Light Pollution and the Greatest Natural Spectacle on the Planet
“The sea-bird wheeling round it,
With the din of wings and winds and solitary cries,
Blinded and maddened by the light within,
Dashes himself against the glare, and dies.”
The skyline of Denver, Colorado is a beautiful sight. But to birds in the midst of their spring migration it can be a deadly attraction. Photo: Frank Turina
Every spring and fall, as you sleep, a truly remarkable event takes place in the skies above you. Across the United States billions of migrating birds are in the air. Tonight, as I write these words, nearly 300 million migrating birds are on the move. Similar numbers have filled the skies every night for the past month or more. On the last night of April, nearly 12 million birds migrated over Colorado and Utah alone. April and May are the heart of the annual spring migration when birds of all sizes, shapes, and colors work their way to their summer breeding grounds. They leave behind places like Mexico, Costa Rica, northern South America, Argentina, and Chile. Some even cross the Atlantic from sub-Saharan Africa to make it to the US and Canada. The distances they cover would exhaust even the most seasoned traveler. One population of Northern Wheatear that winters in eastern Africa flies north across Asia then east over Siberia to its breeding grounds in western Canada, a journey that covers roughly 15,000 km. However, the clear winner of the longest migration contest is the arctic tern. Every year, arctic terns migrate from the Antarctic Circle to the Arctic Circle—a round-trip journey of about 30,000 kilometers (18,641 miles). An impressive feat for a bird that weighs about as much as a deck of cards.
Not long ago, the seasonal disappearance of some bird species from northern environs was a mystery. Until the early 1900s, it was commonly speculated by some scientists that birds hibernated, an idea that may have originated in ancient Greece. Although migration of some birds was observed an understood by the Greeks, the hibernation hypothesis was supported by none other than Aristotle. In his Historia Animalium in the fourth century B.C, Aristotle wrote that “they do not all migrate, as is generally supposed, to warmer countries... a great number of birds also go into hiding… Swallows, for instance, have been often found in holes, quite denuded of their feathers."[1] This idea of hibernating birds (especially Swallows) persisted for millennia. Even renowned 18th century naturalist Gilbert Whiter, who was an early adopter of the migration theory, struggled to let go of the idea of hibernating swallows. He wrote,
“I am more & more induced to believe that many of the swallow kind do not depart from this island; but lay themselves up in holes & caverns; & do, insect-like and bat-like, come forth at mild times, & then retire again to their latebrae.”
However, Whiter was also a pioneer in the study of migratory behavior and paved the way for subsequent banding studies by Audubon and Hans Christian Mortensen. These studies confirmed avian migration and deepened our understanding of this massive, semi-annual spectacle. Today, we can use technology such as weather radar , GPS tracking data, and computer models to monitor the numbers and densities of birds on their pilgrimage. These technologies reveal the unimaginable scale of these events.
Weather radar in Key West USA captured an impressive display of migratory birds. The video shows the birds (biological targets) in green and yellow flying north over the Keys. Rain and showers are depicted in darker blues.
This video shows the recorded movements of 1,654 individual birds that were tracked between 1992 and 2012. The data represent 58 species, over 2 million locations, and 276,800 tracking days. The movements are shown over the course of a single year—see the date in the lower left corner. For more information contact support@movebank.org.
This model of Barn Swallow migration uses eBird data to show where the species may be found at any time of the year. https://cals.cornell.edu/news/ornitho...
Migration is fraught with peril. Flying tens of thousand miles when you weigh just a few ounces is dangerous business. For one thing, these flying powerhouses have to put on some serious weight before their journey, often doubling their mass. Their intestines and digestive organs shrink while their heart, lung, and leg muscles double in size. They also need to time their arrival perfectly. For birds that breed in the arctic, the summer is mercilessly short. Show up a week or two late and they lose the fierce competition for resources. That means less food, reduced reproductive success, and higher mortality. If they don’t establish some territory, find a mate, lay eggs, and raise their chicks in time, the young birds won’t be ready for migration in the rapidly approaching fall. Instead of one long direct flight, most birds tend to hopscotch their way to their breeding grounds, pausing in food-dense regions to refuel and rest. These nutritious stop-over sites are critical for migratory success. The activity at these stops is frenetic; insatiable birds furiously eating as much as they can, as fast as they can, to put on as much weight as they can, to power them through the remainder of their trip. Along the way, they face a host of life-threatening obstacles. Predators, storms, extreme weather, dehydration, and exhaustion are constant risks. Millions of birds won’t survive. To increase their chances, most species migrate at night when the air is cooler, calmer, and there are less predators on the prowl. But flying at night presents a host of other threats, many of which stem from one underlying problem: light pollution.
