Glass is a Conservation Threat to Birds
Bird-window collisions are a leading cause of bird mortality in urban areas. Between 365 and 988 million birds die each year due to collisions with windows and other glass structures (4). There seem to be seasonal trends in bird-window collisions, with rates typically higher in autumn and lower in summer (1). Some bird species appear to be more vulnerable to window strikes, including hummingbirds, thrushes, and migratory species. Juvenile birds may also be more susceptible. Preventing bird-window collision is an important conservation measure to protect birds in urban areas. Below are some causes of window collisions and the solutions to address them.
The Causes of Bird-Window Collisions and Solutions to Address Them
Glass Windows and Structures
The problem with glass is its reflectivity. Bird vision contains little depth perception compared to humans, as most birds’ eyes are on the side of the head. Birds also have poor contrast sensitivity. This means they are less able to distinguish a reflection from reality. To a bird, reflections of sky or habitat in a window may not be distiguishable from the rest of its environment (6). Certain building design elements can also increase collision risk, such as transparent corners, reflective glazing, and extensive glass façades.
There are several ways to adapt buildings to protect birds from window collisions. One method is through collision deterrents. Deterrents include stickers, exterior screens, etched glass, or vertical cords on the outside of windows that break up the reflections and help birds recognize glass as a barrier. Collision deterrents can be beautiful. Seattle Audubon collaborated with a local artist on a design to make the Nature Shop windows bird-safe.
Research shows that adding a pattern to the outside surface of a window can help birds avoid collisions (6). These patterns should be applied following the 2 x 2 rule: there should be no areas of untreated glass larger than two inches wide. This helps protect our smallest birds from colliding with windows.
Collision deterrents can even be a fun DIY project.
Collision prevention can also be incorporated within a building’s design. Architects and engineers can design buildings to be bird-friendly, without sacrificing the value of windows. External shades, sun screens, louvers, and reducing overall surface area of glass can reduce collision risk. Several companies now produce bird-safe glass products, which can replace high risk reflective and transparent glass.
Dozens of jurisdictions have adopted bird-safe building guidelines.
Deterrent stickers placed on the exterior of a window (9).
Interior and Exterior Lights
While the majority of window strikes occur during daylight hours, artificial light, including porch lights, lights in parking lots and in high-rise buildings, can disorient migratory birds that fly at night (6). Birds may become confused or lose their direction during seasonal travel, which can impact the length of time or distance a bird may have to fly. Additionally, reflections in glass from interior and exterior building lights shining on vegetation can lure birds closer to dangerous glass surfaces and increase collision risk, especially during migration periods (5).
The solution to interior and exterior light emission is extremely simple: turn them off. A long term study by the Field Museum in Chicago found that darkening a building by removing and minimizing lights resulted in 6-11 times fewer bird collisions (7). The International Dark Sky Association recommends keeping bright and blue-tinged lights beneath 3000 Kelvins (13). There are several “Lights Out” programs and campaigns across the US, designed to encourage homeowners, businesses, and municipalities to turn off or limit the amount of light emitted during night hours. Since artificial light mostly impacts birds migrating at night, there are forecasts available to monitor for when collision risk is at its highest. Colorado State University and the Cornell Lab of Ornithology have developed BirdCast, a predictive map that shows bird migration across the United States. The action of turning off unneeded or decorative lighting, switching to motion-sensor lights, and reducing interior light emissions with the use of shades can help provide migrating birds with safe passage through our city.
BirdCast migration forecast map (10).
Bird Feeder Placement
Most nature enthusiasts love to have bird feeders in their yards. The problem is not with having a feeder that attracts birds, but rather where the feeder is placed that can increase window collision risk. Avian mortality rates from bird strikes are highest when feeders are placed between 5 and 10 meters away from glass (2). Birds that may be stopping to feed in the middle of a yard, for example, may later fly off in the direction of perceived vegetation reflected in a nearby pane of glass. Or, a bird at a feeder may try to escape a predator and inadvertently fly directly into a window.
