I was horrified at what I had just witnessed. Just as I had pulled into the driveway, a bird flew into the open skylight window, bounced off, tumbled down the roof and landed a few feet away from me.
It was a female rose-breasted grosbeak.
It all happened so fast, I was never able to determine the cause. Perhaps a raptor in the vicinity had caused the bird to fly in panic, and the bird thought it had a foot more of leeway from the roof. The impact killed it instantly.
Some birds aren’t so lucky. These are the ones that initially appear stunned upon impacting a window, only to seemingly recover and fly off. However, often these birds die later from internal bleeding, especially of the brain, according to an article titled, “Keeping Birds Safe around Windows,” by the Cornell Lab of Ornithology.
The article goes on to quote Dr. Daniel Klem of Muhlenberg College, who has researched this issue since the 1970s.
“Intensive studies at single homes reveal one out of every two strikes results in a fatality.” Klem said. “Glass is an indiscriminate killer that takes the fit as well as the unfit of a species population.”
In a 1989 publication of the Wilson Journal of Ornithology, Klem detailed the study methods he used to determine why birds strike windows. He reviewed extensive scientific literature (he writes that the earliest account of a bird-window collision is of a sharp-shinned hawk that had smashed part-way through greenhouse glass in pursuit of prey, in 1832); collected data from museums and individuals, monitored human-made structures, and even conducted field experiments.
The results of his study were clear — birds simply do not see the glass and cannot perceive it as a barrier, regardless of the age of the bird, the type or location of the structure, or the prevailing weather conditions. This refuted a few earlier theories, such as that only young, inexperienced birds struck windows; or those that struck windows had defective eyesight for some reason.
Glass deceives birds in two ways. It reflects the surrounding sky and foliage (even tinted glass does this, Klem discovered), which birds cannot distinguish from the real thing. When windows or glass doors are situated at either side of a building, or at right angles to each other (such as on the corner of a building), it also creates the illusion of a fly-through corridor.
The popularity of bird feeders is an added magnet that draws birds nearer to buildings; ironically, this effort to help birds often puts them in danger of a collision. But this isn’t the only factor in window strikes.
Any building that is surrounded by habitat that provides good foraging, perching, and cover for birds can also present a threat. As illustrated above, any startled bird that flees in panic from a predator (or perceived predator), may view a tree’s reflection in a window as safety.
There are a number of ways to help reduce window collisions or prevent them entirely, however.
First, if you have feeders, relocate them. Placing them a good distance away (at least 30 feet, according to the National Audubon Society), may reduce the chance of a bird flying toward a reflection to seek safety — especially if there is adequate cover nearby.
To learn more about this and how you can prevent window strikes in your home and workplace, go to:
The Web site for Dr. Daniel Klem at www.muhlenberg.edu/depts/biology/faculty/klem/ACO/Klem.htm
The Bird Conservation Network’s “Window Collision Fact Sheet:” www.bcnbirds.org/window_files/WindowCollisionFactSheet.pdf
The American Bird Conservancy’s report on Mortality Threat to Birds: www.abcbirds.org/conservationissues/threats/buildings.html
The National Audubon Society, “Minimizing Window Collisions:” www.audubon.org/bird/at_home/SafeWindows.html
As an alternative, placing a feeder within three feet of a window (or mounting it on or near the window itself), may prevent visiting birds from sustaining injuries if they do hit the window. The thinking here is that the bird will not have enough time or distance to reach top flight speed before colliding with the window.
Contrary to popular belief, one or two hawk silhouette decals do not deter birds from flying toward or into windows, Klem found. Any decal — no matter what it looks like — can only be effective if enough of them are used that they cover the entire window. Spaces between decals need to be no more than 10 centimeters (four inches) apart for this method be effective. The same applies to strings of beads or feathers hung in front of windows.
Keeping any window screens in place year-round, if possible, will cut down on the transparency and reflectivity of glass, as will keeping blinds and shades drawn when not at home, or keeping vertical blinds half-closed. Fine mesh netting stretched tightly in front of windows will also cause birds to bounce away before striking the glass.
Window strike mortality for birds has been a grave concern for many years and continues to grow as more development occurs. Klem and other researchers, after performing extensive, long-term studies, now estimate between 97 million and 975 million birds are killed each year in the United States alone.
If those seem like unrealistically high numbers, Klem clarifies it by explaining that “the roughly 100 million to 1 billion toll is based on the assumption that 1-10 birds are killed at one building in the U.S. each year.”