An estimated 6.8 million birds are killed in the U.S. and Canada each year when they fly into towers or their guy wires, according study published this week in the online research journal PloS ONE.

"If 95 percent of the birds killed were pigeons, we might be concerned about that from a humane standpoint," said Travis Longcore, an associate professor at the University of Southern California and the study''s lead author.

Nearly all the birds killed by tower strikes are passerines, or migratory songbirds important to the health of forest ecosystems and favorites of backyard birdwatchers, but already in jeopardy from declining numbers.

The study was published Wednesday in the online research journal PloS ONE.

Researchers also concluded that taller the tower, the more likely it is to kill birds.

Less than 2 percent of all towers are taller than 900 feet, the study found, but they account for 70 percent of the bird deaths, about 4.5 million a year.