Each year when night falls on Sept. 11 in New York City, twin pillars of sheer radiance powered by 88 high-powered searchlights are blasted into the heavens near where the World Trade Center once stood.

On a clear evening, the cloud-brushing vertical beams — a heart-stirringly emblematic annual art installation known as the Tribute in Light — can be seen from as far as 60 miles away from the site in lower Manhattan.

And on some — but not all — of these nights, hundreds of disoriented birds become trapped within those beams, swirling and swirling in a blinding vortex until they can swirl no more.

Migratory birds get stuck in the Tribute of Light, endlessly circling in the pillars of light. (Photo: Kai Schreiber [CC BY-SA 2.0]/Flickr)

Known as fatal light attraction, this phenomenon occurs when the internal navigating systems of birds — primarily migrating birds making their way from the north to the more temperate climes of Mexico, Central America and South America for the winter — are thrown off by artificial light sources. Like insects swarming a porch light during a particularly buggy summer night, the birds, which are normally guided by the moon and the stars, are lured from their established paths and into the twin beams, at which point they exhaust collide into nearby buildings or exhaust themselves to a point where they can no longer go on.

The Tribute in Light is a rather dramatic example of artificial light causing ill-fated migrating birds to veer off-course. The reality is, this can occur on any night and in any city located along a migration flyway system. But because the Tribute in Light is so big, so powerful and so potentially deadly, it has helped researchers better understand why fatal light attraction occurs — and how it can be prevented. And perhaps most important, it has influenced other cities beyond the Big Apple to flip the switch on bird-disorienting lights during peak migration season.

Reducing the fatal impact of a stunning once-a-night sight

In an op-ed for The New York Times, Andrew Farnsworth and Kyle Horton, both scientists at the Cornell Lab of Ornithology, describe what happens on the ground each Sept 11. to "avert disaster" and minimize the bird-disorienting impact of the Tribute in Light:

New York City Audubon has positioned trained volunteers armed with binoculars on the roof of a parking garage in Battery Park City, at the base of the tribute, to monitor aggregations of birds in the tribute light beams. If densities exceed more than 1,000 birds or if a bird is found dead, the lights are shut down to allow the birds to disperse.

Horton and Farnsworth go onto explain that for several years after the phenomenon was first observed at the Tribute in Light, there was a need to turn off the beams due to adverse weather conditions that kept migrating birds grounded. On Sept. 11, 2010, however, the lights were extinguished five times over the course of the evening. The Tribute in Light was temporarily snuffed out on five of the following seven years. In 2015, the beams were turned off a record nine times over the course of the evening. And the lights never go dark for that long. According to Audubon, shutting them down for just 20 or 30 minutes at a time greatly reduces bird density in the immediate area.

Only two birds have reportedly died since this monitoring practice began.

As one might suspect, the one-night-only Tribute in Light isn't the only colossal illuminated bird magnet jutting high into the New York skyline. Nationwide, skyscrapers are a huge source of bird fatalities — and NYC has a huge number of skyscrapers. (An estimated 90,000 birds die annually due to collisions with New York City buildings.)

New York Gov. Andrew Cuomo announced in 2015 that the state would adopt the Audubon Society's Lights Out initiative, a program already established in several cities nationwide and in a small number of states. As part of the mandatory scheme, all state-owned or state-managed buildings are required to turn off any non-essential exterior lights from 11 p.m. through dawn during peak migration season: April 15 through May 31 and then again from Aug. 15 through Nov. 15.

And on a citywide basis, NYC Audubon has worked with the owners of iconic, non-state-owned buildings like the Chrysler Building to reduce their deadly impact during migration season. In fact, the Lights Out NYC program was established in 2005, predating the state initiative by 10 years.

The Gateway Arch goes dark

The spotlights illuminating the Gateway Arch in St. Louis go dark for a total of four weeks each year so they don't disorient birds traveling along the Mississippi Flyway. (Photo: Matthew Black [CC BY-SA 2.0]/Flickr)

While New York City's Lights Out efforts and the monitoring activities at the Tribute in Light site have been around a good while (and attracted plenty of national attention), the organized push to protect migrating birds from artificial urban light began in 1999 in another big city laden with tall buildings: Chicago. (Toronto's FLAP program, however, predates Audubon's stateside efforts by six years.)

In the years since, local Audubon chapters and partnering organizations have launched Lights Out programs in cities from coast to coast including San Francisco, Detroit, Indianapolis, Baltimore, Boston, Minneapolis/St.Paul, Milwaukee, Portland, Oregon and Charlotte, North Carolina.

