According to the study published Tuesday in the journal Nature communications
, cats kill between 1.4 billion and 3.7 billion birds, and between 6.9 billion and 20.7 billion small mammals such as meadow voles and chipmunks every year.
reports Peter Marra and his colleagues got a grant from the US Fish and Wildlife Service to estimate the number of animals being killed directly and indirectly by people every year. The study considered animals killed indirectly through human activities such as cars, turbines, glass windows, pesticides and finally cats.
co-author Pete Marra, an animal ecologist with the Smithsonian Conservation Biology Institute
, estimated the number of cats, and cat-related bird deaths in the US.
In the attempt to estimate the number of birds that cats kill every year, the researchers compiled information from all previous studies estimating bird deaths and cat population. They arrived at an estimate of around 84 million "owned-cats" (domesticated) and noted that many of them are allowed outdoors on their own.
reports Marra said Americans own about 84 million cats "and of those, about 40 to 70 percent are allowed to go outside." He said: "And we estimate that about 50 to 80 percent of those are actually hunters."
The preliminary estimates led to the conclusion that about 47 million pet cats roam neighborhoods, hunting and killing other small animals, including birds. The study
also considered what the researchers termed "un-owned," cats, that is, "feral cats, barn cats and strays."
The researchers estimated a total of between 30 million to 80 million of "un-owned" cats in the U.S., hunting even more intensively than "owned cats."
They then attempted to estimate how many birds and small animals the cats were killing.
gives insight into the study methodology:
[the] researchers... assembled a systematic review of every U.S.-based cat predation study known in the scientific literature (excluding Hawaii and Alaska). Based on figures the authors verified as scientifically rigorous, they statistically quantified the total bird and small mammal mortality estimate caused by cats, further breaking the categories down into domestic versus unowned cats, that latter of which the authors define as barnyard kitties, strays that receive food from kind humans and cats that are completely wild.
After they had compiled data and done their statistical analysis, the researchers were shocked at the figures they derived:
While some previous studies had estimated that cats kill about 500 million birds a year, Marra and his colleagues, based on available data, estimated a between a staggering 1.4 and 3.7 billion cat-related bird deaths a year. Mara said: "We estimate that cats kill somewhere between 1.4 and 3.7 billion birds a year. For mammals, it's upward of about 15 billion."
comments on the reputation of cats that appear corroborated by the new study estimates:
Cats in particular have earned a nasty reputation for themselves as blood thirsty killers of wildlife. They have been named among the top 100 worst invasive species in the world. Cats have also earned credit for countless island extinctions. Arriving onto the virgin specks of land alongside sailors, the naive native fauna didn't stand a chance against these clever, efficient killers.
[according to Marra et al] around 33 percent of the birds killed are non-native species (read: unwelcome). Even more startlingly, between 6.9 to 20.7 billion small mammals succumb to the predators. In urban areas, most of the mammals were pesky rats and mice, though rabbit, squirrel, shrew and vole carcasses turned up in rural and suburban locations. Just under 70 percent of those deaths, the authors calculate, occur at the paws of unowned cats, a number about three times the amount domesticated kitties slay.
reports Mara said: "A lot of these cats may go outside and go to 10 different houses, but they go back to their house and cuddle up on Mr. Smith's lap at night."
Based on their analysis of data from previous studies, the researchers estimated that each owned free-range cat killed between four and 18 birds a year, and between eight and 21 small mammals per year. Even more threatening to birds and small mammal populations were feral and "un-owned cats" each of which was estimated to kill between 23 and 46 birds a year, and between 129 and 338 small mammals.
reports that the researchers estimated that pet cats are responsible for only about a tenth of mammal deaths caused by cats and about a third of the bird deaths.
pointed out the threat to wildlife such as birds, meadow voles and chipmunks that cats pose based on their estimates.
Marra said that cat-caused mortality far exceeds deaths from other causes such as collisions with cars or wind turbines. He said: "We're pretty confident [about our estimates]. We felt like we only used the best studies out there. We eliminated studies that had small sample sizes or were only conducted for short durations. And we eliminated studies that had really, really high estimates, or really, really low estimates. So we tried to be as conservative as possible."
The authors concluded
Our estimates should alert policy makers and the general public about the large magnitude of wildlife mortality caused by free-ranging cats. Although our results suggest that owned cats have relatively less impact than un-owned cats, owned cats still cause substantial wildlife mortality; simple solutions to reduce mortality caused by pets, such as limiting or preventing outdoor access, should be pursued.
suggested that to prevent wildlife deaths caused by "owned cats," owners should keep them indoors. But "unowned" or feral cats present a bigger problem. The authors
write in their study that "feral cats are the single greatest source of anthropogenic (human-driven) mortality for U.S. birds and mammals."
Previous attempts to capture and sterilize them have not yielded satisfactory results. They authors
, therefore, suggested that feral and "unowned cats" should be a “wildlife management priority,” but they stopped short of suggesting they should be exterminated to save wildlife.
Animal rights advocates, appalled at previous suggestions that wild cats should be exterminated, argue that targeting cat owners who abandon their pets should be the priority