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On the night of October 6, 2023, 1,000 birds were killed as a result of the mostly glass facade of the McCormick Place building along the shores of Lake Michigan. Experts estimate up to 1 billion birds die each year following window collisions in the United States, and although high-rise buildings (12+ floors) contribute to this number, most bird strike incidents are attributed to low-rise buildings (4-11 floors).
Because their visual acuity is different from human eyes, birds don’t see glass, and reflectivity fools them into seeing the blue sky and trees of the surrounding landscape. Glass to glass corners trick them into not realizing that there is a barrier. And nighttime illumination creates a beacon effect for nocturnal migrants orienting themselves by visual cues.
The latter of these points is the easiest to change; motion sensor lighting inside buildings would ensure lights are not on needlessly. Outdoor lighting should be directed downward, where it is also of most use to people.
Substitutions for clear glass exist, but they constitute a cost increase and may not be as aesthetically attractive. Some cities have enacted local laws requiring bird-safe glass for new construction and major repositioning projects; the biggest cities include Toronto, NYC, San Francisco, Chicago and Minneapolis. Glazing is then rated by a threat factor (TF). Where brick/wood/metal are rated at a threat factor of 0, typical transparent glass is assigned TF 100. Most local laws prescribe reducing this threat factor to 25. While some glass manufacturers have pre-tested patterns, such as Viracon and Walker Glass, the American Bird Conservancy has issued prescriptive requirements to meet either TF 25 or 20 to ensure compliance without testing.
Both methods described above need to be applied to glass surface #1 or #2 to reduce reflectivity and create a visible pattern for birds to see. To achieve a TF 25, the American Bird Conservancy prescriptive requirement is a 1/4” dot in a 2” x 4” pattern or 1/8” wide lines either horizontal or angled spaced 2” apart, or vertical spaced 4” apart.
Birds will essentially view these dots like branches, and if too wide apart they calculate that they can in fact fly through. 2” x 4” is the typical height by width of an average sized songbird.
To achieve TF 20, the same dot or line placement, but only 2” apart, would save even the smaller feathered friends such as hummingbirds.
Various options exist for this method, though it is important to note that it is the most expensive of the three, as it also requires a laminated outboard lite. Ideally suited for observation decks and glass balustrades, which typically are laminated without being an insulating glass unit.
The penultimate solution (so far) of the various bird safe requirements: electrochromic glass with acid etch on surface #1. In addition to having the bird safe pattern, this glazing is programmed to tint at night, thereby eliminating the nighttime beacon effect.
As for my private home (it is estimated that approx. 40% of bird strike fatalities happen on 1-3 story buildings) – post installed window markers at the same spacing are a great solution.
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