Over a billion birds die every year in the US alone after colliding with glass windows. 

This number continues to grow—especially in areas where bird-safe design and retrofits haven’t been prioritized. These aren’t just isolated incidents: they involve healthy adults in their breeding prime, fledglings that won’t survive without parental care, and vulnerable species already under pressure from habitat loss and climate change.

The good news? Glass collisions are preventable. If you’re planning a mitigation project, taking the right approach from the start makes all the difference.

Whether you're working on a new build or retrofitting an existing site, here are five key things to consider before you begin:

1a. Reflective Surfaces

About: This is the biggest cause of bird-glass collisions. Birds see reflections of the sky, trees, or the 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. 

Solution: Applying standardized markers/patterns to the outside surface of the glass breaks up the reflection. Note: Applying treatments on any surface other than the external surface (also called the first surface) is substantially less effective. This is because rays from the sun will hit the glass at angles that make interior treatments invisible throughout large portions of the day.  

Why This Works: The bird no longer perceives the space as an open and safe fly-zone. It can now distinguish there is a barrier to the flight path.  

1b. Fly-Through/Transparent Surfaces

About: Some structures, like railings or bus shelters, have transparent or clear glass, also called fly-through 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, habitat does exist on the other side of the glass.

Solution: Applying standardized markers/patterns to the glass will interrupt the transparency. With such surfaces, we recommend applying black markers on the surface that gets more strikes. 

Why This Works: The bird no longer perceives the space as an open and safe fly zone and can “see” the barrier to the flight path. Black markers are critical here to ensure adequate contrast (see below for more on contrast).

2. Spacing

About: Birds are known for their incredible flying skills and can fit through small spaces at high speeds. Therefore, spacing is a critical element of effective prevention. Studies have shown that glass treatment works best when markers are large enough and spaced close enough to be easily seen by birds, enabling them to change their flight path in time to avoid the glass.

Solution: Research suggests that visual markers should be at least ¼ of an inch (6 mm) or larger. The sooner a bird can see the markers, the more time they have to alter their course. Markers should be placed no more than 2 in x 2 in (5 cm x 5 cm) apart. This helps protect even the smallest birds, such as Hummingbirds and Kinglets. Full surface width coverage is recommended for all adjacent glass. Birds are adept at altering course quickly and navigating the tiniest gaps. They will often fly into areas that are left untreated.

Why This Works: When consistent and close spacing is applied, the bird can no longer find enough open space to fit through, and it will avoid the area altogether. 

3. Contrast 

About: An essential factor for any bird-friendly glass solution is visibility under different lighting conditions. In other words, the markers you apply need enough contrast to signal to birds that there’s a barrier ahead. For instance, a green marker might blend in on a window that reflects surrounding trees, while the right colour makes all the difference, helping birds avoid collisions.

Solution: For reflective glass (usually darker), we recommend white markers to stand out against the background.

For clear or fly-through glass, black markers work best to provide the contrast birds need to perceive the glass.

Why This Works: The bird perceives glass as a physical barrier to avoid when a high-contrast treatment is deployed. 

4. Height

About: It’s a common misconception that towering skyscrapers are the main culprits when it comes to bird-glass collisions. In fact, the top floors of high-rises contribute to only 1% of all collisions. 

Most collisions happen between ground level and the height of the tallest trees nearby—since this is where birds tend to thrive in search of habitat and food. It makes sense then that an individual building’s "mortality signature", i.e., the number of birds that will collide with it and die, is determined by its overall glass cover and closeness to habitat. Nevertheless, wherever glass exists, collisions WILL happen, whether it is a low residential building or a high-rise.

Solution: Consider the first 4 to 5 stories of a building or the top of the mature tree canopy, whichever is greater. However, each project is unique and should be analyzed. Therefore, this minimum recommendation may change based on the project.  

Why This Works: Treating the areas of a building or home that pose a danger to birds will save species. 

5. Lighting

About: Birds primarily migrate at night, relying on stars to navigate. Bright artificial lights can disorient and trap them in dangerous loops. Eventually, exhaustion and disorientation may cause them to collide with glass, often fatally.  

Solution: Turning off non-essential lights from 11 p.m. to sunrise reduces this hazard by allowing birds to continue their migratory journeys safely. However, as seen with our recently concluded McCormick Place project, any lights out program must necessarily be combined with glass treatment to be effective. Not doing so leaves birds vulnerable.  

Other Factors that Increase the Risk of Collisions

Several other factors can influence the threat that glass poses to a bird. These factors can include the following: 

Amount of Glass 

Several studies have found the amount of glass on the face of a building to be the best predictor of mortality resulting from a bird collision. A 10 percent increase in glass correlated to a 19 percent increase in collisions (or more, depending on the season).  

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.

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.

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., and several other individuals and organizations in numerous independent studies. 

With almost two decades of 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, including at McCormick Place. 

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

Join the growing movement to make buildings bird-safe!