their ancestors, wildcats, were originally domesticated about 10,000 years ago, most likely in Egypt. Domestic cats still retain a deeply ingrained hunting instinct.

Whether it's well-fed household cats which are allowed outdoors, free-roaming barn cats, or always-on-the-prowl feral cats that are fully independent of people, these are hunting animals. Birds, especially freshly fledged ones, are a favourite for cats. Cat owners won’t have much insight into just how many wild animals their cute little furry pet kills. Scientific studies have confirmed that typically only a fraction of hunted prey is brought back to the house or the farm (23% in one study and only 10% in another). Yet studies in Canada have found that domestic cats kill between 100 and 350 million birds per year there.

Aside from direct predation, the presence of a cat in an area has major indirect impacts on wild bird behaviour. Birds seeing or smelling a cat in their territory are aware of the presence of a mortal threat and become stressed. Their foraging, mating, and reproductive success all decline as a result. In one study, researchers briefly placed a taxidermied (stuffed) cat near a blackbird nest and found that the blackbirds were deterred from feeding their own young properly as a result, reducing the quantity of food by as much as a third.

But cats are opportunistic hunters, not limiting themselves to catching and killing birds. Other prey items include mice, voles, bats, frogs, newts, lizards, fish, and even invertebrates such as butterflies and damselflies. In the USA, the tally runs into the billions, estimated at 1.3–4.0 billion birds, 6.3–22.3 billion mammals, 258–822 million reptiles, and 95–299 million amphibians killed by free-ranging domestic cats each year. This makes cats the highest source of direct, human-related mortality for birds and small mammals in the United States — higher than deaths caused by poisons and pesticides and collisions with cars, buildings, and other structures.

Other scientific research in Britain and in the Netherlands has documented similarly high impacts. Across Britain, 27 million birds were brought home to their owners by cats in just one five-month study period, which means that multiples of this number of birds were actually killed by cats in that timeframe. In the Netherlands, almost one hundred million animals are predated upon by domestic cats each year. In Poland, an assessment of predation by farm cats estimated that an average of 136 million birds and 583 million mammals are killed around Polish farms annually.

To keep context, cats are not the main driver of wildlife loss. Habitat loss, land use change, and resource exploitation are all significantly more damaging to biodiversity than the harm caused by domestic cats. Yet the impact of cats on wildlife is enormous and one that we ought to face up to and take action on, especially given that so many populations of wild birds and other animals are already suffering from so many pressures and threats. Cats are invasive predators that we need to take responsibility for to prevent the damage from continuing.

Removing feral cats from areas containing vulnerable wildlife populations, such as endangered ground-nesting birds and seabird colonies, has proven benefits for those populations. I would hope that in the coming years, we will see growing efforts, here and across the world, to remove feral cats from priority areas such as biodiversity hotspots, critical species habitat or areas of particular vulnerability to the impacts of cats.

Today we published a paper @AnimalConserv finding that pet cats have an ecological impact 4-10X that of a wild predator, although mostly within 100m of their house. (1/n)https://t.co/ZWGmKZJJMl pic.twitter.com/2IWhsKqzDY

— Roland Kays (@RolandKays) March 11, 2020

For free-ranging domestic cats, there are things their owners can do to reduce the extent to which they prey on wild species. Keeping them indoors is the clearest way to reduce risk. Putting a well-fitting collar with a bell on your cat ensures that birds and other vulnerable prey are alerted to their presence in time to take evasive action, though this won’t reduce the indirect ‘fear’ impacts of cats on wild birds, nor does it prevent predation of eggs and nestlings. As long as cats roam free, wildlife fatalities will continue, unless owned cats are only allowed outdoors in cat-proof enclosures or on a leash.

It may seem odd to restrict a pet cat from roaming freely outdoors, because of their instinct and desire to roam, though our perception of what is fair is worth examining. We readily jeopardise the capacity of wild birds to exist, knowing the mortality rates incurred by free- roaming cats. We don’t think twice about putting such restrictions on other pets and domestic animals such as dogs, parrots, chickens, pigs, just a few of the domesticated animals who would gladly embrace any opportunity to wander off and explore by themselves. In terms of societal acceptance and changing values, we have come a long way in making drink-driving ethically and legally unacceptable; ditto for smoking in the workplace and emptying chamber pots in the street. Societal habits can and do evolve.

At least we may begin by restricting cats where endangered birds are nesting, and in proximity to protected areas. Keeping cats indoors in the hour after dawn and after dusk can help minimise impacts too. Wider restrictions are necessary, though they will be unpopular. A likely backlash of public opinion explains, though does not justify, the lack of action on this issue to date. Curtailing cats is a nettle that no-one has yet dared to grasp.