And while some cities located along flyways may not have official Lights Out programs, the owners and operators of individual landmark structures have taken it upon themselves to go dark during migration season.

A notable example is the Gateway Arch in St. Louis, which is tall, brightly illuminated and located directly on the Mississippi Flyway. The Gateway Arch, which was treated to quite the revamp earlier this year, first temporarily turned off its lights during migration season in 2001. It's now become a biannual tradition — the soaring monument's upward-facing spotlights are darkened for two weeks each during May and September to help guarantee that the over 300 North American bird species traveling along the flyway have a safer journey.

"We've often been asked, ‘Why do you bother when you're in a big city that's throwing off all this light?'" Gateway Arch National Park Deputy Superintendent Frank Mares recently told St. Louis Public Radio. "It's because the Arch is possibly the tallest thing any bird will come upon, right on the river."

Over the summer, a $1.2 million overhaul of the Gateway Arch's exterior lighting system was completed. While they'll still be turned off completely for a spell in May and September as is now custom, the new lights are less disorienting to birds than the old ones — just in case.

"The lights are brighter, but there's much less light overspray than there used to be," Mares explains. "There's less light pollution above and around the Arch that could disorient a night-migrating bird."

Houston taps into migration forecasting tool

Houston is one of several cities that have adopted a Lights Out program that helps migrating birds better navigate the nighttime urban sky. (Photo: Katie Haugland Bowen [CC BY 2.0]/Flickr)

Spurred largely in part by a tragic event that occurred in the spring of 2017 when an unprecedented 400 migrating birds fatally collided into a brightly lit high-rise building on a single night, Houston is one of the newest cities to implement a Lights Out program. (The event in question took place at the 23-story One Moody Plaza in neighboring Galveston, which falls under the auspices of Houston Audubon.)

The sprawling Bayou City, located along the Central Flyway, is one of the top five American cities at risk for a high number of bird collisions alongside Chicago, Atlanta, Dallas and New York. This particular stretch of the Gulf Coast is also a veritable bonanza for birdwatchers.

Lights Out Houston includes a notification system for building owners that's based on BirdCast, a migration forecasting and tracking tool by the Cornell Lab of Ornithology. Wildly popular with recreational birders, BirdCast, as it turns out, also serves an even greater purpose: it can help save the lives of birds.

Essentially, participating organizations and individuals can receive alerts when observational data and meteorological conditions predict more intense-than-normal migrational activity in the night sky. This way, building owners know well in advance to flip off the lights, if they haven't already. As Audubon magazine writes, BirdCast can "reliably predict" migration timing up to three days in advance.

A summer tanager, a migratory songbird, pictured in Galveston, Texas. The Gulf Coast resort city was home to a notably deadly bird strike in 2017. (Photo: Greg Schechter [CC BY 2.0]/Flickr)

"This is not just me prognosticating, looking at tea leaves or something," Richard Gibbons, conservation director of Audubon Houston, tells the magazine. "This is based on science."

Interestingly, temperature plays a more significant role during the spring months in predicting which nights will be particularly "busy." And in the fall, there tends to be more young avian travelers in the mix, making it the more deadly migration season for birds. "There may be some learning here," Horton, the Cornell scientist, tells Audubon. "Young birds may be skewed in terms of their attraction to light."

Writing for the Houston Chronicle, Gibbons and his colleague Sarah Flournoy, community programs manager with Audubon Houston, detail why BirdCast is so crucial when protecting avian vagabonds passing through brightly lit urban areas:

Fortunately, the Cornell Lab of Ornithology's BirdCast program that predicts how intense migration will be in a given area launched new tools to support these notifications. If building managers and residents across Houston can turn off lights during migration or design lighting with wildlife in mind, we can transform this threat to birds into rallying recognition that Houston takes pride in its special role anchoring the Central Flyway along the Gulf of Mexico. Practically, it would also save quite a bit of energy.

Audubon emphasizes that although Houston's BirdCast-based alert system is unique, anyone anywhere — including "owners of large lit-up buildings or stadiums that attract and kill migrating birds" — can go online and view the tool's ultra-precise forecast data and then, ideally, take action.

"The more groups, chapters, bird clubs that can help build a groundswell of awareness, the more likely we are to have collective success," says Gibbons.

As for what individual homeowners can do, Audubon Portland has a helpful list of tips on how to reduce bird collisions that goes well beyond the simple but impactful act of turning off unnecessary exterior lights from dusk till dawn during migration season.