On the morning of Sept. 11, 1948, “a good number of dead, dying and exhausted birds” were found at the base of the WBAL radio tower in Baltimore, in what was then viewed as a new and unusual phenomenon. Ever since communication towers began popping up in the United States in the 1940s and 50s, bird bodies have littered the fields below them, especially during migration season.
For the first time, researchers have now quantified this threat to birds in the United States and Canada. In a study published online in the journal PLoS One, they estimate that a whopping 6.8 million are claimed annually by tower collisions.
“Ninety-five percent of birds killed are going to be neo-tropical migrants, and many of them are birds of conservation concern,” said one of the study’s authors, Travis Longcore, the science director of the Urban Wildlands Group and an associate professor of research at the Spatial Sciences Institute at the University of Southern California.
Neotropical migrants like thrushes and warblers breed in Canada and the United States and winter in places south of the latter’s border. Most of the migrants undertake nocturnal journeys, and that’s where the trouble starts. (Daytime bird window collisions, which occur when a bird does not perceive a glass barrier, is a separate problem and not taken into account in this study.)
Scientists are not sure why nocturnally migrating birds seem to have trouble navigating around bright light sources. Some researchers hypothesize that they need a certain wavelength of light to sense the earth’s magnetic field, which they use as a compass. Others think bad weather and low cloud cover obscures the stars and may encourage birds to approach artificial light sources. Still others wonder if birds that happen upon bright light sources become entranced by the stimuli and cannot break away from that zone of influence.
In any case, birds are regularly done in by communication towers, which sometimes reach heights twice that of the Empire State Building and are anchored by miles of cable radiating around them.
To quantify the impact of the towers have on bird populations and to suggest appropriate regulatory actions, Dr. Longcore and his colleagues sought to improve on past estimates from the 1970s. The original study from that time calculated that four to five million birds are killed per year and, until now, has been the figure generally cited by agencies like the F.A.A. But Dr. Longcore and his colleagues considered that estimate to be “seriously back of the envelope” because it was based upon only three data points and assumed that every tower claimed the same number of bird lives regardless of height or location.
To refine this estimate, the researchers compiled data from 38 tower studies. At each study site, they took into account how efficient bird body searchers were, the scavenging rates of animals like rats, raccoons and feral cats, the landscape surrounding the tower, whether the tower was guyed (attached to cables to steady it) and the height of each structure. Based upon those figures, they created a model of avian mortality related to tower height, and then applied it to 70,414 towers exceeding 60 meters (around 200 feet) in height.
Thus they ended up with their estimate of 6.8 million birds killed annually. They also found that the tallest 2 percent of the towers accounted for 71 percent of the mortalities. The Southeastern Coastal Plain, an area stretching across parts of Mississippi, Alabama, Florida and Georgia had the highest mortality rates, and peninsular Florida’s clustering of tall towers accounted for more deaths than all of Canada. Towers with more guy wires exponentially increased the probability of avian collisions as tower height increased, the researchers found, and towers with solid red lights were also especially deadly.
The researchers point out that the 1989 Exxon Valdez oil spill off Alaska, a benchmark of environmental disasters, killed around 250,000 birds, and that communication towers are annually responsible for a figure 27 times that size.
“We’re willing to go out and prosecute an oil refiner, but we have an entire communications tower industry killing millions of birds per year,” Dr. Longcore said. “The least we could do is mitigate it.”
He and his colleagues propose a number of steps, including replacing towers’ steady burning red lights with flashing ones, removing floodlights from the base of towers, avoiding the use of guy wires when practical and encouraging different companies to collocate equipment to minimize new towers being built.
To comply with the century-old Migratory Bird Treaty Act, “I think this should be done as a mitigation on existing towers and not just a condition on future towers,” Dr. Longcore said.
If steady-burning lights were removed from the roughly 4,500 towers that are over 150 meters (492 feet) tall in the United and Canada, overall bird mortality would be reduced by 45 percent, the researchers estimate.
“It seems like the right thing to do given our commitment toward conservation and the laws we have and how we apply them to other industries,” Dr. Longcore said.
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