One point in the case against cats is undisputed: they destroy island ecosystems. A variant of the African wildcat, domestic cats probably first cozied up to humans in Egypt several thousand years ago. They populated the globe by riding the coattails of trade and empire. Cats were welcome members of sailing expeditions because of their small size, their agreeable temperament and their talent for killing shipboard rats. The British navigator James Cook was a veritable Johnny Appleseed of the cat. Cook brought tabbies on his 18th-century voyages around the Pacific, many of which were dropped off or stolen along the way.

The newcomers came ashore teeth first. Most oceanic islands had no mammalian predators before human contact, so native birds evolved with little ability to elude cats. Many were ground-nesters. Some lost, or never gained, the ability to fly. In Hawaii, at least 30 species or subspecies of forest birds were decimated or extirpated between 1870 and 1930, partly because cats ate them. In 1894, a lighthouse keeper’s cat on an island off New Zealand proudly presented his owner with dead specimens of a bird then unknown to science, thus discovering and extinguishing the Stephens Island wren in a single year. Forty years ago, when the Swiss ecologist Vinzenz Ziswiler added up the number of birds wiped out by introduced predators, he found that cats were implicated in 17 of 43 extinctions. The cat’s only rivals were the rat (14 extinctions) and the mongoose (9). As the naturalist Christopher Lever once wrote, “The list of species they have helped to exterminate or endanger reads like a roll call of avian disaster.”

But continents and islands are different. Continental birds had defenses against clawed mammals, so cats weren’t a problem. Or such was the comfortable conventional wisdom until recently in the United States, which has 1 in 4 of the world’s cats. The idea was challenged in 1987, when two biologists found that cats in a small English village were killing a surprising number of birds — nearly 300 by 78 cats in a single year. American biologists followed up with a study of cat kills in rural Wisconsin. John Coleman, a wildlife ecologist with the Great Lakes Indian Fish and Wildlife Commission, and Stanley Temple, a University of Wisconsin professor, estimated that the state’s 1.4 to 2 million rural cats were killing between 8 million and 219 million birds every year.

Though that range was so large as to be of limited value, alarms went off in the birding world. If cats were taking out that many birds in a single state, imagine the carnage nationwide! Conservation groups urged cat owners to keep their pets indoors. The American Bird Conservancy suggested cat leashes.

Cat defenders scrambled to stay ahead of the story. They pointed out deficiencies in the Wisconsin study and said, correctly, that the situation couldn’t be simplified into a kills-per-cat formula. Some cats were expert hunters. Others didn’t hunt at all. The Wisconsin study dealt only with rural cats, and the authors didn’t look at whether the cats were taking down rare species or common starlings.

Cat advocates love to attack the Wisconsin study, but the more you delve into the scientific literature, the more the Wisconsin study looks like a red herring used by cat defenders to divert attention from more grounded research. In the past decade, at least a dozen studies published in top scientific journals like Biological Conservation, Journal of Zoology and Mammal Review have chronicled the problem of cat predation of small mammals and birds. The takeaway is clear: cats are a growing environmental concern because they are driving down some native bird populations — on islands, to be sure, but also in ecologically sensitive continental areas. At hot spots along the Pacific, Atlantic and Gulf Coast, cat predation is a growing threat to shorebirds and long-distance migrants. And as wild habitat becomes more fragmented by human development, even some inland species are under increasing pressure from both house cats and their feral cousins.