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SCIENCE science.org 22 APRIL 2022 • VOL 376 ISSUE 6591 341

PHOTOS: MELISSA BREYER; (OPPOSITE PAGE LAWRENCE LUCIER/FILMMAGIC VIA GETTY

E

very 11 September at dusk, in mem-

ory of the 2001 attacks, New York

City mounts the Tribute in Light,

an art installation in lower Manhat-

tan. And every year, as twin tow-

ers of light bloom skyward, they

attract thousands of migrating birds,

sucking in warblers, seabirds, and

thrushes—along with predators such

as peregrine falcons eager to take advantage

of the confusion. On each anniversary, bird

conservationists wait below, counting and

listening to disoriented chirps. If the observ-

ers report too many birds circling aimlessly

in the beams, organizers flip off the lights.

In recent years, on-site observers have also

used a complementary tool to quantify the or-

biting birds: weather radar, which bounces off

birds as well as raindrops. In 2017,

a group led by Cornell University

ornithologist Andrew Farnsworth

found that during seven previ-

ous anniversaries, the once-a-year

installation had attracted a total

of about 1.1 million birds. Within

20 minutes of lighting up, up to

16,000 birds crammed themselves

into a half-kilometer radius. But

when the lights flicked off, the

dense clouds of birds on the radar

screen dissipated just as fast, a

finding later confirmed by on-site

thermal cameras.

“That was really illuminating—

pun not intended,” says that

study’s lead author, Cornell eco-

logist Benjamin van Doren. “It re-

ally gives you a sense of the scale

that light can impact bird migra-

tion.” The circling birds burn

through time and precious body fat, are easy

prey, and worst of all, can brain themselves

on the windows of nearby buildings.

The finding came as scientists grew in-

creasingly worried about slumping bird

numbers. The skies above North America

host some 3 billion fewer birds today than

in 1970, according to one 2019 analysis. The

flocks have faced death by a thousand cuts,

including not just light pollution, but climate

change, vanishing habitat, and pesticides.

Ornithologists fear each added insult could

be enough to bend once-abundant bird pop-

ulations toward extinction (Science, 20 Sep-

tember 2019, p. 1228).

The radar studies at the Tribute in Light

helped lay the groundwork for a tool that

could ease the toll: a program Farnsworth’s

team calls BirdCast, which incorporates

continent-scale weather radar and machine

learning to forecast the exact nights when

hundreds of millions of migratory birds will

torrent over U.S. cities. The team then feeds

those findings to conservationists and policy-

makers desperate to help the birds survive

the journey by dimming lights along the way.

Such research, paired with existing con-

servation efforts, is beginning to make a dif-

ference: New York City recently passed an

ordinance requiring city buildings to turn

down lights during migration season, the

world’s largest city to take such a step. It joins

dozens of growing campaigns across the

United States that aim to save tens or even

hundreds of thousands of birds per city each

year. “Given declines over the last 50 years,

and what’s happening on the planet, every-

body needs a win,” Farnsworth says.

LONG BEFORE THOMAS EDISON turned on a

light bulb, people were recording anecdotes

of nighttime lights ensnaring birds, espe-

cially during migration. In 1880, for exam-

ple, the Bulletin of the Nuttall Ornithological

Club queried lighthouse keepers across the

United States; one respondent, in Florida, re-

ported shielding his face with his hat against

an onslaught of “more than a million” birds

heading south. He carried away two bushels

of crumpled bodies the next morning.

Today’s researchers keep more detailed

records. In New York City, for example, vol-

unteer crews patrol city sidewalks morning

after morning during migration seasons, cat-

aloging birds that thwacked into buildings.

Among them are songbirds, light as quarters,

that are adapted to every part of their yearly

trek between the Amazon and Canada’s bo-

real forests—except for human cities.

“It’s really bad to see injured birds that you

know are about to die,” says Kaitlyn Parkins,

a science consultant for New York City Audu-

bon, which has tracked fallen birds since

1997. “I just—I can’t internalize it anymore.”
      Some 3.5 billion birds fly through the
      southern border area of the contiguous
      United States each spring, then head north
      through the center of the country, Cornell
      ecologist Adriaan Dokter found in a 2018
      study using weather radar. Come fall, after
      breeding season, roughly 4.7 billion birds
      travel south, this time along routes that clus-
      ter more toward the eastern United States.
      Collisions with buildings take out anywhere
      from 365 million to 1 billion of them each
      year, according to one landmark meta-
      analysis of papers and bird collision data sets
      published in 2014. That study put buildings
      behind only domestic cats on the list of top
      anthropogenic bird killers.
      As biologist Pete Marra at Georgetown
      University puts it, big glass build-
      ings might be more “efficient from
      an energy perspective [but] are
      extremely efficient from a killing
      perspective.”
      In the daytime, reflective glass
      can trick birds into flying into
      what seems like clear, open space,
      which often ends in death by
      brain hemorrhage. Overnight, the
      chief killer is artificial light, which
      harms birds in several steps. First,
      when the cumulative glow of light
      above cities is bright enough to
      outshine the Milky Way, it sucks
      in passing migrants from up to
      200 kilometers away, a 2018 study
      using weather radar found. Once
      in a city, birds are subject to de-
      graded habitats and are prey for
      housecats, and individual light
      sources can trap them in pointless,
      calorie-wasting circles or tempt them head-
      long into windows. Exposure to light at night
      has also been linked to disruption of birds’
      immune systems, microbiomes, foraging be-
      havior, and of course sleep cycles.
      To this day, researchers don’t know why
      so many birds find artificial light so alluring.
      Insect researchers face the same puzzle, de-
      bating why, exactly, moths and other insects
      are drawn to flames and streetlights (Science,
      7 May 2021, p. 556). “If I could answer that
      question, I’d be happy,” says Kyle Horton,
      a BirdCast team member now at Colorado
      State University, Fort Collins. He speculates
      only that the impulse to fly to light must have
      once conferred an evolutionary advantage.

WHATEVER THE REASON light lures birds,

Farnsworth has toiled since the 1990s to

keep them safe from it by tracking and pre-

dicting their movements. He aimed to map

migration not just as numbers of birds cross-

ing a particular field site, but as flows of bio-

mass operating on regional, even continental

These warblers and other birds died at the World Trade Center in

New York City on 20 September 2021.

The Tribute in Lights installation in New York City

lures thousands of migratory birds (bright specks)

into twin towers of light.