In recent years, downtown office towers have increasingly been blamed for their role in the deaths of the migratory birds that fly into them each year. But according to a landmark series of Environment Canada studies released today, the impact of tall buildings is dwarfed by another killer: cats.

The numbers are striking. Each year, over 276 million birds are killed in Canada due to "human-related activities." About 196 million of those deaths are due to cat predation, 116 million from feral cats and 80 million from domestic ones. And the annual death tally from tall buildings? About 64,000.

“Cats are the biggest killer of our bird life in North America," Bruce Di Labio, a longtime Ottawa birder and guide, told the Ottawa Citizen. “People let them out. They’re responsible, and indirectly they are leading to the killing of these birds by letting the cats out."

Other major causes of human-related bird deaths include power transmission lines, responsible for about 25.6 million deaths; houses, which kill 22.4 million; road vehicles, which account for 13.8 million; and hunting, which kills 4.7 million birds each year.

The nine individual studies collectively make up the largest-ever investigation into the human-related causes of bird deaths in the country. “It’s very hard to come up with these numbers, and that’s why it hasn’t really been done till now,” Richard Elliot, Environment Canada's director of wildlife research, told the Citizen. In a release, the agency calls the estimates "a positive first step in understanding the relative importance of different causes of mortality," and says they'll be used to help focus its conservation efforts.

Via Ottawa Citizen