Photo credit: Ted, CC BY-SA 2.0 , via Wikimedia Commons

by Anita Hagy Ferguson

Window decals intended to reduce incidents of bird window strikes—one of the largest human-
made causes of bird mortality— are only effective if decals are placed on the outside of the
window, a new study shows.

Researchers found that the patterns on the films and decals placed on the internal surface of
windows do not reduce collision because they may not be sufficiently visible to birds.
Bird window strikes occur when a bird flying near a building cannot perceive a glass
windowpane and flies into it. These strikes are a significant concern for bird enthusiasts and
conservationists, many of whom advocate for applying visibly noticeable films, patterns, and
decals on surfaces of windows to alert birds of the glass.

Many people sympathetic to the potential of bird strikes around their homes or offices tend to
apply decals to the inside of their windowpanes, primarily because external application is often
logistically difficult and economically prohibitive, especially if the windows are above the first
floor of a building. However, the results of this new study show that only external application of
these decals can be associated with greater reductions in both window collisions and avian

Dr. John P. Swaddle, of William & Mary’s Institute for Integrative Conservation, worked with
students Blythe Brewster, Maddie Schuyler, and Anjie Su, to perform the first experimental study
to compare the effectiveness of two distinct window films when applied to either the internal or
external surface of double-glazed windows. The research team tested two different window film
products: BirdShades and Haverkamp. These products were selected for the test because they
engage with different wave lenths of light and colors visible to many songbirds.

Funding for the test of the BirdShades window film was provided by BirdShades Innovation
GmbH. Using these films, the research team tested the avoidance of window collisions by zebra
finches through controlled aviary flight trials. The team employed a design that allowed isolation
of the effect of the window treatments on avoidance flight behaviors. A fine mist net in front of
the windows prevented actual bird collision during the tests.

The team found consistent evidence that when applied to the external surface of windows, the
films resulted in reduced likelihood of collision. However, neither product was effective when
the films were applied to the internal surface of windows. Therefore, the results of this research
demonstrate the imperative that installers apply these products to exterior surfaces of windows to
maximize their protective benefits and reduce the risk of daytime window collision.

“Many people want to reduce bird-window collisions, as these unfortunate events kill hundreds
of millions of birds each year,” says Dr. Swaddle. “There are lots of decals and window films
that will likely make glass surfaces more visible to birds, decreasing collision risk. We were able
to show that people must apply decals and films to the external surface of their windows to benefit the birds. We want people to know this as we want their time and money to be well
spent—protecting the birds we all love.” Swaddle added.