Giant TV and radio towers that have been blamed for nearly 7 million bird deaths each year are doing the most damage to species that can hardly afford the loss, according to a new study.
At least 97 percent of the birds that crash into the towers, or the guy wires that hold them up, are the tiny songbirds – mostly warblers – that are considered "birds of conservation concern" in the United States and Canada, according to the study, published in the journal Biological Conservation. Each year, according to the study, the species yellow rail loses about 9 percent of its total population because of communications towers, many of which are taller than the Empire State Building.
The latest study comes from the same researchers, members of the Los Angeles-based Urban Wildlands Group, that warned last year of the spiraling mortality of birds that are attracted to the lights, usually red, atop the towers. The lights are required by the Federal Aviation Administration for any tower over 200 feet tall, and there are thousands in North America that are more than 10 times that height.
The deaths usually occur during the nocturnal migration of songbirds, especially when the cloud ceiling is low and there is fog or rain. The lights create an illuminated area around the tower and it is thought the birds become confused, switch off their night navigation and begin to spiral around the tower. Some run into the support cables, or into each other, and plunge to the ground.
Numerous organizations and governmental agencies, including the U.S. Fish and Wildlife Service, have documented that, although their numbers do not always agree.
Travis Longcore, lead author of the current study, who is also with the University of Southern California, said his group wanted to look beyond the sheer numbers and focus on which types of birds were suffering the greatest loses, based chiefly on the number of deaths compared to the overall population estimate of each species.
"Many bird species are killed at towers disproportionate to their abundance," the study says.
"This lets us look at this (the tower deaths) as a factor in the trajectory of the population," Longcore said in a telephone interview. The study indicates the towers are a very significant factor for a number of species, especially small songbirds, some of which are declining in numbers overall.
The researchers found that 58 percent of the birds killed by towers each year are warblers, those colorful little singers that are found in many urban backyards. High on the list is the plain but lyrical Swainson's warbler, which loses 8.9 percent of its population each year, and the colorful black-throated blue warbler, 5.6 percent.
In addition to communications towers, however, the birds have to fend off cats and other predators, and many are killed when they crash into windows, as urban dwellers know so well. So towers are only part of the problem, but this study suggests they may be more significant than had been thought, at least for certain species.
There has been some progress since the researchers published their first report last year. Some experiments have shown that a blinking light attracts fewer birds than a light that remains on. The FAA recently ruled that tower operators may switch to blinking lights, and some have done so.