Beneath massive communication towers, fallen bird bodies pile up like confetti. They collide with the steel structures—which can reach heights twice that of the Empire State Building—or fly into the miles of cables radiating around the beacons. Each year, nearly 7 million birds lose their lives to these web-like traps of wire and metal—27 times more birds than were killed in the infamous 1989 Exxon Valdez spill.
The killing season peaks during the time nocturnal migratory birds make their way between Canada and the U.S. Flying in the darkness, they spot the tower lights, become disoriented and begin circling the beams. After a storm, when natural navigational cues like the stars or moon are obscured, mortalities are particularly high.
While the magnitude of causalities is worrying, until now researchers did not know whether or not the avian victims were species of conservation concern or just common sparrows. Research recently published in the journal Biological Conservation, however, confirms scientists’ fears. Members of thirteen threatened North American species succumb each year to the towers. The fallen birds represent between 1 and 9 percent of those species’ total population numbers.
“Certain species of birds, including many already in decline, are killed at communication towers in far greater proportions than their abundance would suggest,” said lead author Travis Longcore, the science director of the Urban Wildlands Group and an associate professor of research at the Spatial Science Institute at the University of Southern California, in an email. “And it’s not just these thirteen species we have to worry about—they’re just the ones being killed at the highest rates,” he continued. “Many more species of concern are killed at lower rates, too.”
To figure out mortality by species and regions, Longcore and his co-authors constructed a database of species deaths based on verifiable, available records. Then, they calculated the mean proportion of each species killed and compared those statistics with overall mortality rates for each species’ total population in the U.S. and Canada.
All in all, they found, 97 percent of the birds being killed are passerines, or songbirds. Among the threatened birds that are dying are the Yellow Rail, with 2,200 annuals mortalities, representing 8.9 percent of the species’ total population; the Golden-winged Warbler, with 5,300 annual deaths, representing 2.5 percent of the population; and the Swainson’s Warbler, with 7,500 annual deaths, representing 8.9 percent of the population. Other species, though not currently of conservation concern, still suffer formidable losses. Red-eyed Vireos, for example, relinquish 581,000 lives to communication towers each year, and around 499,000 Ovenbirds die this way, too.
Last year, the same team found that around 1,000 of the towers, used for television and radio broadcast, are responsible for 70 percent of the bird deaths. Those 1,000 towers, the team noted, stand 900 feet or higher, representing the largest of North America’s 70,000-odd communication towers included in the original study. In their follow up study, they identified the deadliest sites, which are in Texas, Louisiana, Florida and the Midwest. The findings are no surprise; the Southeastern coastal plain and the Midwest regions contain the highest concentrations of the tallest towers on the continent.
The Migratory Bird Treaty Act of 1918 makes it illegal to kill migratory birds in the U.S., so the researchers hope their findings may be used to better regulate communication towers. Eliminating the steady-glow red lights from the towers and replacing them with blinking lights—the same fix adopted by the Federal Aviation Administration—may reduce bird mortality by 50 to 70 percent.
The study also carries another lesson, Longcore said. Simply counting up the total number of birds killed by wind turbines, cats, windows, pesticides or communication towers across the country and then making crude comparisons between mortality sources can be misleading, he pointed out. The most impactful data—the types of species killed, and where, and when and how—often lurk beneath those surface figures. “Simple estimates of total ‘bird’ mortality are insufficient; it matters which species are being killed,” he said. “Each mortality source may be significant, but for different species and in different places.”