WASHINGTON –

Outdoor cats account for the leading cause of death among both birds and mammals in the U.S., according to a new study, killing anywhere between 1.4 billion and 3.7 billion birds each year.

The mammalian toll is even higher, concluded researchers from the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service, ranging between 6.9 billion and 20.7 billion annually.

The analysis, published in the journal Nature Communications, suggests that feral and pet cats pose a far greater threat than previously thought. One study in 2011 estimated cats in the U.S. kill roughly 500 million birds annually.

Peter Marra, the paper's senior author, said he and his colleagues "pulled together all the best estimates" from 90 different studies to reach their estimate, taking into account the difference in behavior between owned and unowned cats.

Researchers estimate that one pet cat kills between one and 34 birds a year, while a feral cat kills between 23 and 46 birds a year. As a result, the new study provides a wide range of the total bird death count.

Cats pose the greatest danger to birds and mammals living on islands, because there are fewer opportunities for these animals to escape. Cats are responsible for helping drive 33 species of birds, mammals and reptiles to extinction on islands, including the Stephens Island wren from New Zealand in the 1800s, according to the International Union for Conservation of Nature.

Scientists have a hard time measuring the impact of cats on small mammals in the U.S. because they lack precise population counts for these species, Marra said. "We don't know how many Eastern cottontail rabbits are out there," he said. By contrast, researchers estimate the U.S. is home to between 15 billion and 20 billion adult land birds. Cats kill about 10 percent of them each year, according to the analysis.