During their flight, birds are attracted to lights from cities and towns on the horizon. These lights disorient and divert birds from their migratory route, drawing them in like moths to a flame. Once these birds arrive in brightly lit urban areas, they are often reluctant or unable to leave. They become trapped where food and other resources are scarce. There, they also face a variety of threats from human development. Chief among those threats is collisions with buildings. It’s estimated that up to one billion birds die from building collisions every year. The injuries sustained from window collisions are often horrific. The most common external injuries are broken bills. Evidence of internal injury usually includes blood and fluid leaking from the mouth and nostrils. Internal examinations revealed that nearly every fatality sustains some bleeding in the brain. Those that survive the initial collision are often knocked unconscious or stunned. Shaken, many eventually fly off where they later succumb to their injuries or fall victim to predators.
Birds killed in collision with American National Insurance building in Galveston, Texas in 2017 Photo: National Audubon Society
The relationship between light and bird collisions has been known for a long time. In 1918, Walter Squires and Harold Hanson published “The Destruction of Birds at the Lighthouses on the Coast of California.” In this groundbreaking work, they referred to a lighthouse in Mendocino County that saw at least 1000 collisions per month during migration season. They also referenced reports of birds perishing in much greater numbers at lighthouses in Europe, the Great Lakes, and the Atlantic Coast. The scale of destruction from collisions can be shocking. On September 12, 1937, 576 birds slammed into the Washington Monument in the space of 1.5 hours just before midnight. More recently, nearly 400 birds were killed in a single night in 2017 when they collided with the 23-story American National Insurance building in Galveston, Texas.
But perhaps the most notorious building with respect to bird collisions is McCormick Place in Chicago, a glass-sided convention center along the shores of Lake Michigan. In 1978, ornithologist David Willard received a tip from a colleague about birds hitting windows at the sprawling complex. Willard investigated and found a few birds at the base of the structure. That started a forty-year study of bird collisions at the site. From 1978 to 2016, scientists from the Chicago Field Museum walked around the facility during spring and fall migration seasons, gathering casualties and sealing them in plastic bags for the museum’s collection. Over the years, the team collected over 30,000 specimens of dead birds that had crashed into the glass facade of the building, a small portion of the overall death toll. Subsequent studies of collisions pointed to lighting in and around the structure as the root cause. While some research identified other contributing factors, after 40 years of studying the issue, Willard understands the primary cause: “I’m pretty convinced that it’s almost 100% about lights.” The good news is that in recent years both the American National Insurance building and the McCormick Center have improved their lighting and the number of collisions has plummeted.
McCormick Place and dead birds collected by the Chicago Field Museum Photos: Birds - Roger Hart/University of Michigan (Birds); McCormack Place - Getty Images, Retrieved from https://www.audacy.com/wbbm780
How birds navigate long distances is still a bit of a mystery. There are several factors that seem to play a role, including an internal magnetic “compass”, an uncanny sense of smell, and the stars. In 1967, Cornell University scientist Stephen Emlen used Indigo Buntings to show that the night sky is an important factor for maintaining direction during long migrations. Emlen raised bunting chicks under the dome of a planetarium and varied the point around which the stars rotated throughout the night. Under natural conditions, this point of rotation is the North Star. In the experiment, some chicks were raised under a sky that rotated around the star Betelgeuse. When Emlen released them in a special cage within the planetarium, the birds headed in a direction completely opposite of Betelgeuse. The research demonstrated that young birds learn a north-south orientation from the rotational patterns in the night sky. They learn direction by watching the stars. Light pollution makes this task more difficult.
