On October 13, 2021

Minimizing migration’s perils

By Brett Amy Thelen

One morning in early autumn, I was running errands in downtown Keene, New Hampshire, when I was stopped in my tracks by a flash of yellow. Crouching down, I found a gorgeous, palm-sized bird, olive above, with a belly as gold as sunshine. It was a species I’d never seen before, and it was lying dead on the ground beneath a store window.

Submitted.

After consulting several field guides and two friends who know the intricacies of warbler identification far better than I ever will, I deduced that it was a mourning warbler — a secretive songbird of boreal and high-elevation forest clearings, with a migration route that tracks to Central and South America. Imagine flying all that way — a wonder of beauty and color — only to be felled by a run-in with a pane of glass.

Sadly, the warbler is part of a much larger pattern. Migrating birds face many hazards, including storms, habitat loss and predation. But two threats working in concert have proven particularly deadly: artificial light and glass windows.

Many birds fuel up by day and migrate by night, when temperatures are cooler and aerial predators are less active. Starlight may also aid in navigation. Artificial lights can both attract and disorient migrating birds, drawing them to cities — and, for many, into collision courses with windows.

One of the most well-studied instances of the effects of bright lights on migrating birds is the Tribute in Light, the powerful twin beams that illuminate the New York City skyline each September as a memorial to those lost in the terrorist attacks on Sept. 11, 2001. September is peak time for songbird migration, and when weather conditions align just so, thousands of birds can become “trapped” in these beams, circling until they drop from exhaustion or strike the sides of nearby buildings. Studies have shown that bird densities in lower Manhattan are up to 150 times higher when the Tribute lights are on. NYC Audubon now coordinates volunteers to monitor the memorial; when too many birds become caught in the lights, Tribute operators turn them off for a brief time to allow the migrants to disperse.

New York is not alone. Intensive monitoring at a 40-story convention center in Chicago, for instance, has documented 40,000 window-strike fatalities since 1978 — but also showed that numbers dropped from thousands to hundreds of birds per year when the building began shutting its lights off at night in the early 2000s.

Birds can hit windows of any height, at any time. However, during migration, window strikes commonly occur in early morning, when hungry migrants drawn toward buildings by nighttime lights search for food and instead encounter glass — imperceptible to birds — that reflects the sky or nearby vegetation.

Somewhere between 365 million and 988 million birds — from songbirds to raptors — fall victim to building collisions in the United States each year, second only to outdoor cats as the largest source of direct, human-caused mortality for U.S. birds. Despite the attention garnered by high-profile skyscrapers, a 2014 study in the journal Ornithological Applications found that high-rises were responsible for fewer than 1% of window-strike deaths, with 56% attributed to low-rise buildings and 44% to residences. The researchers also determined that structures that kill only a small number of birds annually — like my local shopping center in Keene — account for a large proportion of total window-strike mortality.

Thankfully, this is a problem with solutions. We can all help by turning off exterior lights and unused interior lights overnight, especially during peak bird migration season in spring (April 1 to May 15) and fall (September 1 to October 31). When you must use indoor lights after dark, opt for lamps that illuminate a small area instead of overhead lights that brighten the whole room, and keep your curtains or blinds closed.

In addition, if you live or work in a building with large windows, keep an eye out for window-strike victims, then apply screens, stickers, or decals to break up the reflections of any problem glass. For helpful guidance on design considerations for buildings big and small, visit the American Bird Conservancy’s Bird-Friendly Building project at abcbirds.org/glass-collisions.

Migration has always been fraught with danger, but today’s glass-hardened landscape presents a novel and extraordinary threat. With a bit of effort, we can soften that landscape, and help the birds that grace us with their presence move safely through it.

Brett Amy Thelen is science director at the Harris Center for Conservation Education in Hancock, New Hampshire (harriscenter.org). Illustration by Adelaide Murphy Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation: nhcf.org.

Do you want to submit feedback to the editor?

Send Us An Email!

Related Posts

Pies, parades, and porch chats

July 2, 2025
“America is a tune. It must be sung together.”—Gerald Stanley lee The month of July is the height of summer, bringing a spirit of celebration to all of us. Our town of Killington may be small, but we know how to celebrate the 4th of July. We start early with the annual book sale at…

Inventing a better ski day: the innovations that drew crowds to Killington

July 2, 2025
By Karen D. Lorentz Editors’ Note: This is part of a series on the factors that enabled Killington to become the Beast of the East. Quotations are from author interviews in the 1980s for the book Killington, A Story of Mountains and Men. “We’ve got a million dollars that says you’ll learn to ski at…

‘Almost Heaven’

July 2, 2025
The stage was simple, designed to resemble a wooden board that resembled the siding of any barn, anywhere across America. It could have been the barn behind my house, or the one that my cousins have down in Georgia. It could have been a barn in Colorado or even West Virginia.  Nothing remarkable at all,…

Getting away from it all

July 2, 2025
My family and I went to the beach this past week. The temperatures were hot, and the weather was sunny, making for a classic seaside vacation. The house we rented was in the harbor of the town where we were visiting, so while we didn’t stare out at the ocean, we were able to sit…