Window Strike Reduction

Reducing Bird Collisions with Windows: How you can help!

By Richard A. Novak

Have you ever been at home and heard a “thud” on your window? Maybe you have seen a bird at the base of a building and wondered what happened to it. While birds striking windows may not be something you observe every day, it’s something that happens quite frequently.

Figure 1: Imprint left on a window from a bird collision (Credit: Creative Commons)

Unfortunately, bird collisions with windows is a leading cause of mortality among birds nationwide. Estimates of annual bird deaths from window collisions in the United States vary from 365 million to 988 million birds per year (Loss et al. 2014). Many species of birds are vulnerable to window collisions, including some species that are rare and of conservation concern. Bird collisions with buildings occur most often during the spring and fall periods when birds are making their migrations. 

Why do birds collide with windows? There are several factors that make a building’s windows risky to birds. First, if a window is reflective, a bird perceives that reflection as space that it can fly into. For example, if a window is reflecting a tree line that’s across from the building, a bird flying towards that window may not realize that image of a tree line it is seeing in the window it is seeing is actually not there, and unfortunately collide with the window. 

An additional risk to birds comes when windows create a pass through effect. This occurs when windows on both sides of a building allow seeing through to the other side. This also occurs when windows or large glass faces are situated on corners of buildings. Birds do not perceive glass as a barrier, and see these large window spaces as something they can fly through. 

Figure 2: This building has both risk factors: a tree line is visibly reflecting on the window surface, and a "pass-through" space is on the corner, giving the appearance that a bird could fly through (Credit: USFWS)

What can you do to help? Luckily, there are low-cost methods that homeowners can adopt in order to reduce the risk of birds colliding with their windows. These methods involve reducing reflectivity on the outside face of a window, or creating the effect of a barrier in pass-through spaces. These treatments include a variety of films, stickers, hanging strands, screens or other physical barriers, and related treatments that can be applied on or in front of a window in order to create a barrier that birds can perceive. More information about how these methods are used, where to purchase materials, and other information about reducing the risk of bird collisions can be found here.

When applying tape, strips, stickers, or paint to windows in order to create a barrier, the markings must be spaced in a manner that creates a space small enough that birds perceive it as a something that they cannot fit through while flying. The figure below provides a rule of thumb spacing for adequate window marking in order to create a visual barrier to most birds: 

Figure 3: Appropriate spacing of markings to create a barrier to birds (Credit: ABC & Roy Hancliff)

This guideline suggests that a spacing of 2” for horizontally lines, and 4” for vertically spaced lines will create enough of a “barrier” to birds that most will not attempt to fly through it, greatly reducing collisions. This spacing can be accomplished with tape strips, dots, or stickers applied directly to the outside surface of the window, or with paracord strips hanging outside of the window. 

Actions as simple as keeping blinds or curtains closed in your windows can have an effect on reducing bird collisions. Keeping blinds closed during the day can significantly cut down on the reflectivity of windows, and may have the added benefit of keeping your house cooler in the warmer months. 

While these actions may seem small, they can have a big effect on protecting bird species. The bird strike issue is very widespread, and every little bit of attention helps address the bigger problem. I encourage you to consider taking action at your home or business by identifying risky windows in your home or place of business and perhaps making some changes to the windows to reduce the risk to birds. If you have any questions or would like more information, please contact Richard Novak of the US Fish & Wildlife Service at richard_novak@fws.gov.