They’re lethal killing machines, and they could be lurking in your house or back yard.

Cats aren’t likely to harm you, of course, but they’re putting a serious dent in Canada’s bird population. According to a recent Environment Canada report that examined how human-related activities affect bird mortality, feral and domestic cats are the biggest killers of birds, claiming 196 million lives every year.

Richard Elliot, director of Wildlife Research for Environment Canada, says he thinks people will be surprised to see cats at the top of the list. “We think of a cat as a nice, quiet animal. It’s actually a pretty effective predator.”

Power-transmission lines were the next biggest bird killers, responsible for 25.6 million deaths per year, followed by houses (22.4 million), vehicles (13.8 million), hunting (4.7 million), agricultural pesticides (2.7 million) and low, mid-rise and tall buildings (2.5 million).

Recent news stories have highlighted the dangers posed by structures such as buildings and wind turbines. But Bridget Stutchbury, a York University biology professor, says the threat of cats is well known among those who study birds. “Part of the reason we don’t hear about it is because cat owners don’t want to hear about it,” she says. “When it comes to issues with pets, a lot of cat owners know cats eat birds, but they want their cats outdoors.”

Elliot says there are things people can do to help reduce bird deaths, such as putting wind turbines in locations away from peak bird migration areas and using reflectors on windows to prevent impacts with tall buildings and homes.