Over a billion birds die every year in the US alone after colliding with glass windows. Inevitably, the question that must be answered here is why birds collide with windows and how we can prevent this.

Before we answer that question, it is important to ask the following questions:

Can birds see glass or glass windows?
Can birds see through glass?

Birds don’t ‘see’ glass the way humans do. Even while we cannot see glass on its own, we are aided by visual cues that surround it and can detect its presence as a solid material. Because glass either reflects habitat or is transparent (or both), birds are unable to recognize it as a solid object and fly directly into it, often with fatal results.

They either die on the spot or suffer serious internal injuries that result in death later. Even birds with less serious injuries will be susceptible to predation and other dangers that lurk nearby.

Very rarely will a bird that has collided with glass escape healthy and unscathed.

Dr Daniel Klem. Jr., has pioneered research on bird-glass collisions since the 1970s and says, “Glass is an indiscriminate killer that takes the fit as well as the unfit of a species’ population.”

Let us explore in detail why birds collide with windows or glass and then consider the best ways to prevent collisions.

Main reasons for birds colliding with windows

Reflection: Birds see reflections of the sky, trees, or surrounding landscape in glass. This reflection appears to birds as an extension of their environment, and they try to fly to it, not realizing there is a barrier. In this case, an illusion of habitat is being created.

Transparency: Some structures, like railings or bus shelters, have transparent or clear glass. When such glass aligns with greenery or open spaces, birds try to fly through it, assuming that there is a clear path. In this case, there is actual habitat on the other side of the transparent glass.
Artificial Lights at Night: Most bird species migrate at night. Artificial light at night (ALAN) attracts birds and disorients them, sometimes ‘trapping’ them. Tired and confused, they fly about and often collide with glass. Learn more about ALAN at the BirdCast project and the Fatal Light Awareness Program.

Factors that increase the chances of bird-glass collisions

Closeness to Habitat: Glass surfaces that reflect nearby dense vegetation, especially up to treetop height, will experience more bird collisions. Aquatic habitats also draw birds, adding to collision risks during migration. Birds often fly between habitats, encountering glass structures like bus shelters on the way.
Migratory Behavior: Migrating birds, especially in fall, face higher collision risks due to larger flocks and inexperienced juveniles. Many collisions occur at dawn as birds search for landing spots, often unaware of nearby glass structures.
Type of Structure: Residential homes, even while causing lower individual collisions, contribute most to overall bird strikes due to their abundance.

Amount of Glass: More glass on a building correlates with more collisions. A 10% increase in glass area can result in up to a 19% rise in bird strikes, especially during migration.

Why should we prevent bird-window collisions?

Birds are a key element of all ecosystems, offering services such as controlling pests, pollinating plants, and spreading seeds. Safeguarding them from window collisions is critical to preserving their populations, which directly impacts the overall health of ecosystems. Healthy ecosystems benefit all species, including humans.

What makes a bird collision deterrent effective?

Any solution that can help birds ‘see’ glass as a solid barrier will prevent collisions. Visual markers on glass make windows more noticeable to birds. Cords, tape, paint, or any other pattern that disrupts the reflectivity or transparency of glass can also be used if the following key spacing, contrast and coverage guidelines are followed:

Ensure the pattern covers the entire glass surface.
Gaps between each marker or decal should be no larger than 2 inches (horizontally and vertically).
The pattern should be applied to the outer surface of the window or glass.
Each individual dot or marker should be at least 5 mm wide.
The pattern should contrast well with the surrounding habitat to be easily visible.

Can decals help prevent bird collisions?

One or two decals stuck on windows will signal the presence of glass to humans, but not to birds. Birds are attuned to flying through gaps and will simply try to fly around the visible obstacle, eventually colliding with the invisible obstacle, i.e., glass. To avoid this, decals or any other collision mitigation solution must be placed densely and follow the abovementioned guidelines.

Best ways to prevent bird collisions with windows

There are several effective ways to prevent bird collisions with windows. You can use one or a combination of the following effective methods to reduce bird-glass collisions on your property:

Using external screens or netting: Covering your windows with mosquito screens is very effective, as long as the screen is on the outside and covers the entire surface. A drawback of using screens is reduced visibility from the inside.
Installing bird collision deterrent markers: One of the most effective solutions is applying bird collision deterrent markers to windows. Products like Feather Friendly Small-scale DIY, Feather Friendly Medium-scale Pro DIY, Feather Friendly Large-scale Pro markers, and Bird Divert UV markers are scientifically proven to prevent collisions by making glass visible to birds. These markers break up the reflection and transparency issues, significantly reducing the risk of collisions. Moreover, these markers do not obstruct the view from the inside. Only 6-8% of the glass is covered and there is no solar load.

Turning off or dimming lights at night: During peak migration, dimming or turning off lights can reduce bird collisions. It is advisable to use motion-sensor lighting, or dim unnecessary lights, especially during fall and spring migration. While turning lights off at night is important, this should be combined with a bird collision deterrent on windows since the glass eventually kills birds.

Using bird-safe glass: Bird-safe glass is specially designed with frit/etch patterns or UV coatings that birds can see, significantly reducing bird strikes. While retrofitting every window may not be practical, new construction projects should consider bird-safe glass from the outset. It is critical that the fret/etch pattern should be on the outside (first) surface exterior.
Moving bird feeders away from windows: While bird feeders attract beautiful bird visitors to your yard, placing them too close to windows can increase the risk of collisions. Feeders must be placed either very close (within 3 feet) or far away (at least 30 feet) from windows to reduce impact risk

What should you do if you find a bird that has hit a window?

If you find a bird near a building that stays still when you approach it might be injured.

While ensuring that the bird is safe from predators and pedestrian traffic, find a cardboard box or unwaxed paper bag to contain the bird. Poke a few holes in the cardboard box.
Gently place the bird in the box or the paper bag.
Ensure that the bag or box is closed and that the bird does not escape.
Even if it seems OK, it is vital that it be assessed by a wildlife rehabilitator because the effects of internal injuries can show up later. Do not feed the bird or give it water.

Ready to protect birds?

Feather Friendly is the trusted choice for preventing bird collisions. Our markers are scientifically tested and approved by the American Bird Conservancy, Dr. Daniel Klem Jr. (the leading authority on bird collisions), numerous independent studies, and several other individuals and organizations.

With over 18 years’ experience in the industry, we have the knowledge and expertise to provide you with the solutions that work for your unique project.

We understand that no two projects are the same, so we always take the time to understand your distinctive needs before recommending an appropriate solution. We have worked with small residential projects to some of the largest commercial projects ever executed.

We’d love to help you decide which of our solutions best suits your next project. Contact us today to get started.

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Why Birds Collide with Windows and How Can We Prevent It?