Deadly Pillars of Light

To make a safe landing, one of the many pieces of data that pilots need is the height of clouds above the landing strip. Because clouds block a pilot’s vision, a low cloud bank can make it difficult, even dangerous, for planes to approach an airport. In 1946, a new machine was put into widespread use that could calculate the height of the cloud base, helping to guide planes safely home. This device, called a ceilometer, shone an intensely bright spotlight directly up at the clouds, while a second receiver a little ways away measured the angle at which the spotlight hit. Meteorologists could then use simple trigonometry to figure out the height of the cloud floor, or bottom layer of clouds.  

Ceilometers were so bright that they could make a spot of light appear on clouds 15,000 feet above ground. They were so bright that on cloudy nights, their narrow, blueish beam could be seen from miles away. They were so bright that if you looked directly at the light source, they could blind you for several hours. Touching the lantern could leave you with third-degree burns.

As long as they were used properly, these powerful machines didn’t pose any real threat to people. But in 1948, Charles Linville discovered that they could be extremely dangerous to birds. During the night of September 10, while he was serving as the caretaker of the ceilometer at the Nashville airport, Linville began noticing birds begin to gather in and around the spotlight’s intense beam. As he raised his eyes upwards, he saw scores of birds tumbling towards the ground. Some of the songbirds that crashed to earth were merely stunned, but most of them fell to earth dead. Linville watched this confusing and tragic spectacle for more than an hour, but as soon as larger birds like grebes and bitterns began smacking into the tarmac around him, he quickly took shelter.¹

The next morning, Linville gathered up all 248 dead birds he found scattered around the ceilometer and took them to the Nashville Children’s Museum for the resident naturalist to study. When they were reported in an ornithological journal soon after, these “apparently singular circumstances” caught the attention of the scientific community. People had known for more than a hundred years that migrating birds, sometimes by the hundreds, could be killed by flying into tall buildings, especially when they were brightly illuminated. But birds being killed by a spotlight shining into thin air? This was new. 

For two years, it seemed as if the incident at Nashville was unique, one of nature’s rogue and unfortunate mysteries. But on September 17, 1950, a ceilometer in Roanoke, Virginia killed 165 birds in much the same way as it had done in Nashville. Just three days after that, an airport in Long Island found 15 dead birds around their ceilometer. In 1951, bird deaths started piling up. The Nashville airport saw another 476 birds killed on October 7, and the same day the ceilometer in Smyrna, Tennessee killed over a thousand birds. The following night, 1,044 birds were killed at the Knoxville airport and a hundred were found dead in Louisville.² 

With each of these incidents, it was becoming more clear that bird death around ceilometers wasn’t a freak accident but a predictable tragedy. There were not always scientists on hand to record the birds’ death, but airport workers had gotten familiar with the carnage. One clean-up crew later told an ornithologist that they had “half-filled a 55 gallon drum with birds ten days before, and that it was usual to pick up several handfuls.”³

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Every fall, billions of birds sweep down from their breeding grounds in the north toward their wintering grounds in the south. Most of these birds do their migrating at night, flying dozens or hundreds of miles non-stop before landing to rest and refuel during the day. Incredibly, birds are able to travel thousands of miles and still return to the same patch of field or forest year after year. It’s still not fully understood how they do it, but we now know that they navigate using the earth’s magnetic field and natural light from the stars.

Possibly because they navigate by starlight, many birds’ eyes and brains are vulnerable to artificial light. As they migrate at night high overhead, they can be pulled off course by lights in cities below, and driven to crash into illuminated windows or monuments. 

Evidently, they could also become almost trapped by vertical beams of light. When an object passes through a ceilometer’s beam, it immediately shines a brilliant blue-white. One glowing bird, fluttering frantically in the beam, seemed to draw more birds to the light, who would themselves draw more birds in. These confused and careening birds would circle lower and lower until they’d crash into a building, crash into the ground, crash into each other, or simply drop from exhaustion. 

Ceilometers kept shining their bright beams skyward every day and every night at most weather stations and airports across the country. And most of the time, these beams did not kill birds. Lights were only a threat at night, and only then, during fall migration — birds seemed to be much less affected flying north in the spring. But on fall nights with a cold front and overcast skies, ceilometers could kill dozens, hundreds, sometimes thousands of migrating birds. 

The true magnitude of the threat that ceilometers posed finally became clear on October 8, 1954. That night, a huge flock of migrating birds had flown over the Warner Robins Air Force Base, south of Macon, Georgia, and gotten caught in the beam. Workers arrived at the base the next morning to find that “dead birds were strewn by the hundreds over the runways, taxi strips, grassy plots, and tops of buildings.” A pair of ornithologists rushed to the base to survey the damage, and found that on the roof of just one building, workers had shoveled and raked together 2,552 birds, representing 53 species. In total, they estimated that more than 50,000 birds were killed at the airport that night.⁴

