Why Birds Hit Glass

Three Main Threats

Transparent Glass

The atrium at the Thurgood Marshall Federal Judiciary Center in Washington, DC lit up inside at night shows trees and vegetation inside a large glass facade. — Photo by Nick Lund

Birds don’t see glass as solid. They try to fly through it, usually in pursuit of food or shelter.

Reflective Glass

Reflection of vegetation in glass — Photo by John Benam

Birds see reflections as real habitat. They fly toward reflections unaware they are about to collide with glass.

Artificial Light

San Francisco skyline at night — © POND5

Light draws birds toward buildings at night as well as increasing the reflection and transparency of glass.

Glass Is Not The Only Threat

Although bird-glass collisions are by far the most common cause of collision for birds, birds can also collide with non-glass structures and off-shore structures. Non-glass collisions are generally associated with structural non-visibility (such as the moving blades of wind turbines), the disorientation of birds from artificial light disturbance, and/or birds’ fatigue from long-distance flight. To prevent collisions, we must prioritize making threatening structures visible to birds and reducing light pollution both on land and at sea.

Learn How to Make Glass Safe for Birds

The Birds Most at Risk of Collision

Songbirds

Varied Thrush © Mark Chavez / Macaulay Library

Songbirds are often attracted to trees and plants near buildings. Their tendency to move in rapid bursts increases their risk of collision.

Migrants

Male Common Yellowthroat — Common Yellowthroat © Brad Imhoff / Macaulay Library

Collisions spike during spring and fall migrations when large numbers of birds stop over in urban areas. Artificial light often draws migrating birds toward buildings and glass.

Juveniles

Juvenile White-throated Sparrow © Tom Edell / Macaulay Library — Juvenile White-crowned Sparrow ©Tom Edell / Macaulay Library

Juvenile birds may be especially vulnerable to window strikes as they learn to fly, navigate their surroundings, and embark on their first migrations.

Songbirds are Not the Only Birds at Risk of Collision

Around 300 species in the United States and Canada have been documented in window strikes including hawks, grouse, kingfishers, hummingbirds, woodpeckers, and birds of conservation concern such as Golden-winged Warblers and Painted Buntings.

Why Songbirds, Migrants, and Juveniles are Most at Risk

Proximity

Grasshopper Sparrow © Simon Tolzmann / Macaulay Library

Songbirds often forage and move below the tree canopy, putting them in the same zone as buildings with glass. Many songbirds migrate at night and are strongly attracted to light which draws them off their flight paths and toward buildings.

Behavior

Eastern Phoebe perched on a branch having caught an insect — Eastern Phoebe © Robert Stone / Macaulay Library

Research has shown that collisions are greatest for migratory, insectivorous, and woodland bird species. This includes songbirds, whose movement while hunting and seeking shelter increases their risk of collision.

Vulnerability

Brown Creeper stunned on the street after window strike — Brown Creeper stunned by window strike (Photo by Bird Safe Maine)

Migratory songbirds hit glass in urban and suburban areas where they come down to rest. Hungry, tired, and in new surroundings, they may hit windows when seeking food, evading predators, or skirmishing with other birds.

How Artificial Light Increases Collisions

NYC Bird Alliance scientists observe the southern Tribute in Light beam through a camera scope and binoculars. © NYC Bird Alliance — NYC Bird Alliance scientists monitor The Tribute in Light beam in New York City, known to trap migrating birds. When too many birds enter the beam, monitors signal to turn the lights off and let the birds disperse.
© NYC Bird Alliance

Did you know that more than 80% of migratory bird species in North America migrate at night? As they fly, light from buildings can attract them from more than 3 miles away into areas where they face higher risk of collision, light-entrapment, predation, and other urban threats.

There are many recorded instances of birds getting drawn off their migration paths and becoming “entrapped” by artificial lighting. They often fly into the building when circling, sometimes even dropping out of the sky out of pure exhaustion. Even if they survive their long journey, being drawn off course, weakened, and confused by artificial light reduces their chance of survival when they reach their final destination.

Birds that migrate or forage offshore can also be affected and disoriented by nighttime lighting from coastal cities and offshore vessels and structures.

Nighttime lighting can also over-illuminate areas where birds might normally fly through, rest, or forage. Bright light means higher visibility to predators and can have a negative impact on birds’ optimal foraging decisions.

Map of the United States' light pollution at night — This NASA Earth Observatory Image shows light pollution across the U.S. at night. © Joshua Stevens

Turn Your Lights Out During Migration

Reducing light pollution doesn’t just benefit birds; it is important for both human and environmental health. We can all do our part to reduce harmful artificial light by turning our lights off during spring migration (March-June) and fall migration (August-November). Check BirdCast to see when birds are migrating near you.