Un-owned and owned free-ranging domestic cats kill between 1.4 and 3.7 billion birds and between 6.9 and 20.7 billion small mammals each year in the contiguous United States, according to Smithsonian Conservation Biology Institute scientists who have completed the first systematic review of publications that estimate cats’ predation rates. The findings in the study, published online in Nature Communications and since covered worldwide by the media, suggest that free-ranging cats may be the largest source of human-related mortality of U.S. birds and mammals.
“Our study shows that the issue of cat predation on birds and mammals is an even bigger environmental and ecological threat than we thought and one that deserves conservation attention,” said Scott Loss, SCBI postdoctoral fellow and lead author of the study. “Our study provides motivation for further research and for incorporating cat impacts into conservation and management efforts.”
The paper’s authors built a mathematical model based on 21 publications that estimated free-ranging cat predation in the United States and Europe. The researchers took a rigorous and conservative approach, excluding studies that did not distinguish between owned cats and un-owned cats and studies that were based on a small sample size or a short sampling period.
“When we ran the model, we didn’t know what to expect,” said Pete Marra, SCBI research scientist and the study’s senior author. “We were absolutely stunned by the results.”
Up to this point, it was often assumed that cats killed many fewer birds than other human-related threats, such as building collisions and pesticides, and therefore were unlikely to have a significant effect on mainland vertebrate populations.
“Given these results, free-ranging cats are likely having a population-level impact on native species of birds,” Marra said.
The study finds that it is un-owned cats—such as farm and barn cats, strays, colony cats, and feral cats—that cause the majority of the mortality, roughly 69 percent of bird deaths and 89 percent of mammal deaths.
Free-ranging domestic cats on islands have contributed to 14 percent of all documented extinctions of birds, mammals and reptiles. Reflecting this toll, domestic cats are on the IUCN’s list of the top 100 World’s Worst Invasive Alien Species. The study states that “despite these harmful effects, policies for management of free-ranging cat populations and regulation of pet ownership behaviors are dictated by animal welfare issues rather than ecological impacts.”
The next step in the research, according to Marra, is to further refine the estimates of how many birds, mammals, reptiles and amphibians are killed by feral cats, including those in Trap-Neuter-Return (TNR) colonies. The researchers would also like to determine which wildlife species are most affected by free-ranging cats, the precise numbers of feral cats throughout the country and where they are more and less abundant.
The research is also part of a larger three-year Fish and Wildlife Service-funded effort to arrive at more rigorous estimates of the number of birds killed by other human-related threats, including pesticides and collisions with vehicles, wind turbines and windows.
In addition to Loss and Marra, the paper’s other author is Tom Will in the Division of Migratory Birds at the U.S. Fish and Wildlife Service.
The Smithsonian Conservation Biology Institute plays a key role in the Smithsonian’s global efforts to understand and conserve species and train future generations of conservationists. Headquartered in Front Royal, Va., SCBI facilitates and promotes research programs based at Front Royal, the National Zoo in Washington, D.C., and at field research stations and training sites worldwide.