UPDATE September 2024: Thirty wildlife conservation organizations from 24 states filed a legal petition that asks the U.S. Fish and Wildlife Service to establish a process for commercial buildings to protect birds from deadly window collisions. It proposes a permitting process under the Migratory Bird Treaty Act that would require building owners to use proven techniques that make glass visible to birds, in order to reduce collisions.
When I read a recent post from my local wildlife rehab center announcing that they’ve been caring for four varied thrushes in their facility—all injured by window collisions—it got me thinking. This winter I’ve seen just one of these gorgeous birds in our yard. Might others have been victims of window collisions? I certainly hope not, but the rehab center reportedly takes in several hundred window victims each year, and it’s not hard to imagine that countless others die out of sight, often slowly and painfully. Certain species—such as thrushes, cedar waxwings, warblers and woodpeckers—are more likely to fly into reflective glass, and migratory species are also at high risk, as well as birds like herons and owls. According to the Bird Alliance of Oregon, “Whether the species is rare or common, young or old, resident or migratory, most birds are at risk of collision-related injury or death.”
Studies conclude that the more glass on a structure, the greater the chance of mortality, and windows that reflect surrounding vegetation create three times more risk than those that do not (Kummer et al. 2016b). And since birds are attracted to native plants, the risk increases.
Photo courtesy Cornell Lab of Ornithology
A billion deaths a year
Contrary to popular belief, it’s not unusual for birds to collide with windows. In fact, ornithologists say that bird fatality by collision with manmade structures is second only to habitat loss that’s brought on by agriculture, industrial forestry, urban development, invasive species, and climate change. The number of deaths due to window strikes is appalling: An estimated one billion birds die each year from encounters with reflective surfaces in North America! Birds who don’t die quickly from injury may suffer slow, painful deaths or become easy prey for predators. Many bird species, such as the elusive varied thrush, are already in steep decline, and deaths by collision only exacerbate the problem. And it’s getting worse—as urban areas grow, the quantity and size of obstacles increase and natural habitats degrade. Stopover habitat for migratory birds is getting smaller and smaller and more fragmented as humans encroach on what once was grassland, wetland, shoreline, and the like.
Large urban buildings may be the most notorious killers, but any unobstructed, reflective window can kill and large rural structures are the most problematic: A study in Biological Conservation confirmed that rural buildings are worse than urban skyscrapers because they happen to be right where birds forage. The authors surveyed 40 college campuses across the continent and discovered that sites with abundant shrubs and trees in a 160-foot radius were the deadliest. Furthermore, since many birds travel along undeveloped migration routes, well lit towns and office parks they come across have a greater chance of distracting them. There is also speculation that there may be an innate behavioral difference among rural and city bird populations, with urban birds possibly having learned to avoid windows and other structures following a few non-fatal crashes. Rural birds would lack that training, which could make them more vulnerable. This would explain why thrushes and woodpeckers would be some of the most vulnerable species, since they adapted to forest environments.
What they see
Birds don’t see window glass and shiny or mirrored office buildings like we do. They see a reflection of trees, shrubs, and sky that appears to be a clear path, and consequently fly into it. 
Moreover, some fruit-eating species may get intoxicated by eating fermented berries and are more likely to hit windows while flying “drunk.”
Or, birds may see through clear glass (such as two corner windows perpendicular to each other, a solarium, or a bus shelter) and are deceived into flying right through as they try to get to vegetative cover that they see beyond the glass. Reportedly, this can also happen if indoor plants are situated right next to windows.
Some species (such as robins and bushtits) see their reflection during breeding season, view it as an intruder to their territory, and actually attack the glass—I’ve seen it happen. This territorial behavior can be intense, but they usually aren’t seriously injured (unlike the other situations). These territorial strikes can also happen at car windows.
How you can help
Because windows are everywhere, it’s easy to think that the problem is too overwhelming to do anything about. But any bird-friendly change you make to your property’s windows can help. Especially if your good intentions attract birds to your yard—with feeders and/or native plants—or you’ve noticed birds hitting your windows, it ought to be compulsory.
Bird strikes often follow a pattern, with the same windows repeatedly struck. If you have a lot of windows, take some time to identify which windows are problematic, paying attention to bird attractants like food, water, and cover. Look at your windows from a bird’s point of view.
