Tweety Bird had it right. His biggest enemy was a cat, though cats are far more efficient killers than Sylvester, the cartoon "puddy tat" who made a fool of himself in his futile pursuit of one canary.

But Tweety’s instincts were spot on.

An Environment Canada study released Tuesday shows that more than 270 million birds are killed in Canada every year from human-related activity, which includes deaths caused by cats owned, or not controlled well, by humans.

Richard Elliot, director of wildlife research for Environment Canada, said in an interview the estimated figure of 270 million is out of a total of 10 billion birds. "We've got a lot of birds, and that's probably a good thing because we're killing a lot."

The report looked at wild birds and not the millions of chickens, turkeys and other birds that are raised to be slaughtered for the food industry.

Read the Environment Canada report on avian mortality (pdf)

After cats, both domestic and feral, the biggest bird-killers are collisions with tall structures and road deaths. Combined, these three causes are responsible for 95 per cent of deaths.

Somewhat surprisingly, the oil and gas industry and wind turbines, which have both been blamed for causing bird deaths, didn't make it onto the list of top killers.

Birds such as this American goldfinch are most threatened by cats or collisions with buildings. (Amy Sancetta/AP Photo) This the first study of its kind conducted in Canada, though similar studies have been done in the U.K., the U.S. and New Zealand.

Because the research is so new, and there is still uncertainty about how exactly to determine the cause of bird deaths, the numbers are estimates, though scientists at Environment Canada are confident of the range of the numbers. The study was conducted using data from numerous other studies and then applying models to those numbers based on other information such as the number of cats in Canada.

Most birds in Canada are protected by the 100-year old Migratory Bird Conventions Act, as well as the Species at Risk Act and various provincial wildlife acts that prohibit destroying nests or killing birds, but little is being done to shield them from the following top killers.

1. Domestic and feral cats: 200 million

There are about 8.5 million domestic cats in Canada, and 1.4 to 4.2 million wild or stray cats. Although feral cats are smaller in number than house cats, they're responsible for twice as many bird kills. Even so, cats by nature can be serial killers and don't just kill when they're hungry.

Elliot said kitty-cams attached to cats' collars reveal that even house cats are avid hunters. "A cat you think is just out wandering around the premises would be killing 10 or 12 birds a night."

Ian Davidson of Nature Canada said in an interview with CBC, "Our pets don't really understand the difference between an endangered bird species or not, so we strongly recommend people keep their cats indoors, especially around dawn or dusk."

2. Power lines, collisions and electrocutions: 25 million

Wind turbines accounted for only 16,700 kills. But wind power is expected to grow tenfold over the next decade.

3. Collision with houses or buildings: 25 million

Between two and five per cent of nuthatches, chickadees and pigeons may be killed after striking houses or buildings, the report estimates. Davidson suggests turning off lights in large municipal buildings, since birds are attracted to bright light, as well as muting reflections on the windows so they don't appear transparent to birds

4. Vehicle collisions: 14 million

"Striking things," Elliot said, is a huge killer of birds.

5. Game bird hunting: 5 million

The report says extensive programs are in place throughout North America that ensure that any population-level effects of hunting are sustainable in the long term.

6. Agricultural pesticides 2.7 million

Electrical power and agriculture represent the largest industrial sources of bird mortality.

7. Agricultural mowing: 2.2 million young birds, equivalent to one million adult birds

One example cited are bobolinks, a protected bird, which nest in grasses and are killed every year by the cutting or clearing of grasses.

8. Commercial forestry: 1.4 million nests, equivalent to 900,000 adult birds

Activities that alter habitat during the breeding season, such as forestry and agricultural mowing, tend to destroy nests, eggs and young birds.

9. Communications towers: 220,000

Birds killed by flying into communications towers include kinglets and warblers.