A quarter of a million birds died after the Exxon Valdez oil spill off the coast of Alaska in 1989, marking it as one of the nation's most serious environmental disasters. But that pales compared to the number of birds that die each year after colliding with one of the 84,000 communication towers scattered across North America.
A new study by researchers from nine institutions puts that number at about seven million. Every year.
But it doesn't have to be that way, according to the research, published in the science journal PLoS One. Simply changing the lighting system on the tallest TV and radio towers -- which account for most of the deaths -- could save about 2.5 million birds every year, according to the study.
It turns out that a solid red light attracts and disorients more birds as they approach the towers, some of which are nearly 2,000 feet tall, than a flashing red or white light.
The victims are mostly small songbirds, like Tennessee Warblers and thrushes, "that would fit in the palm of your hand," research scientist Travis Longcore, lead author of the study, said in a telephone interview. Longcore, who holds positions with both the University of Southern California and the Urban Wildlands Group, said "the bulk of the mortalities happen during the annual migration" from Canada to Latin America and points south.
Environmentalists have talked about the problem for years, but this study looked at a wide range of factors -- height of the towers, lighting systems, nearby terrain, and number of birds killed -- and identified specific areas of greatest concern.
The tallest towers, reaching over 900 feet, killed 70 percent of the birds, or about 4.5 million per year. Yet those super-towers are only 1.6 percent of the total number of towers. They are used as platforms for television and radio antennas. The smaller cell towers were not included in the study.
Most birds did not run into the towers themselves, but instead collided with the long guy wires that are required to keep the metal towers upright.
But why would a solid red light be more deadly than a flashing light?
"So far we've been unsuccessful in getting into the mind of a bird," Longcore said. "We can only know what happens from our observations."
Ornithologist Sidney Gauthreaux Jr. of Clemson University, a member of the research team, spent long nights lying on his back near communication towers in South Carolina, observing nocturnal birds migrating through the area.
According to his observations, as the birds approached a tower with a static red light they went into a circular pattern, leaving them vulnerable to the guy wires as they soared around the tower. They were far less likely to do that when approaching a tower with a flashing light.
Gauthreaux has speculated that a bird's magnetic compass breaks down in the presence of a static red light.
If the light is flashing, Longcore suggested, perhaps the stimulus of the light is interrupted enough that it allows the bird to escape.
Of course, towers are only part of the problem for migrating birds. They also have to avoid power lines, windows, cars and even predatory house cats, which together may account for far more deaths than towers. The numbers are hard to come by, and based on a wide range of assumptions.