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:

Birds don’t ‘see’ glass the way humans do. Even while we cannot see glass on our own, we are aided by visual cues surrounding it and can detect its presence as a solid material. Because glass reflects habitat or is transparent, birds cannot recognize it as a solid object and fly 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 in his groundbreaking book Solid Air, “Glass is an indiscriminate killer that takes the fit as well as the unfit of a species’ population.” 

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

Why do birds collide 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. 

• Fly-through: 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. 

• Artificial Light 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. 

• 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 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:  

• The pattern should cover the entire glass surface.
• Gaps between each marker should be no larger than 5 cm by 5 cm (2 inches by 2 inches). 
• The pattern should be applied to the outer surface of the window or glass. 
• Each individual marker should be at least 6 mm (1/4th of an inch) 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 try to fly around the visible obstacle, eventually colliding with the invisible obstacle, i.e., glass. Decals or any other collision mitigation solution must be placed densely and follow the above-mentioned guidelines to avoid this. 

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 Large visual 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. 

• Installing 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. 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 (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 is alive but stays still when you approach, it might be injured. Follow these steps to help save its life: 

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

Read more detailed instructions from Fatal Light Awareness Program (FLAP) Canada. 

You can also report bird collisions on the Global Bird Collision Mapper. 

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

With over 18 years 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 earlier this year. 

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!