"As many as two billion birds die in North America every year as a result of smashing into windows, according to a recent re-analysis of window-kill avian losses. Much of the mortality goes unnoticed because many of the victims' carcasses are rapidly removed by crows, cats, raccoons, rats, gulls and other scavengers before they are seen by human passersby.
A preliminary 1990 analysis of the number of birds killed by flying into window panes, by Daniel Klem of Muhlenberg College, suggested the North American toll probably tops one billion birds. That figure was based on per-building avian deaths of up to 10 per year. But recent studies indicate actual per-building avian losses are four to six times greater than that.
Dozens of recent studies suggest the per-building avian toll is 29 to 60 per year. Although there are no concise figures for the Prairies, data from the Fatal Light Awareness Program suggests up to 10 million birds strike buildings in the Metropolitan Toronto area every year. The New York City Project Safe Flight confirms some downtown buildings kill more than 250 birds each year. One study estimates one million of the eight million birds that migrate annually across the Chicago area are killed as a result of smashing into buildings in that city."