Bird-Window Collisions and How to Help

Scope of the Bird-Window Collision Problem

Each year up to 1 billion birds die after hitting glass surfaces in the United States. Why do birds hit windows? Bird-window collisions occur for two primary reasons:

  • They perceive glass reflections of vegetation, landscapes, or sky to be real.
  • They attempt to reach habitat, open spaces, or other attractive features visible through either glass surfaces or free-standing glass.

Collisions in the U.S. are most frequent during spring and fall migration, as enormous numbers of birds move between northern breeding grounds and southern wintering grounds. The largest number of collisions occur during the fall, when migrants include both adult birds and their offspring.

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.

Artificial lights at night influence collisions, too. Many species migrate at night, and lighting draws them to developed areas. This happens 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.

You can read more about the problem here.


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.


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 read the results of their tests here.

Bird Safe Philly

Bird Safe Philly is a partnership led by the Academy of Natural Sciences of Drexel University, Audubon Mid-Atlantic, Delaware Valley Ornithological Club, National Audubon Society, Valley Forge Audubon Society, and Wyncote Audubon Society. The partnership is designed to help protect native birds in the Philadelphia area from a variety of issues that can harm birds in urban areas, especially issues that can cause birds to collide with buildings and other human made structures. 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.

The ”Lights Out” Program runs from April 1 to May 31 (Spring Migration) and August 15 to November 15 (Fall Migration).

Learn More About Bird Safe Philly and how to get involved.

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