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.
Longcore said the most impressive change so far is at the Federal Communications Commission, which has authority over the towers. The FCC has hired avian ecologist Joelle Gehring, who lead a large study of tower deaths in Michigan that found that when a steady light is replaced by a blinking light, the birds simply leave.
Gehring contends it's not expensive, and the results are immediate.
Of course, all of this is based on estimates, some of which wildly disagree with estimates from other researchers, and that may be partly because the situation varies so dramatically across the country. The songbirds suffer the most losses in the South and Midwestern U.S., so those estimates are not going to coincide with estimates from the far north.
And just simply collecting the data is difficult. Longcore said a dead bird doesn't hang around very long.
"Scavengers and predators and decomposers are incredibly effective at recycling spare nutrients lying around," he said. In one case an owl was spotted as it zipped through the night sky and grabbed small birds before they even hit the ground.
Even professional researchers probably only find about 20 percent of the victims because the bird deaths take place at night and scavengers are on the job.
However, sometimes it's easier to find how many birds have died. Scientists found that an estimated 5,000 to 10,000 birds, mostly Lapland Longspurs, were killed on Jan. 22, 1998, near a 420-foot tower in western Kansas. It happened, as is often the case, during a heavy snowstorm.
And the number of kills from towers still lags far behind the estimated number of deaths caused by birds hitting windows. And still, the estimates vary widely. One group of researchers, for example, estimated that the number of window-kills was somewhere between 97.6 million to 976 million annually.
Even Longcore admits that his own group's estimates are just that, estimates.
"We frankly and completely acknowledge that this is a sophisticated back of the envelope estimate," he said.
So if the numbers are right, why don't we sometimes find ourselves up to our eyeballs in dead birds?
"Don't forget these numbers come out of the sheer number of towers and windows across the continent," he said. In most cases, the number of deaths at any one site on any one evening might be low, but when multiplied by the number of towers in America (about 84,000, and growing each year) the aggregate can be huge.
By the way, the tallest towers claim the most birds. Towers over 900 feet tall are blamed for 70 percent of the deaths.
Longcore said he believes that if most of those are modified, just by switching to flashing lights, many more songbirds will be around to serenade us in our gardens.