The 84,000 communication towers that dot North America, some rising as high as 2,000 feet, are proving deadly to birds migrating from Canada and the United States to Central and South America, a study in the journal PLoS ONE reported.
And the taller the tower the greater the threat, the study found; the approximately 1,000 towers above 900 feet account for only 1.6 percent of the total number of towers but killed 70 percent of the birds, about 4.5 million a year, the researchers said.
"This is a tragedy that does not have to be," lead author Travis Longcore at the University of Southern California said.
The majority of the birds are not killed by running into the towers but rather in collisions with the dozens of cables, known as guy wires, that prop up the thin, freestanding structures, Longcore said.
During bad weather birds are pushed down by cloud cover and fly at lower altitudes, deprived of navigation cues such as stars, leaving only the blinking or static red lights found on most towers, he said.
The lights often confuse the birds, he said, causing them to circle the towers.
"In the presence of the solid red lights, the birds are unable to get out of their spell," Longcore said. "They circle the tower and run into the big cables holding it up."
Longcore suggests businesses share towers to reduce their number and build more freestanding towers to reduce the need for guy wires.
"One of the things this country has been great about is saying we care about not losing species on our watch," he said. "With these towers, we are killing birds in an unnatural way. This is senseless."