Authored by Dr. Daniel Klem, world-renowned ornithologist and expert in the area of bird deterrence. 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 45 years, I have and continue to study, write, and teach about the threat that sheet glass and plastic pose to free flying birds worldwide. It was during those formative investigations in the 1970s that what is now called tunnel and field testing was invented and used to evaluate bird-window collision prevention methods. The 2×4 rule was first determined using incremental line spacing and tunnel tests, then called flight cage experiments that were conducted outdoors.

These tests were later repeated, which confirmed and validated field tests that accurately simulated windows installed in actual inhabited buildings. The experimental results of what became 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), but its wide-spread use in commercially available bird strike prevention products has only recently occurred within the past several years.



The 2×4 rule describes the distance between elements making up a pattern applied to windows for the purpose of preventing bird strikes. To be effective, the pattern must uniformly cover the entire window and consist of elements of any shape (lines, dots, other geometric figures, etc.) separated by no more than 5 cm (2 inches) if oriented in horizontal rows, or by 10 cm (4 inches) if oriented in vertical columns. These patterns eliminate bird-window collisions when applied to the outer surface (Surface #1) of reflective panes, or when applied to any surface of multi-pane see-through windows. Greater spacing between pattern elements increases the risk of a strike and casualties. Reducing the spacing between pattern elements retains effective bird-window collision prevention, and the Canadian Standards Association’s Bird-friendly Building Design (A460:19) recommends 5 cm (2 inches) spacing between pattern elements, labeling such patterns as following a 2×2 Rule.

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