That figure comes from a 2013 study published by Environment Canada scientists in the Avian Conservation and Ecology journal, which lists cats as the number one killer of birds in Canada.
Death by window collision is a distant second claiming about 25 million birds per year.
Canada's birds dying off
Canada's wild bird populations are estimated to have declined about 12 per cent in the last 40 years, with some populations decreasing by over 95 per cent.
"If we even step back from Canada and we look globally, we know for a fact that cats have contributed directly to the extinction of 34 species of birds. Next to rats and humans, of course, that's the biggest factor," said Ted Cheskey, a conservationist with Nature Canada.
Globally, the number of at-risk bird species has increased from 47 to 87 between 2001 and 2015.
The causes of decline are complex, but cat predation introduces a significant additional pressure to populations that are already facing challenges such as loss of habitat, pesticides and factors associated with global warming, Cheskey said.
Okanagan valley most at risk
"British Columbia has a very high proportion of species at risk. The Okanagan Valley being perhaps the place where there's the highest density of species at risk in all of the country," Cheskey said.
Cheskey also noted that free-roaming cats pose a significant threat to the province's island-based bird populations.
"Island populations are pretty insular, and often they lack predators as well. When a new predator like a cat is introduced to an island, [it] can be totally devastating."
Well-fed, domesticated cats who have ample toys to play with still pose a danger to avian populations, Cheskey said.
"The hunting action is an instinctive action. They may not eat the birds, but they will still hunt."
"The cats, they're indiscriminate on who they choose to kill. It can be a common bird, but it can be a bird that's perilously close to being wiped out as well. It's a big problem."
National campaign to launch
"Ideally, cats wouldn't be allowed to roam freely. We know that for now that is unacceptable to many cat owners," Cheskey said, which is why Nature Canada plans to launch an awareness campaign later this year.
Many municipalities in Canada already have bylaws that obligate residents to keep their pets indoors, Cheskey said.
"We know it's better for birds and better for cats," said Cheskey, who said the average life span of an outdoor cat is two to five years, compared to fourteen years for an indoor cat.
The campaign will also focus on the importance of spaying and neutering cats to reduce the feral cat population.
To hear the full interview with Ted Cheskey, listen to the audio labelled: Keep your cats indoors, say conservationists.