Thousands of years after first becoming domesticated, the house cat remains a beloved family pet. (Photo: Dave Morris/Flickr via Creative Commons)

Most people view cats as harmless pets, but for birds and small mammals, the domesticated feline can be a deadly killer.

A new study in Nature Communications finds bird and mammal mortality caused by outdoor cats is much higher than has been previously reported. Annual bird deaths in the United States are now estimated to be 1.4 to 3.7 billion, while mammal mortality runs from 6.9 to 20.7 billion individual animals.

Stray cats, as opposed to those that are owned and cared for by humans, cause the majority of these deaths, according to researchers from the US Fish and Wildlife Service (FWS) and the Smithsonian Conservation Biology Institute, who conducted the study.

The study, by two of the world’s leading science and wildlife organizations, offers the one of the most comprehensive analyses on outdoor cat predation.

The ancient Egyptians may have been the first to domesticate the cat more than 4,000 years ago. The animal became a favored household pet over the centuries, in large part due to their natural ability to hunt and eradicate household pests and vermin, such as mice and rats.

But that natural hunting ability could be a key reason cats are listed as one of the 100 worst non-native invasive species in the world, according to the Global Invasive Species Database, which was published in 2000.

A cat with its catch, a bird called the American Coot (Photo: Debi Shearwater)

A separate 2011 study found free-ranging cats have caused or contributed to 14 percent of modern bird, mammal and reptile extinctions on islands. The same report also found that feral cats cause a substantial proportion of total wildlife mortality.

Dr. George Fenwick, president of American Bird Conservancy, a group which has called for action on this issue for many years, reacted to this latest report. “This study, which employed scientifically rigorous standards for data inclusion, demonstrates that the issue of cat predation on birds and mammals is an even bigger environmental and ecological threat than we thought. No estimates of any other anthropogenic [human-caused] mortality source approach the bird mortality this study calculated for cat predation.”

However, the Humane Society of the United States questioned the report’s numbers.  “The HSUS values both cats and wildlife. There is a legitimate issue with free-roaming cats preying on birds and other wildlife, and we are working to change that in a meaningful way,” said Wayne Pacelle, president and CEO of the Humane Society of the US. “Despite the scientific rigor with which this report was prepared, like others recently published, it tries to attach a number to something that is almost impossible to credibly quantify.”

The larger issue, according to the humane society,  is finding practical and humane actions to mitigate the impact of cats in communities.

Current policies for managing feral cat populations and regulating pet ownership usually focus more on animal welfare rather than ecological impacts, according to authors of the latest study.

The domesticated cat’s natural hunting abilities help homeowners keep their homes pest and vermin free. (Photo: Cobalt123/Flickr via Creative Commons)

One of the more popular forms of managing free-ranging cats includes Trap-Neuter-Vaccinate and Return colonies,  a method of humanely trapping feral cats, spaying or neutering them as needed, vaccinating the animals for diseases like rabies, and then releasing them back to the same location where they were collected.

The study asserts these methods of managing feral cats are employed throughout the US without widespread public knowledge, consideration of scientific evidence, or the environmental review processes typically required for actions with harmful environmental concerns.

The researchers recommend scientifically sound conservation and policy intervention to reduce the negative impact of feral cats on wildlife.