Authored by Dr. Daniel Klem, Jr., Ph.D., D.Sc. Sarkis Acopian Professor of Ornithology and Conservation Biology at Muhlenberg College, Allentown, Pa.

Dr. Klem is a renowned ornithologist who literally wrote the book on bird-glass collisions with his 2021 title, Solid Air – Invisible Killer: Saving Billions of Birds from Windows. For more information about Dr. Klem and his work, please visit

Blazing a trail in bird-glass collision research

As my doctoral dissertation topic from 1974 to 1979, I conducted the first comprehensive study of bird-window collisions and how to prevent them. From then to the present, a span of nearly half a century, I have continued to study, write, and teach about the threat that sheet glass and plastic pose to free flying birds worldwide. I have witnessed the growth of bird friendly building regulations across the globe and acted as an expert consultant on many occasions, including during the development of the CSA A460:19 standard for bird friendly building design.

It was during those formative investigations in the 1970s that my team and I developed an early form of field testing to evaluate the effectiveness of bird deterrence technologies. Our initial flight cage experiments eventually led to the two dominant bird safety measures used today: the Prescriptive Method which incorporates the 2×4 rule, and Tunnel Tests.

The origins of the 2×4 rule

We first determined the 2×4 rule using incremental line spacing and outdoor flight cage experiments. Later versions of the same experiments confirmed and validated a related set of field tests, which simulated windows installed in inhabited buildings. These window simulations were significant because they gave us a glimpse of how bird-deterrent glazing would perform in actual buildings, not just in testing scenarios.

The experimental precursors of the 2×4 rule were first published in peer-reviewed scientific literature in 1990 (Klem, D., Jr. Collisions between birds and windows: Mortality and prevention. Journal of Field Ornithology 61:120-128). However, it would be another two decades before bird strike prevention products became commercially available. Several years after that, they are seeing widespread use in architectural glazing and home improvement projects.

What is the 2×4 rule?

The 2×4 rule describes the maximum distance between elements in bird-deterrent patterning. Pattern elements may be any shape (lines, dots, other geometric figures, etc.), but they should not be separated by more than 50 mm (2 in) if oriented in horizontal rows, or by 100 mm (4 in) if oriented in vertical columns. In other words, the horizontal spaces between elements should be 10 cm (4 in) or narrower, and the vertical spaces should be 5 cm (2 in) or shorter.

bird-safe glass 2x4 rule

Rules of thumb for bird friendly glazing

  • Coverage: Bird-deterrent patterning should cover a window’s entire surface.
  • Surface placement: Bird friendly markers can dramatically mitigate bird-window collisions when applied to the outer surface (surface #1) of reflective panes. They can also be helpful when applied to surfaces 2, 3, or 4, provided there is no reflection on surface 1.
  • Spacing: Greater spacing between pattern elements increases the risk of strikes and casualties, while reduced spacing between pattern elements decreases the risk.
  • 2×2 rule: The Canadian Standards Association’s Bird-friendly Building Design (A460:19) recommends 5 cm (2 inches) spacing between pattern elements both vertically and horizontally. This stipulation is known as the 2×2 rule.

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