Birds hit glass because they can't see it. Film only helps when it puts a visible pattern on the outside, with gaps no wider than 2 inches.
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More than 1 billion birds die from collisions with glass every year in the US alone, the American Bird Conservancy (ABC) estimates. It isn't only towers: ABC points to the clear glass in homes, schools and stores. The fix that works is not darker glass. It's a pattern the bird can see in time to turn away.
Why birds hit glass
Clear glass is invisible to birds, just as it can be to us. Reflective glass is worse in a different way: birds treat the reflected trees and sky as the real thing and fly toward them, ABC's collision scientists explain. A single decal doesn't fix either problem. As ABC puts it, a decal may warn an alert human, but for a bird it is just an obstacle to fly around.
What makes film bird-safe
- Spacing. Early research found that horizontal lines 2 inches apart, or vertical lines 4 inches apart, significantly reduced collisions. Later monitoring showed 4-inch spacing doesn't stop the smallest birds, especially hummingbirds, so ABC now describes the old "2x4 rule" as the "2x2 rule": no gap wider than 2 inches either way. Patterns on 2x4 spacing can still earn a bird-friendly rating.
- The outside surface. ABC says the best place for markers is the outside of the window. Strong reflections can hide a pattern on an inside surface for part or all of the day. If you put film inside, ABC suggests checking it from outside every hour or two, starting in the early morning.
- Visible from about 10 feet. Songbirds have much poorer eyesight than people, so ABC judges patterns from about 10 feet (3 meters) away. A pattern that looks obvious to you may not register with a bird.
How to read a Threat Factor
ABC rates glass and film in a flight tunnel. At least 80 wild songbirds fly toward two panes, one clear and one patterned, and a net stops each bird safely before the glass. The Threat Factor is the share of birds that flew toward the patterned pane: if 20 of 80 did, it scores 25. Lower is better.
ABC calls a material bird-friendly at a Threat Factor of 30 or lower, which it estimates cuts collisions by at least half in real conditions. The scale isn't linear, so 15 isn't twice as good as 30, and a score is not a guarantee for a given building: orientation, what the glass reflects and how the facade is built all change the result.
Films with a published rating
| Film | Pattern | Threat Factor |
|---|---|---|
| Solar Gard byDESIGN Bird Strike Horizontal | 1/8-inch horizontal stripes, 1 inch apart | 10 |
| Solar Gard byDESIGN Bird Strike Dot | 1/4-inch dots, 2 inches apart | 15 |
| Solar Gard byDESIGN Bird Strike Vertical | 1/8-inch vertical stripes, 3 inches apart | 20 |
All three scores come from an ABC and Bird-Safe Buildings Alliance review of Solar Gard's samples, dated February 14, 2022, which found all three bird-friendly. XPEL's VISION decorative series also lists a Bird Safe Dotted film, which lets in 88% of visible light with 10% exterior reflectivity, but its spec sheet gives no Threat Factor or dot spacing. Ask the maker for the ABC rating before you rely on it.
What film can't do
Tint darkness and reflectivity are not bird protection. Reflections are part of the problem: ABC won't tunnel-test glass that reflects more than 15% of light at its outside surface unless the markers are on that outside surface or its contrast analysis shows they show through. A dark or mirrored film with no pattern adds nothing a bird can see.
Where it has worked
After about 1,000 birds died in one night at Chicago's McCormick Place in October 2023, dotted film went on its 120,000 square feet of glass. The Field Museum's count that fall showed collisions down by more than 95%. Chicago is now weighing a bill that would count patterned film on the outside of glass toward bird-friendly building rules; our Chicago bird-safe glass post has the details.
Sources
- American Bird Conservancy, About the ABC Rating System: What is a Material Threat Factor? (updated January 23, 2023): tunnel testing, the Threat Factor of 30 or lower, the 2x2 rule, reflections and the 15% limit, the 10-foot viewing distance
- American Bird Conservancy, Frequently Asked Questions About Birds and Glass (Christine Sheppard and Bryan Lenz): why birds hit glass, single decals, outside versus inside markings
- American Bird Conservancy, Preventing Collisions: more than 1 billion birds a year in the US
- American Bird Conservancy, Evaluation Report for Saint-Gobain Solar Gard, February 14, 2022 (PDF): byDESIGN Bird Strike patterns and Threat Factors
- XPEL VISION Decorative Series spec sheet (R0824V2, PDF): Bird Safe Dotted
- WBEZ / Chicago Sun-Times, January 9, 2025: bird collisions down at McCormick Place after film; and our Chicago bird-safe glass post with the ordinance text
