Glass—An Unintended but Catastrophic Hazard for Birds

by Daniel Klem, Jr., Ph.D. Acopian Professor of Ornithology and Conservation Biology, Muhlenberg College, Allentown, Pennsylvania

A clear window with no light from behind appears as a mirror when viewed from outside this rural home.


An adult male Rose-breasted Grosbeak killed from striking a clear patio glass door in a suburban home.


Wild birds are killing themselves by flying into buildings, power lines, cell, radio and television towers, and motor vehicles. Bird strikes at buildings can occur during the day or night. Nighttime strikes are restricted to the migratory season, the fall and spring in North America, and like tower strikes, they typically occur under inclement weather when cloud cover forces birds to fly at lower altitudes. Lights associated with tall structures such as skyscrapers in cities attract migratory birds in passage, especially under inclement weather. Seemingly like moths to a flame they enter and exit beams of light that appear to confuse them. In their confusion, some individuals actually strike the opaque surface of structures. Most, however, fly about and eventually flutter to the ground exhausted and vulnerable in a city’s landscape of asphalt streets and concrete canyons. At first light these birds become vulnerable to the clear and reflective glass panes, present in just about every human structure, in sizes a few inches on a side to entire walls covering multistory buildings. Nighttime collisions can be simply eliminated by turning off lights illuminating tall structures. When followed, organized “lights-out programs” have proven successful in the metropolitan areas of Chicago, New York, and Toronto where thousands of birds, among them the rare, threatened and endangered, have been saved by darkening the skyline.

Daytime strikes occur exclusively at clear and reflective glass, and the overall bird kill is estimated to be greater than any other human associated avian mortality factor, greater than pesticides, greater than that attributable to cats, exponentially greater than any other collision source, and only exceeded in its potential threat to bird populations by the destruction of the very habitat required for the fundamental survival of a species. To dramatize and put the attrition at glass in perspective, consider that the world would have to experience 333 Exxon Valdez oil spills every year to equal the most conservative glass kill figure estimated for the U.S. alone. The Exxon Valdez released 260,000 barrels of crude oil into Alaska’s Prince William Sound on 24 March 1989, and the spill was estimated to have killed from 100,000 to 300,000 marine birds.

From 100 million to 1 billion birds are annually estimated to be killed striking clear and reflective windows in the U.S. The yearly death toll is in the billions worldwide. Observations and experiments over more than 30 years have revealed that birds act as if clear and reflective panes are invisible. They attempt to fly to habitat seen through a clear glass corridor or to vegetation or sky mirrored in reflective panes. The result is that there is no time of day, season, location, window orientation, or weather condition in which birds are able to elude the hazard. Lethal collisions are possible whenever and wherever birds and glass mutually occur, and the best predictor of the number of fatalities at any one site is the density of birds in the vicinity of windows.

Media attention to this unintended slaughter is increasing but typically occurs in the fall and spring in North America when migrants are killed on passage and their bodies are most visible in front of stores on barren sidewalks of populated cities. But, in fact, most fatalities occur in the non-breeding winter period at north temperate latitudes when large numbers of birds are attracted to feeders that are almost exclusively placed between zero and 30 feet from windows so homeowners and those at park building centers can see feeder visitors up close.

A dark tinted wall of glass in a suburban college campus building shows the reflective effect by mirroring the facing habitat and sky.

A corridor with walls of clear glass in a rural university campus building shows the see-through effect with habitat visible from either side. photos by Daniel Klem, Jr., Ph.D.

From the continuous monitoring of individual homes and other experiments it is known that one out of every two glass strikes result in a fatality. The handling of freshly killed collision victims most often suggests that death was caused by a broken neck. However, detailed examinations of glass fatalities that include over 250 x-rays reveal no broken necks. Birds die from striking windows for the same reasons humans succumb to collisions resulting in severe head trauma. Death results when the brain swells within the skull causing internal tissue damage and bleeding.

Currently, there are many solutions that reduce or eliminate daytime bird strikes, but none are universally applicable or readily acceptable for all human structures. Protective measures range from physical barriers such as netting that keep birds from striking to detractants that protect by transforming the area occupied by glass into uninviting space or a recognizable obstacle to be avoided. The manufacture of new varieties of sheet glass is recommended: panes having external patterns that alert birds to its presence but retain the current unaltered view from inside. The angling of windows at 20 and 40 degrees from vertical reduces the number of lethal strikes, but these steep orientations are likely to be practical only at single story buildings or at ground level of multistory structures. Placement of bird feeders within three feet of the glass surface eliminates the hazard because birds cannot build up enough momentum to injure themselves.

Guilt and anxiety are common feelings among an increasing number of people who discover an accidental fatality beneath the window of their home, workplace, or any other structure. Those interested in protecting birds as a vital part of the Earth’s natural resources and those architects, developers, glass manufacturers, and landscape planners seeking to accommodate human needs and interests must work together if we are to effectively address this unintended and indiscriminate slaughter of one of nature’s most exquisite creations.

You Can Make a Difference
Help reduce the number of fatal bird strikes by contacting Fatal Light Awareness Program (FLAP) at www.flap.org or call (416) 366-FLAP. As more buildings arise it is important for developers to consider the impact on birds and their migratory paths. Please contact the following groups, which do not currently have a policy regarding bird strikes, and urge them to develop programs that promote bird friendly buildings.

• S. Richard Fedrizzi, President, U.S. Green Building Council, 1015 18th Street, NW, Suite 508, Washington, DC 20036; email: rfedrizzi@usgbc.org.  USGBC is a building industry coalition to promote structures that are, among other things, environmentally responsible.

Eugene C. Hopkins, President, The American Institute of Architects, 1735 New York Ave., NW, Washington, DC 20006-5292; email: arutledge@aia.org. AIA serves as the “voice of the architecture profession” and hopes to inspire “creation of a better built environment.”