Bird feeder placed appropriately near a window (12).
Feeders placement within 1 meter of a window is considered the safest for birds, though mortality rates also begin to drop when a feeder is placed beyond 10 meters away from glass (2). Given the fact that most homes and businesses in Seattle do not have exceptionally large yards or green spaces, placing bird feeders either right next to window, or even attached to a window like a suction suet feeder, may help protect birds from window strikes. Alternatively, placing a feeder as far away from glass structures as possible, such as the end of a yard, can also decrease the risk of flight into glass panes.
Proximity to Habitat
Green spaces provide habitat and food for birds, but the risk of collision increases when these green spaces are located near reflective structures. Birds are often unable to distinguish between real vegetation and the reflected image. Vegetation within 50 meters of a window can cause reflections that attract birds closer to glass surfaces and increase collision risk (3). This is not to say do not have a beautiful yard or garden overflowing with plants, but that having a green space in an urban area includes a responsibility to protect local bird populations as well.
Vegetation reflected in a window (11).
Given that we share our urban spaces with many different bird species, individual actions and company decisions can prioritize protecting our feathery friends. Assessing windows for reflections and treating them can be a first step. All of the solutions previously discussed including window deterrents, limiting lighting, and carefully placing feeders, can help to address problems with bird-window collisions caused by human structures in the built environment. Humans and birds will continue to coexist, side by side in every city space, therefore applying conservation methods to minimize the risk of human-related bird deaths from glass and window strikes can make an important impact.
Photo: Judy Bowes
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1. De Groot, Krista L., Porter, Alison N., Norris, Andrea R., Huang, Andrew C., Joy, Ruth. “Year-round monitoring at a Pacific coastal campus reveals similar winter and spring collision mortality and high vulnerability of the Varied Thrush.” Ornithological Applications, Volume 123, Issue 3, (2021).
2. Klem Jr. et al. “Effects of Window Angling, Feeder Placement, and Scavengers on Avian Mortality at Plate Glass.” Wilson Bullitin, 116(1), pp 69-73, (2004).
3. Loss, Scott R., Will, Tom, Marra, Peter P., “Direct Mortality of Birds from Anthropogenic Causes.” Annual Review of Ecology, Evolution, and Systematics. 46: 99-120. (2015).
4. Loss, Scott R., Tom Will, Sara S. Loss, and Peter P. Marra. “Bird–Building Collisions in the United States: Estimates of Annual Mortality and Species Vulnerability.” The Condor 116, no. 1 (2014): 8–23.
5. Lao, Sirena, Bruce A. Robertson, Abigail W. Anderson, Robert B. Blair, Joanna W. Eckles, Reed J. Turner, and Scott R. Loss. “The Influence of Artificial Light at Night and Polarized Light on Bird-Building Collisions.” Biological Conservation 241 (2020): 108358.
6. Sheppard, Christine, Phillips, Glen. “Bird-Friendly Building Design”. American Bird Conservancy, NYC Audubon. 2nd Edition. (2015).
7. Van Doren, Benjamin M., Willard, David E., Hennen, Mary. “Drivers of fatal bird collisions in an urban center.” Proceedings of the National Academy of Sciences of the United States of America (PNAS). 118: 24. (2021).
8. Guardian Glass, LLC. “Expert Series: Bird-Friendly Glazing.” Guardian Glass. pp. 5-6. (2020).
9. Feather Friendly. “Bird Friendly Windows.” New York City Audubon. (2020).
10. Migration Tools, “BirdCast.” Cornell Lab of Ornithology, Colorado State University, University of Massachusetts Amherst. (2021).
11. Susan Spear. “Why Birds Hit Windows – and How You Can Help Prevent It.” All About Birds – Cornell Lab. (2017).
12. Dominic Arenas. “Ruby-throated Hummingbird Visits Feeders – The Strawberry Plains Hummingbird Migration and Nature Celebration.” Strawberry Plains Audubon Center. (2019).
13. IDA. “LED: Why 3000K or Less.” International Dark Sky Association. (2021).