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To stop bird-window collisions on glass facades, certain well-researched* rules of thumb should be followed – most notably the “2x4” rule: two inches or less of vertical space or four inches or less of horizontal space should be left untreated. Research has also confirmed that visual markers are more effective for birds when they are on the outside surface (first surface) of the glass, giving birds a greater chance to identify the surface from all angles and in varied weather conditions. Many regulations in place today refer to the 2×4 rule and this first-surface treatment.


Threat Factor

The National Glass Association adhere to the 2x4 rule. 

The 2x4 Rule

What to do?

Today’s glass structures kill billions of birds worldwide each year, and up to one billion in North America alone. When faced with glass, birds don’t see it as a solid surface the way we do. Instead, they’re more likely to mistake it for an opening. When the glass reflects surrounding vegetation, birds mistake the reflection for an actual environment and may try to fly into it, with deadly results.

The threat factor is a score on a scale from 0 to 100. Glass with a score of 30 or below is considered safe for birds. The tests are conducted by the American Bird Conservancy (ABC) in a tunnel test after capturing birds in nets during the spring and fall migrations. To conduct the test, a bird is released at one end of the tunnel and its flying path is scored based on the bird’s tendency to avoid collision with a glass sample inserted at the far end.  According to the ABC, the “threat factor” is defined as: “a way to assign scores that measure birds’ ability to see and avoid patterned glass and other materials.” This scoring method is used in various legislations and in the LEED credit for (Bird Collision Deterrence).

For the most part, bird deterrence legislation is enforced at the municipality level. In North America, there are two trains of thought to avoid bird collisions against glass. Depending on the region and legislation in question, they will either adhere to the 2x4 rule or require a threat factor.  


As a part of our standard offering, Tristar carries two AviProtek® patterns from Walker Glass, both of which meet legislations found in North America. They are available in monolithic sheets or with Solarban® solar control coatings from Vitro on surface 2. With these options available, Tristar can answer bird safety requirements for the entire building envelope of any given project. 

Other patterns available upon demand- See them on the Walker Glass website.

Product Specifications

Thicknesses: With low-e 1 /4’’ (6mm) 

Monolithic from 5/32” to 3/4” (4mm to 19mm)

Dimensions: Standard 96’’ x 130’’ 

Substrates: Clear glass. Other substrates such as Starphire Ultra-Clear™ glass are available on demand. 

Energy: Standard offering with Solarban® 70. Due to the low density of AviProtek® patterns, they have no significant impact on solar control performance. Other coatings are available on demand.

Warranty for the low-e coating: For more information, please communicate with the Vitro Glass Customer Service Department. 

Warranty: 10-year limited warranty against surface degradation. For all terms and conditions regarding warranty, please contact Walker’s Customer Service Department. 


AviProtek® patterns are acid-etched on the exterior surface of glass, which helps birds to identify the glass as a solid barrier. Grit, dust, and fluids slide easily off the non-porous surface of acid-etched glass, just like they would on unetched glass, making it a simple matter to keep the glass clean. Regular maintenance, the same as for un-etched glass, will keep your bird-safe glass immaculate. For more details, download the Handling Booklet

Sustainable Materials

AviProtek® is the ONLY bird-friendly glass product available with both EPD (Environmental Product Declaration) and HPD® (Health Product Declaration). This environmentally responsible solution can help projects qualify for LEED® points such as Optimized Energy Performance and Bird Collision Deterrence, as well as many others. AviProtek® E also meets the Buy Clean California Act, AB-262.

Pattern 227

5mm squares at 2” x 4” apart 

Total coverage : 0.70% 

Meets the 2x4 rule 

Threat Factor of 25 

Pattern 217

5mm dots at 2” apart 

Total coverage: 1.94% 

Meets the 2x4 rule 

Threat Factor of 15 


Contact us today for
a free quote!

© 2022 Tristar Glass, Inc.

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