Why we oppose the glass walls next to habitat. Information on Bird Mortality and glass surfaces.

Windows: The Bane of Birds (or ‘What a Pane!’)


By John Martin, Wildlife Biologist, U.S. Fish and Wildlife Service, San Diego National Wildlife Refuge 

On a Saturday morning, as I walk into the kitchen to top off my coffee, an unmistakable thud takes precedence over all the other little sounds in the house. I move into the living room, quickly running through and instantly dismissing a series of alternatives: a log settling in the stove, something on the radio, a distant car door…. No, that thud was a bird hitting the window, and I can see the ghostly splayed imprint of feather dust on the glass as I approach. I look over the sill, and on the ground beneath the window, on its back, wings quivering, is a splendid splash of black, white and red: a red-breasted sapsucker. I rush out the door, to either help the bird or recover the specimen, but in the seconds it takes for me to get there, the bird had regained its equilibrium enough to see a big, rapidly-approaching person, and fly across the road to disappear into the oaks and chaparral.
A happy ending? Probably not. How can a body as complex and delicate as a bird’s hit a solid object hard enough to make that sound, and fly away unscathed? If the woodpecker sustained any injury, he’d now be in the unforgiving woods, with his ability to keep warm, find food, and avoid predators compromised. It’s a grim outlook for this woodpecker, a continued risk for the rest of the birds in my yard, and a risk around the world wherever birds and glass share the airspace.
In 2014, scientists from U.S. Fish and Wildlife Service and the Smithsonian Institution analyzed 23 recent studies of bird/window collisions, to better estimate the magnitude of the problem. They estimated that window collisions kill 365-988 million birds annually in the United States. Of these, 44 percent occurred at residences, presumably because windows of homes are within the range of heights of vegetation that essentially defines habitat for most songbirds in North America. Only 44 percent? That’s still hundreds of millions of repetitions of the sapsucker collision at my house, every year, across the nation.
A depressing statistic, especially when I think of the enjoyment I get out of birds. When I’m out in the yard, or the oaks and chaparral in the neighborhood, it’s the birds that are the most conspicuous sparks of life, another song or flash of motion every few seconds, livening up the landscape.
Is anyone studying the mechanics of bird/window collisions, to better understand not only the size of the problem, but how windows kill birds, and what we can do to reduce the problem? Surprisingly few biologists have made bird/window collisions a focus of their research. But Dr. Daniel Klem Jr., Professor of Biology at Muhlenberg College, has built his professional career on this issue, publishing over a dozen papers elucidating the mechanisms of injury, the factors that affect the likelihood of collision, and what we can do to reduce collisions and injuries.
Dr. Klem has taken a closer look at exactly what happens to birds that strike windows, and found a surprising result. Though it’s commonly assumed that birds that die striking windows die from a broken neck, this isn’t the case. It makes sense: breaking a bird’s neck is like trying to break a rope by pushing it into a solid object. Birds have 13-27 neck vertebrae (compared to 7 in mammals such as ourselves), and the articulations between them are especially flexible. Dissection of hundreds of birds killed by window strikes reveals that none of them had broken necks. But essentially all of them suffered ruptured blood vessels in the cranium, and died of intracranial bleeding and/or associated damage to the brain. Most birds that have apparently “recovered” after striking windows also suffered intracranial bleeding. A bird that regains consciousness and flies off after hitting a window isn’t necessarily in the clear: it may have recovered enough to seek shelter, and die later of its injuries.
What can we do to reduce bird mortality at our own windows? One approach is to reduce the likelihood that birds will fly into the window. Birds presumably strike windows either because the transparent glass creates the illusion that they can fly straight to the habitat that they see beyond the invisible barrier, or because they see habitat reflected in the smooth surface of the glass. In either case, they don’t perceive the glass as a barrier. To allow birds to recognize the glass as a barrier, people have tried applying opaque objects or patterns to windows. Isolated objects or patterns (such as falcon or owl silhouettes) don’t work: the bird sees the object, but not the window. The birds attempt to fly around the falcon graphic, and in avoiding the object they smack into the adjacent glass. Experiments by Dr. Klem and other researchers indicate that to deter birds in an aviary from flying through an empty window frame, the critical dimension for visual obstructions (objects or patterns on the glass) is about 2 inches apart horizontally, and 4 inches apart vertically. Windows marked with patterns of dots, stripes, or other shapes (at least ¼ inch wide) effectively show birds that the window is a barrier, and greatly reduce or eliminate window collisions. There are several brands of commercially available window films that apply a pattern sufficient to deter birds, or you can design your own with opaque tape (such as American Bird Conservancy’s BirdTape). It’s important to apply the pattern to the outside of the glass, rather than the inside, to allow the pattern to disrupt the reflected image of an unobstructed flight path, as well as the transparent window.
Unfortunately, obscuring the window sufficiently to eliminate bird strikes contradicts the whole point of having glass windows: we want to see through them. Yet it may be possible to make a pattern on the window that birds can see and we can’t! Generally, birds can see a wider spectrum of light than humans, including ultravio-let wavelengths. Experiments have been conducted to determine if glass marked with a pattern of materials that absorb or reflect ultraviolet light (invisible to us) dissuades birds from flying into the glass. Results of laboratory experiments are promising, but investigators note that under natural, outdoor light, the ultraviolet-marked glass may not function as it does in the laboratory. Until bird mortality at glass windows is more widely-recognized as a problem, glass manufacturers are unlikely to invest in the research, development, and marketing of bird-friendly glass. But ornithologists continue to research this potential solution.
A different approach to reducing bird mortality due to window strikes is to reduce the force with which birds hit windows. Kinetic energy (say, of a flying bird) = ½ (mass)(velocity)2. Can’t do much about the mass, but we can help reduce the velocity at which birds are moving when they hit the window, reducing their kinetic energy and thus the damage that occurs inside their skull when it abruptly stops at the glass. If you have a bird feeder, place it within 3 feet of a window. Birds leaving the feeder will not have built up sufficient speed to hit the glass hard, so are less likely to get hurt striking the window. Researchers have also investigated reducing bird mortality by installing windows at an angle (20-40˚ from vertical), such that birds are deflected when they hit the window. Under laboratory conditions, birds are indeed less likely to die when flying into an angled window, but in nature, where birds can approach the glass from a variety of angles, it’s clear that fatal collisions still result from birds striking angled windows. Installation of bird netting (such as that used to protect fruit trees from birds) over the entire window several inches out from the surface, has the potential to reduce the force of impact of a bird striking the window. Netting may also reduce the likelihood of a bird striking the window in the first place, because the birds may see it.
If you’re looking for more detailed information on reducing bird mortality on glass in your home, a good place to start is the American Bird Conservancy’s web page devoted to the issue:www.abcbirds.org/abcprograms/policy/collisions/glass.html. At my house, I believe a pattern of tape on my living room window is in order. Sapsuckers are back for the winter, and I don’t want to hear that unmistakable thump again.

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