As light pollution increases, more and more stars vanish under a milky haze. Scientists estimate that the Milky Way is no longer visible to fully one-third of humanity, including 60 percent of Europeans and 80 percent of Americans. In Australia, the Southern Cross, arguably the most well-known constellation in the southern hemisphere, is disappearing. Epsilon Crucis, the fifth star in the Southern Cross can no longer be seen, and Delta Crucis, the righthand star is barely visible in the skies over major metropolitan areas such as Sydney. In the worst polluted cities across the globe, only a handful of the brightest stars and planets are visible. In most urban areas, fewer than 100 stars can be seen, and the degradation of the night sky and the loss of stars extends far from urban centers. It’s much more difficult to learn from the stars when the heavens are disappearing.
Light pollution is one of many stressors affecting migratory birds. In addition to the dangers associated with stray light, birds are facing habitat loss in breeding, wintering, and stop over sites. Climate change is also creating a new migratory landscape. Spring and summer are arriving earlier and mild weather is creeping further into the fall and winter. Trees, flowers, and other plants necessary for survival are blooming earlier. Insects, a major food source for many species, are hatching earlier in the spring. These environmental changes are occurring quickly and many species are struggling to keep up. In the past five decades, bird populations have plummeted by nearly 30 percent, a loss of nearly three billion across North America alone.
We can re-claim the stars and save countless birds from a gruesome death at the foot of our buildings with a few simple actions. The solutions to light pollution are not technically challenging and they’re not cost prohibitive. To address the issue, the Audubon Society has created the “Lights Out” program. The program works by convincing building owners and managers to turn off excess lighting during the spring and fall migrations. That’s all it takes! This year, Lights Out programs are happening in dozens of cities across the U.S.
Broad-Tailed Hummingbird at the feeder Photo: Frank Turina
On a cool cloudless morning last week, I retrieved our hummingbird feeders from the storage bin in the pantry, filled them, and hung them on our deck. I hadn’t seen a hummer yet, but I knew they should be arriving any day. As I was hanging the feeder closest to the house, a big Broad-Tailed zipped right past my head. He was making a commotion, apparently annoyed at my indolence when it came to putting out the feeders. I picked up the iridescent flash of color as the morning sun glinted off of its head, chin, and back. This was probably the same bird that drank from my feeder last summer, finally home after his winter in Mexico or Guatemala. Although some species like the Ruby Throated will fly non-stop, day and night, over the Gulf of Mexico, hummingbirds migrate primarily during the day. However, even these daytime migrators are affected by light pollution. Hummingbirds have been known to feed well into the night in the vicinity of outdoor lights. Under normal conditions, Hummingbirds sleep during the night and often go into a state of torpor in which their body temperature and metabolic activity drops dramatically. The unnatural feeding behavior encouraged by outdoor lighting interrupts their sleep patterns and places additional stress on the birds.
Scott Weidensaul, prominent ornithologist and author of A World On the Wing, believes “If you could strip away the night sky, and make [nocturnal migration] visible, it would be the largest, greatest natural spectacle on the planet.” He also believes that “the one area that probably has the greatest implications for bird conservation in North America is our understanding of how urban lights are changing bird migration.”
Simple steps are all that is needed to reduce bird mortality during migration and help populations rebound. During migration season turn off any nonessential outdoor and indoor lights; shield exterior fixtures to eliminate horizontal glare and any upward light; install automatic motion sensors, timers, and controls wherever possible; when lights are used, reduce the brightness to the lowest level possible; and install lights with warmer tones (lower CCT). These measures will help ensure that the greatest natural spectacle on Earth continues unabated for current and future generations.
[1] From Historia Animalium: “Swallows, for instance, have been often found in holes, quite denuded of their feathers, and the kite on its first emergence from torpidity has been seen to fly from some such hiding-place… the stork, the owzel, the turtle-dove, and the lark, all go into hiding. The case of the turtledove is the most notorious of all, for we would defy any one to assert that he had anywhere seen a turtle-dove in winter-time; at the beginning of the hiding time it is exceedingly plump, and during this period it moults, but retains its plumpness. Some cushats hide... The thrush and the starling hide; and of birds with crooked talons the kite and the owl hide for a few days.”