A tragedy on that scale got national attention. It was no longer just a phenomenon to study, but a threat to conservation that needed to be addressed, and ornithologists, meteorologists, and engineers got to work on a solution. Within a year, a Tennessee naturalist named John Terres had developed a filter that could be placed over a ceilometer’s beam to block out all but blue light, leaving the beams almost imperceptible to birds.⁵ These filters were first installed at Knoxville and Nashville, and the Army Times reported that “bird lovers are making plans for obtaining more filters for other Weather Bureau stations.” For its part, the National Weather Bureau instructed its stations to turn off unfiltered ceilometer beams when birds were in flight as long as it didn’t endanger air travel, while the Air Force made plans to install filters at each of their bases.⁶

These solutions were simple, and operators who put them to the test immediately saw how effective they were. A meteorologist at Berry Field in Nashville saw birds concentrating in a ceilometer beam at 2AM one September night, and as soon as he placed a filter over the light the birds immediately dispersed. At the Duluth airport, a ceilometer beam killed 82 birds in a half hour, but by turning off the light the operator prevented further casualties.⁷ Within a year of the first filter’s installation the US Secretary of the Interior released a statement, writing that “the toll at ceilometer stations in recent years has been alarming … but with all of us working together we think we have the big problem around ceilometer stations pretty well licked.”⁸

Once the problem and the solution became well known, bird deaths became completely preventable. At airports and weather stations where filters were installed — at a low cost and with little effort — bird deaths ceased entirely. Even where this investment was not made, meteorologists who knew of the problem and cared enough to address it turned off the spotlight when birds passed through or got caught in the beams, saving even more lives.

Yet as simple and inexpensive as these solutions were, not every airport and weather station adopted them. And predictably, tragedies continued to strike. In Albany, New York in September 1956, 313 birds were killed. In Portland, Maine in September 1958, 200 birds were killed. In Madison, Wisconsin in October 1959, 117 birds were killed. At Laughlin Air Base in Texas in September 1960, over 6,000 birds were killed. In Duluth, Minnesota in 1962, at least 1,000 birds were killed.

It wasn’t until a newer model of ceilometer that used rotating beams was rolled out (which “doesn’t seem to interest these southbound vacationeers”, according to the Montgomery County Sentinel) that ceilometers entirely ceased to be a threat to birds. These new machines were not adopted because they were bird-friendly, but rather because the old models had simply grown outdated.

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I’m not sure whether to be heartened or discouraged by this story. After the threat that ceilometers posed to birds attracted national attention, it took less than a year for a solution to be developed that protected birds without compromising the safety of pilots. The Air Force, the Department of Interior, and the National Weather Service all cared enough about birds’ welfare to push for the installation of filters, while the ordinary folks working at weather stations went out of their way to install filters or turn off lights once they became aware that doing so could save birds’ lives.

At the same time, many weather stations didn’t adopt the solutions. For more than a decade, ceilometers kept killing birds, even though extremely simple and cheap solutions were available. No laws were passed to require the use of filters, so out of ignorance or indifference many stations never updated their practices. It wasn’t behavior change that finally ended the threat of ceilometers to birds, but obsolescence. Eventually, rotating-beam or laser ceilometers replaced the machines that relied on visible light. Goodwill can get us a long way toward protecting wildlife, but it can’t get us all the way.

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For the next several decades, we did not have an occasion to regularly shine incredibly bright spotlights upwards during heavy migration periods, undoubtedly to birds’ relief. But after September 11, 2001, we found a reason to put those spotlights back to use. On each anniversary of the tragedy, the city of New York shines spotlights skywards where the Twin Towers once stood. And just like they got caught in ceilometer beams decades ago, today’s migrating birds are caught and killed by the memorial spotlights. But the lessons from ceilometers have not been totally forgotten. Volunteers wait under the lights and watch for birds to get caught in the beams. If they see that enough are being trapped, the spotlights are shut off for twenty minutes to give the birds a chance to disperse. It’s not a perfect system. Some birds are still killed by the memorial lights, though not nearly as many as if they had no opportunity for escape. In a world that we insist on shaping to our needs and whims, we’re still figuring out how to live alongside birds. 

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1. Spofford, Walter R. 1949. “Mortality of Birds at the Ceilometer of the Nashville Airport.” The Wilson bulletin 61(2), 86–90. ↩︎
2. Avery, Michael L.., Springer, Paul F.., Dailey, Nancy S.. Avian Mortality at Man-made Structures, an Annotated Bibliography. United States: Fish and Wildlife Service, U.S. Department of the Interior, 1978. ↩︎
3. Howell, Joseph C., Amelia R. Laskey, and James T. Tanner. “Bird Mortality at Airport Ceilometers.” The Wilson Bulletin 66, no. 3 (1954): 207–15. ↩︎
4. David W. Johnston, T. P. Haines, Analysis of Mass Bird Mortality in October, 1954, The Auk, Volume 74, Issue 4, 1 October 1957, Pages 447–458, https://doi.org/10.2307/4081744 ↩︎
5. Time. “Science: Birds in Trouble.” Time Magazine. October 31, 1955. ↩︎
6. “‘Black Light’ Makes Bird Migration Safer.” Army Times  December 10, 1955: Vol 16 Iss 18. ↩︎
7. Birds in Our Lives. United States: Edited, Stefferud, Alfred. U.S. Government Printing Office, 1966. ↩︎
8. “‘Black Light’ Makes Bird Migration Safer.” Army Times  December 10, 1955: Vol 16 Iss 18. ↩︎