More than 6 million birds die every year as they migrate from the United States and Canada to Central and South America, according to a new study published in the open access journal PLoS ONE. The birds are killed by the 84,000 communication towers that dot North America and can rise nearly 2,000 feet into the sky.

"This is a tragedy that does not have to be," said lead author Travis Longcore of the University of Southern California. The taller the tower the greater the threat, the researchers found; the tallest 1.9 percent of towers account for 71 percent of the mortalities.

The birds are generally killed not by running into the tower itself, but by getting caught in the dozens of cables that prop up the thin freestanding structures. During bad weather, the birds are pushed down by cloud cover and fly at lower altitudes. The clouds also remove navigation cues such as stars, leaving only the red lights of 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."

Towers with blinking red lights, on the other hand, cause fewer deaths. The authors estimate that replacing the steady-burning lights with blinking lights on the roughly 4,500 towers greater than 150 meters tall could reduce mortality by about 45 percent, or about 2.5 million birds.

The study also recommends that 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," Longcore said. "With these towers, we are killing birds in an unnatural way. This is senseless."

Longcore T, Rich C, Mineau P, MacDonald B, Bert DG, et al. (2012) An Estimate of Avian Mortality at Communication Towers in the United States and Canada. PLoS ONE 7(4): e34025. doi:10.1371/journal.pone.0034025.