According to the American Bird Conservancy and the Smithsonian Institution, an estimated 330 million to one billion birds die annually in the USA from window strikes. In the UK, the British Trust for Ornithology estimates up to 30 million birds die this way every year. This scale of loss contributes with climate change and other factors to an alarming global loss of wildlife and degradation of ecosystems.In addition, window strikes are distressing for building occupants. As architects, we often use glass to connect people with nature, but when done wrongly, that glass can literally kill the nature we seek to connect with.A multi-faceted problemWindow strikes mostly occur near where birds rest and forage, from the ground up to tree canopy level at buildings, highway noise barriers, or any glass structure. Birds assume they can fly to greenery and sky reflected in the glass or, where interior planting is used, behind it. Variables such as siting, lighting, weather and time of day can also make glass appear fully reflective or transparent.Bird strikes are especially problematic during migratory seasons, when species such as sparrows, buntings and warblers pass through towns and cities. Built environments increasingly occupy or interrupt migratory routes, and artificial lighting can attract birds which migrate at night.In response, New York City passed new regulations in January 2020 requiring all new buildings and major renovations to use ‘bird-safe’ construction from early 2021. Specifically, 90% of wall surfaces below 75 feet (23 m) and glass surfaces one storey above green roofs needs to be bird-safe, among other requirements. Similar but less stringent legislation has already been adopted in Minnesota, Oakland, Portland, San Francisco and Toronto.Retrofitting patterned glazing to the Javits Center in New York reduced bird deaths from window strikes by 90% (Chris Cooper, FX Collaborative)<!--clear div-->Bird-safe constructionThe main option for bird-safe construction is patterned glass. Patterned glass deters collisions by making glass visible to birds. Effective patterns have a density with spacing under 50 mm pacing, with markings at least 5 mm around. Denser patterns can also potentially reduce solar heat gain, which can help offset extra material costs by reducing cooling costs.Externally etched or frit surface patterns (a finely porous glass through which gas or liquid may pass) are generally better as they are always visible, even with strong reflection, but patterns on inside glass surfaces can also be effective. New products that are clear but reflect light in the ultra-violet spectrum are available, but currently expensive. Perforated screens and tightly spaced louvres can also be bird-safe. For retrofits, lightweight netting or cut vinyl patterns can be applied to existing windows.The cost of adding bird-friendly frit to new glass adds only a fraction of a percent to overall building construction costs. Furthermore, louvres, patterned glass and other bird-friendly measures are often used for aesthetic reasons or to reduce energy, such that meeting the new regulations can represent no additional cost.A case studyIn 2014 my firm renovated the Jacob K Javits Convention Center in New York. At the time, it was the worst, bird-killing building in the city. We replaced the original tinted and reflective glass with a less-reflective type that had a frit pattern of tiny dots. We also replaced a significant portion of glass with opaque panels.While adding bird habitat in the form of a 2.7 ha green roof, the changes reduced bird fatalities by more than 90% − and they also helped save energy by reducing air-conditioning loads. In addition, because the glass has a high level of light transmission yet reduces glare, the building now feels more transparent and welcoming.This article is based on the authors’ briefing article in the latest issue (173 CE2) of the ICE Civil Engineering journal.