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| Estimated annual avian mortality from communication towers by Bird Conservation Region. High mortality estimates in Peninsular Florida and Southeastern Coastal Plain reflect the more numerous and taller communication towers in these regions. (Credit: Longcore et al. PLoS One; doi/10.1371/journal.pone.0034025.g006) |
Every year nearly 7 million birds die as they migrate from the United States and Canada to Central and South America, according to a new USC study published on 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."
Placing that figure
in context, the Exxon Valdez oil spill killed 250,000 birds 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,
associate professor in the USC Spatial Sciences Institute at 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 accounted for only 1.6 percent of the total number of towers. Yet these
skyscraper towers killed 70 percent of the birds, about 4.5 million a year,
Longcore said.
Most of the birds
spent winter in places like the Bahamas and summer in Canada. With names like
the Common Yellowthroat and the Tennessee Warbler, they could fit in the palm
of one's 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 were pushed down by cloud cover and flew at lower altitudes. The
clouds also removed navigation cues, such as stars, leaving only the blinking
or static red lights of towers.
The blinking did
not fool the birds, but towers with a static red light resulted in 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 estimated
that changing the steady-burning lights on the 4,500 towers greater than 490
feet tall (about 6 percent of the total) could reduce mortality about 45
percent, or about 2.5 million birds. The study also recommended 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 height, the guy wires
and the types of lights affixed to the tower.
The team then
matched up tower types, sizes and attributes of 38 tower studies, applying
those findings to the 84,000 towers across Canada and the United States in
preparation for the new publication, which also was 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 typically are 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.
Story Source:
The above story is reprinted from materials provided by University of Southern California. The original article was written by Eddie North-Hager.Note: Materials may be edited for content and length. For further information, please contact the source cited above.
Journal Reference:
- Longcore T, Rich C, Mineau P, MacDonald B, Bert DG, et al. An Estimate of Avian Mortality at Communication Towers in the United States and Canada. PLoS ONE, 2012; 7(4): e34025 DOI: 10.1371/journal.pone.0034025
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