Bird-Window Collisions and How to Help
Scope of the Bird-Window Collision Problem
Up to one billion birds are estimated to collide with buildings and other human made structures in the U.S. every year, and billions of additional birds probably collide with buildings each year in other parts of the world. These collisions occur because glass and/or artificial light at night cause birds to fly into buildings and other human made structures. Unfortunately, most of these collisions are fatal.
Glass
Reflective and transparent glass exteriors, indoor plants, and greenery close to buildings can all create confusing situations for birds who often mistake the reflections they see on windows and other glass surfaces for real scenes, or mistake transparent glass surfaces for clear passageways to fly through. Wild birds, unlike most household pets, have never had an opportunity to learn what glass is, so they need strong clues on or around glass to warn them that it’s there.
The largest numbers of individual birds and variety of species generally occur each year during the spring migration (mid-February through early June) and the fall migration (late July through late November), but the number and variety of migratory birds that occur during fall is always larger than the number and variety that occur during the spring.
However, because birds can’t see glass, dangers exist for birds year-round, especially glass near bird feeders and bird baths. Resident birds, particularly juveniles, tend to collide more often with glass in summer as they explore local environments. Collisions also happen all winter long.
You can read more about the problem here.
Lights
Most species of birds migrate at night using a variety of cues to find their way, including the earth’s magnetic field and the position of the stars. Because birds are particularly sensitive to light when they are migrating at night, artificial lights at night can disorient them, particularly during cloudy or rainy weather when the celestial cues they use to navigate may not be visible. Under these circumstances birds may congregate around artificial lights causing them to collide with various parts of the buildings associated with lights. This happens not only in large cities but also in areas with much less pervasive lighting, like highways, parking lots, and suburban malls. Because light attracts migrating birds, they often stop to rest in developed areas during their journeys. This means that when these birds forage the next day, they are more likely to encounter, and collide with, glass. Turning lights off overnight, or at least before birds descend in the early morning, can reduce the number of birds near buildings and therefore reduce collisions.
Solutions
One easy way for homeowners to prevent birds from hitting windows is to apply visible patterns to the outside of the glass. Patterns on the inside are often ineffective because they can be hidden by reflections on the outside of the glass.
Most birds will avoid glass with vertical or horizontal stripes (or other markings) spaced 2” apart. Stripes should be at least 1/8” wide. A good rule of thumb — you should be able to see the pattern clearly from ten feet away. White stripes tend to perform better because they reflect most light and so are visible against more types of background reflections. A translucent line won’t show up as well. Patterns of dots can also work if the diameter of the dots is at least ¼”. Of course, since the spacing is what’s important, it doesn’t have to be dots or stripes — you can use your imagination.
Also, keep in mind that bigger is better: The wider the lines/dots, the more effective they will be because birds will see them from greater distances. Again, the critical issue is the spacing. Until recently, we talked about the ‘2×4 rule’ because research by Klem and Roessler showed that vertical lines spaced 4” apart would stop a lot of collisions – and that’s still true. However, many small songbirds, and especially hummingbirds, can and will fly through a 4” gap.
Products
Among the products that have been shown to be effective in preventing collisions include:
- Acopian Bird Savers
- Bird Tape
- CollidEscape
- Tempera Paint
Read more about these products here and here.
The American Bird Conservancy has evaluated many products that homeowners can use to prevent bird collisions. You can find the products here.
Bird Safe Philly
Bird Safe Philly was created in response to a mass collision event that occurred on October 2, 2020 in which thousands of migratory birds died after colliding with buildings in Center City Philadelphia. Bird Safe Philly is a partnership led by the Academy of Natural Sciences of Drexel University, Audubon Mid-Atlantic, Delaware Valley Ornithological Club, Liberty Bird Alliance, National Audubon Society, and Valley Forge Audubon Society.
The Bird Safe Philly partnership aims to create safe spaces for birds by developing awareness of the issue of bird collisions and implementing solutions, working in partnership with the Philadelphia community. Through Lights Out, Collision Monitoring, and Taking Action, Bird Safe Philly strives to engage individuals, organizations, and the community in better understanding bird collisions and the solutions that can be implemented to help prevent these collisions. The ”Lights Out” Program runs from April 1 to May 31 (Spring Migration) and August 15 to November 15 (Fall Migration).
Produced as part of an environmental science animated series, “A WonderGrove for Birds” by Wonder Media. “A WonderGrove for Birds” demonstrates ways in which young learners in grades K-6 can develop an interest in their environment by focusing on the needs and behaviors of birds in and around their home.
Additional Information
If you want to learn more about this topic, Dr Daniel Klem Jr has written a book Solid Air: Invisible Killer. Saving Billions of Birds from Windows
VFAS has done a presentation on the Bird Collision issue and the Bird Safe Philly program, which you can view on You Tube.
Sources: Accopian Center for Ornithology at Muhlenberg College, American Bird Conservancy, Audubon Mid-Atlantic
