The sprawling networks of towers that broadcast TV and radio signals across Canada and the U.S. kill nearly seven million migratory birds each year, according to a new study.

"This is a tragedy that does not have to be," says Travis Longcore, an associate professor at the University of Southern California and the study’s lead researcher.

About 6.8 million birds die annually as they make their way between North America and the warmer climates farther south, the researchers estimate.

The continent’s 84,000 communication towers are to blame, according to the study, which was published Wednesday in the journal PLoS ONE.

The tallest towers — up to 600 metres high — were reported to be the most dangerous for birds.

Researchers say that when migrating birds are forced to fly at lower altitudes in bad weather and cloud cover blots out the stars, lights on TV and radio towers become more attractive.

"In the presence of the solid red lights, the birds are unable to get out of their spell," Longcore says in a press release. "They circle the tower and run into the big cables holding it up."

The dangers of communication towers are becoming "an issue of pressing conservation," the study says.

"With these towers, we are killing birds in an unnatural way," Longcore says. "This is senseless."

About 2.5 million birds could be saved each year if blinking, not solid lights, were used on towers that stand at least 150 metres tall, the authors say.

They're urging businesses to share communication towers to keep their numbers down, put limits on the height of new towers being built, where possible, and opt for freestanding towers to keep the bird deaths to a minimum.

Researchers used numbers from previous studies on birds killed around 38 communication towers to come to their overall estimate for Canada and the U.S.

The study’s co-authors included Pierre Mineau, of Environment Canada's National Wildlife Research Centre.