- Created on Wednesday, 25 April 2012 22:29
- Written by USC
Los Angeles, California - More than 80,000 towers, some almost twice as high as the Empire State Building, across the US and Canada.
Every year more than 6 million birds die as they migrate from the United States and Canada to Central and South America, according to a new USC study published April 25 in the 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, according to the authors of "An Estimate of Avian Mortality at Communication Towers in the United States and Canada."
In contrast, the Exxon Valdez oil spill killed 250,000 birds total and the Empire State Building is 1,250 feet high.
"This is a tragedy that does not have to be," said lead author, Travis Longcore, an associate professor in the USC Spatial Sciences Institute in the USC Dornsife College of Letters, Arts and Sciences.
The taller the tower the greater the threat, the study found. The 1,000 or so towers above 900 feet account for only 1.6 percent of the total number of towers.
Yet these skyscraper towers kill 70 percent of the birds, about 4.5 million a year, Longcore said.
Most of the birds winter in places likes The Bahamas and summer in Canada. With names like the Common Yellowthroat and the Tennessee Warbler, they could fit in the palm of your hand.
"These birds eat insects and keep our forests healthy," Longcore said. "They are quite beautiful. We have a long history of appreciating birds. Millions of people watch birds."
However the birds are not generally killed by running into the tower itself, but the dozens of cables, known as guy wires, that prop up the thin, freestanding structure, Longcore said.
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 blinking or static red lights of towers.
The blinking doesn't fool the birds. But towers with a static red light have dramatically more dead birds.
"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 estimates that changing the steady-burning lights on the 4,500 towers greater than 490 ft tall (about 6 percent of the total) could reduce mortality 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.
In 2005, Longcore and his colleagues started collecting and analyzing data from field studies that counted the number of bird kills at communication towers across the United States. The team only used findings that documented bird kills for at least a year and in some cases for several decades.
The numbers were scrutinized to find the average bird mortality based on based on height, the guy wires and the types of light affixed to the tower.
The team then matched up tower types, sizes and attributes of 38 tower studies and applied those findings to the 84,000 towers across Canada and the United States in preparation for the new publication, which has also been submitted to the Federal Communications Commission.
"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."
The study, which does not include shorter towers that are typically used for mobile telephone transmission, focused on towers taller than 180 feet, which typically provide TV and radio frequencies.
The study's authors included Catherine Rich and Beau MacDonald of The Urban Wildlands Group, Pierre Mineau, Daniel G. Bert, and Erin Mutrie of Environment Canada, Lauren M. Sullivan of UCLA, Sidney A. Gauthreaux of Clemson University, Michael Avery of the U.S. Department of Agriculture/Wildlife Services, Albert M. Manville of U.S. Fish and Wildlife Service, Emilie Travis and David Drake of the University of Wisconsin and independent scholar Robert L. Crawford.
The study was funded in part by The Urban Wildlands Group, Environment Canada, the American Bird Conservancy, and Defenders of Wildlife.