Domestic cats kill many more wild birds in the United States than scientists thought, according to a new analysis. Cats may rank as the biggest immediate danger that living around people brings to wildlife, researchers say.
For wild mammals, the annual toll lies between 6.3 billion and 22.3 billion, Marra and his colleagues report along with the bird numbers January 29 in Nature Communications. The majority of these doomed mammals and birds fall into the jaws of cats that live outdoors full-time with or without food supplements from people.
“The results are remarkable, not only for the big number, but also for the proportion of deaths from feral cats,” says Gary M. Langham, chief scientist for the National Audubon Society. The study assigns 861 million to 3.3 billion bird deaths a year to these wild cats. “These numbers really elevate this threat to a new level.”
To figure out how much wildlife cats catch, Marra and his colleagues combed the scientific literature for the best assessments of how many cats live in the United States and of what cats there and in similar climates hunt. Roughly 114 million cats live in the contiguous United States, 84 million of which share people’s houses. Forty to 70 percent of those household cats do at least some roaming outside. Between half and 80 percent of those outdoor cats hunt.
Marra says scientists have difficulty judging what proportion of total populations the cat catches represent. Comprehensive mammal numbers are a deep mystery, and estimates for U.S. land birds from volunteer counts lie between 10 billion and 20 billion adults.
“Cats are a nonnative species,” he notes, and multiple studies have shown that their hunting often targets natives. In his own research, Marra has shown that hunting cats can transform places that would normally be sources of young birds into sinks that drain birds from neighboring populations.