Most of the following remedies work either by blocking reflective glass or making it visible to birds by giving them visual cues. Sheer curtains and blinds closed part way may help cut down on reflection, but they don’t fully eliminate it, so don’t rely on interior remedies. Silhouettes placed on the inside of windows do not work because birds still see the reflection.
DIY suggestions:
♦ Locate all bird feeders and bird baths at least 30 feet from windows, a distance that allows birds to see that windows are part of a house. Or, keep them very close—within 2 feet—to reduce the chance of high impact collisions. If that doesn’t help, either add additional protections or remove the feeders or baths altogether.
♦ If any of your windows have a clear view through your house to another window, create an obstruction (such as curtains) that blocks what may appear to birds to be a flight path.
♦ Keep taut window screens on year round if you have them, or consider adding them. Screens block reflections considerably and soften any impact. Keeping your windows dirty may also help!
♦ Make your own “zen wind curtains,” which are practical and effective and don’t look the least bit odd.
♦ Apply patterns (a few inches apart) with soap on the outside of windows—use stencils found at craft stores, or make your own. The patterns can be wiped off and redone when necessary. They are very inexpensive but may may be impractical for windows that receive rain or are hard to reach.
♦ For birds who fight with their reflection, simply hang a cloth or apply some masking tape to the area for a few days to break the bird of the habit.
♦ Be sure that blinds, shades and/or shutters are in place and closed to reduce nighttime light that attracts and confuses birds.
Products for purchase:
♦ Decals that reflect ultra-violet wavelengths of light—which birds can see but we can’t—are applied to the outside of windows. Follow manufacturers instructions for adequate coverage (aim for 80%), generally a few inches apart. Some examples include Window Alert (pictured) and BirdTape, which provide a stoplight for birds. In direct sunlight, decals will need to be replaced more often than in shade, so be sure to keep track of when you put them up. If you have a lot of windows to cover, BirdTape is more economical and may last longer. 
♦ Films like CollidEscape, that appear opaque to birds but transparent to you, are applied to the outside of windows.
♦ Feather Friendly adhesive dots are applied on the exterior of windows in a “frit” pattern.
♦ External awnings or sun shades help minimize both reflection and transparency.
Architectural solutions:
Planning on remodeling or building a new home? Are you an architect or developer? The Resource Guide for Bird-Friendly Building Design is a comprehensive publication that offers excellent info and workable solutions for reducing collisions in commercial areas as well as residential. Also check out the American Bird Conservancy’s Bird-Friendly Building Design and the City of Toronto’s Bird-Friendly Best Practices: Glass. All are well worth a read.
Other important recommendations:
At night, turn off lights in office buildings (all levels), especially during spring and fall migrations. At home, pull your shades or draw draperies, and install motion censors on outdoor lighting, rather than leaving lights on at night. All of this prevents disorientation of migratory birds traveling at night and cuts down on other negative effects of artificial light pollution.
If you find a bird on the ground near a window: Slowly and gently cover and catch the bird with a lightweight, soft cloth and carefully place it in a small box (such as a shoebox) that has air holes and is lined with a soft cloth or paper towels rolled into a doughnut shape to keep the bird upright. Handle the bird as little as possible and keep the box securely closed. Do not give food or water. Place the box in a quiet, dark, and pleasantly warm place, away from other animals, noise, and children. If the bird has an obvious injury like a cracked bill or dangling wing, transport it immediately (in the darkened box ) to a licensed wildlife rehabilitator—broken bones need attention quickly. If there are no obvious injuries, quietly check on the bird several times over one to two hours—outside and away from human activity and buildings in case the bird can fly—but don’t touch it. If the bird develops swollen eyes or becomes unresponsive during the hour, quickly transport it to a wildlife rehabilitator. If the bird seems alert and can stand on its own, place the box in a quiet spot and open it. Move away, remain still and out of sight, and wait. If s/he doesn’t fly away within 5 or 10 minutes, carefully and quietly take the bird to a wildlife rehabilitator. Remember that, other than transporting a bird to a rehabilitator, it is illegal to handle migratory birds without a license.
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