Bird Migrations: Timeless and Threatened 

Across the setting sun, migrating birds fly high in the sky toward a distant destination. It’s an iconic image we’re all familiar with, heralding the arrival of autumn. Many well-known species of birds migrate night and day between August and November, including hawks, geese, thrushes, warblers and sparrows. As these birds journey through the darkness, however, they often face unexpected, dangerous and unseen challenges — distracting bright lights, invisible glass windows and human-made buildings.  

Since the launch of Lights Out Philly, the Philadelphia community has come together in a remarkable way to help protect our feathered friends in flight. The program — a coalition led by the Academy that includes Delaware Valley Ornithological Club, Audubon Mid-Atlantic and two local Audubon Society chapters, Valley Forge and Wyncote — now has dozens of commercial buildings in center city flicking switches off at night for these migrating birds, making their long-distance travels a little safer. But there’s still more work to do.

The Academy reached out to Scott Weidensaul, New York Times bestselling author of A World on the Wing: The Global Odyssey of Migratory Birds and winner of the Academy’s Richard Hopper Day Memorial Medal for his lifelong contributions to interpreting and communicating natural science, to learn more about the true importance of a migrating bird’s marvelous journey and what we can do to help. 

Canada goose, Branta canadensis © A. Morris/VIREO

Tell us more about the real wonders of bird migration. 

At a global level, bird migration is utterly ubiquitous, continuous and universal; at any given moment, day or night, year-round, there are millions of birds in migration somewhere in the planet’s skies. At certain times and places, the numbers are astronomical — on a single night during the peak of spring migration, there may be three-quarters of a billion birds migrating across the Lower 48. Most of them migrate at night, even species that are normally active by day, because the night sky is cooler, more humid and less turbulent. 
Birds migrate because the resources they need vary with the seasons, and migrations, be they short or epic, allow them to take advantage of the best a particular place has to offer. Because most of 10,500 or so species of birds live in the tropics, the majority of birds at a global scale don’t migrate (though we’re finding more and more evidence of seasonal movements even there, for example following seasonal rains and fruiting patterns). But here in North America, about 500 of the 800 or so species of birds are migratory to some degree. 
Some birds make extraordinary long-distance migrations where to falter is to die, like bar-tailed godwits flying more than 8,000 miles, over the course of up to 11 days, from Alaska to New Zealand and Australia. About two billion songbirds, some as small as ruby-throated hummingbirds, each spring fly north 600 miles across the Gulf of Mexico — nonstop, obviously. But those migrating across land more usually break up their periods of active migration with days of rest and refueling, known as stopover periods. 

Bar-tailed godwit, Limosa lapponica © B. Chudleigh/VIREO

How are these bird migrations impacted by human action? 

In 2019, a team of the top ornithologists in North America, using an array of long-term monitoring data (including decades of Doppler weather radar, which is so sensitive it can allow us to calculate with remarkable precision just how many birds are flying through the night skies at any given moment), found that North America has lost about a third of its birds since 1970 — almost 3 billion fewer individual birds today than then. 
That loss is the result of a bewildering variety of human pressures, from habitat loss and environmental toxins such as pesticides to free-running cats, window and building collisions, and now climate change, which is altering the abundance and timing of seasonal food resources, intensifying wildfires and droughts and changing the wind and weather patterns on which the migrants depend. All of these are on top of, and in addition to, the many natural hazards that birds face from storms, exhaustion and other dangers. 
The ripple effects through natural systems can be profound, because birds provide countless ecosystem services and functions, from eating insects, to pollinating plants (not just hummingbirds — many of our “insect-eating” songbirds feed heavily on nectar while wintering in the tropics) to spreading the seeds of the fruits they eat. Sometimes the impact on us is economic, like the studies that have shown birds provide pest-control services worth thousands of dollars per acre in hardwood forests. And for people like me, the notion of spring without the return of colorful, vocal songbirds is too depressing to contemplate. 

Red knot, Calidris canutus © D. Tipling/VIREO

What can we do to help?  

 Between 1 and 2 billion birds a year die from colliding with buildings, hitting reflective glass by day and especially being confused by urban lights at night. By some estimates, nearly 10 percent of all the migrants that die each year in the U.S. do so by hitting buildings, making collisions second only to free-ranging house cats as the biggest cause of human-related bird mortality. 
But there’s a simple solution: Turn off unnecessary lights during the migration period. This is particularly important in autumn, when most of the birds migrating are youngsters on their first trip; the juveniles seem to be especially prone to being drawn in by urban light pollution and crashing into buildings. Programs like Lights Out Philly are a win-win, because they both protect vulnerable migratory birds, and save building owners money on otherwise wasted electricity. 
Even if you’re not a birder, think about this: Every clear night from August through mid-November, there are tens to hundreds of thousands of birds passing through the Tri-State area. Some have come from the far Arctic, en route to wintering areas deep in the tropics of Central or South America. Many of them weigh so little you could mail two of them for a single first-class stamp. They’ve been making these journeys since time immemorial, keeping faith with the seasons and the far horizon, but today they need our help.  

Compared with what they are facing, how hard is it to flip a switch, turn off the lights, and give them a break? 

Purple martin, Progne subis © Doug Wechsler/VIREO

All migratory bird images were provided by Visual Resources for Ornithology (VIREO), the worldwide bird photograph collection of the Academy of Natural Sciences. More than 700 photographers from around the world have contributed to VIREO, with over 100,000 images accessible online for educational, scientific and commercial use for students, birders and researchers alike.


  1. Thank you, Scott! As always, you are so eloquent and so adept at conveying maximal info with your well-honed phrases. And, thanks to the Academy for co-sponsoring Lights Out Philly with DVOC and our Audubon partners. This is one situation when “being in the dark” is a wonderful way to be–for the birds & for their human friends.

  2. Love this! And the idea of turning off more lights during migration periods is a fabulous idea, one I had never heard before!
    Definitely also concerned about the placement of many wind farms along our Eastern seaboard within the flight path of many species. Not convinced it’s the best choice for renewable energy.

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