246. Invasive Species: The Case of the Domesticated Cat

By Elisabeth Rosenthal, The New York Times, March 20, 2011

While public attention has focused on wind turbines as a menace to birds, a new study shows that a far greater threat may be posed by a more familiar antagonist: the pet house cat.

A new study in The Journal of Ornithology on the mortality of baby gray catbirds in the Washington suburbs found that cats were the No. 1 killer in the area, by a large margin.

Nearly 80 percent of the birds were killed by predators, and cats were responsible for 47 percent of those deaths, according to the researchers, from the Smithsonian Institution and Towson University in Maryland. Death rates were particularly high in neighborhoods with large cat populations.

Predation was so serious in some areas that the catbirds could not replace their numbers for the next generation, according to the researchers, who affixed tiny radio transmitters to the birds to follow them. It is the first scientific study to calculate what fraction of bird deaths during the vulnerable fledgling stage can be attributed to cats.

“Cats are way up there in terms of threats to birds — they are a formidable force in driving out native species,” said Peter Marra of the Smithsonian Conservation Biology Institute, one of the authors of the study.

The American Bird Conservancy estimates that up to 500 million birds are killed each year by cats — about half by pets and half by feral felines. “I hope we can now stop minimizing and trivializing the impacts that outdoor cats have on the environment and start addressing the serious problem of cat predation,” said Darin Schroeder, the group’s vice president for conservation advocacy.

By contrast, 440,000 birds are killed by wind turbines each year, according to the United States Fish and Wildlife Service, although that number is expected to exceed one million by 2030 as the number of wind farms grows to meet increased demand.

The American Bird Conservancy generally supports the development of wind energy, but it argues that wind farms should be “bird smart” — for example, positioned so that they do not interfere with major migration paths or disturb breeding grounds, with their power lines buried to prevent collisions.

“I’m excited about wind; we just have to be careful where and how we put the turbines,” said Dr. Marra, who studies threats to birds, including from climate change and habitat loss. He said the leading cause of bird deaths over all, as opposed to the catbird fledglings in the study, remained collisions with buildings, windows and towers, followed by predators.

Yet wind turbines often provoke greater outrage than cats do, said Gavin Shire, vice president of the Bird Conservancy. “The idea of a man-made machine chopping a bird in half creates a visceral reaction,” he said, “while the idea of a predator with its prey in its mouth — well we’ve seen that on the Nature Channel. People’s reaction is that it is normal for cats to kill birds.”

Household cats were introduced in North America by European colonists; they are regarded as an invasive species and have few natural enemies to check their numbers. “They are like gypsy moths and kudzu — they cause major ecological disruption,” Dr. Marra said.