Authored by Irene Fedun

Bird collisions with glass are as old as the product itself (according to historical data, it was likely made as early as 290 C.E.).  But until the late 1900s, no efforts were made to reduce the incidence of often-deadly strikes on a large scale.  Dr. Daniel Klem Jr., Sarkis Acopian Professor of Ornithology and Conservation Biology at Muhlenberg College in Pennsylvania, was the first to comprehensively study the problem, recognizing that millions of birds around the world die unnecessary deaths by hitting windows.

Dr. Klem, what made you decide to do your doctoral study on bird-window collisions?

In 1974, my supervising advisor, Dr. William G. George of the Department of Zoology at Southern Illinois University at Carbondale, suggested that I dedicate my doctoral dissertation to the issue of window-killed birds.  On my first research day, I arose before dawn, sat in front of the all-glass façade of our chemistry building where others had collected birds. As the dark gave way to light, a Mourning Dove flew through the upper branches of the mature, leafless oak about 10 metres (30 feet) from the glass wall and hit it with full force. The bird died on impact. Beneath the glass panels of other campus buildings I found the remains of what appeared to be previous strike victims. My curiosity turned into a commitment to find out more.

You have done numerous tests to find sustainable glazing solutions, starting with tunnel tests and then introducing field tests. What are the biggest differences between the tests and what are your key findings?

As part of my doctoral research, I designed a flight cage to test if birds could discriminate between clear air space and clear glass. The results indicated they could not. I then set up the flight cage to evaluate if birds could discriminate the clear air space on one side of the cage from an obstructed side which had various items on the window. A hawk silhouette and other objects were suspended on monofilament line to appear as if they were affixed to a window. Incremental numbers of white stripes with varying spaces between them were tacked to the top and bottom of the window-space frame to determine the distance between stripes that resulted in birds avoiding the obstructed side of the cage. I wanted to determine what, if any, items could be used to deter bird strikes. Birds that were being tested were released at a narrow end of the cage and stimulated to fly to the opposite, more lighted end toward what appeared to be a flight path to safety. This flight cage design to test bird-window collision (BWC) preventive methods is called tunnel testing.

Tunnel testing is valuable in that it can test several potential preventive measures at one time, but it does not accurately reflect what a bird experiences when faced with a real window.  Birds are released in a dark area of the tunnel and stimulated to fly to the bright end, making a choice to move to safety on either side of the escape route. In real life, birds are in the bright environment and fly into darker areas trying to reach habitat and sky on the other side of clear panes or they fly into reflections of habitat and sky in mirrored panes. The test’s limitations encouraged me to develop an experimental design – a field test – that would accurately simulate windows in real buildings.

A field test consists of comparing the rate of bird strikes at conventional clear and reflective windows which serve as controls, and windows that are modified to deter bird strikes.

I’ve used both testing methods and judge that tunnel testing is useful, but not as a final assessment. It should not be used as an industry standard to evaluate the effectiveness of BWC prevention.

The Canadian Standards Association recently published a bird-deterrence glazing standard, and you were asked to participate in the process as an expert. What are the key elements of this standard?

I endorse the following recommendations of Bird-friendly building design (CSA A460:19) to transform sheet glass into barriers that birds will see and avoid: use acid etching, ceramic frit, or various markers printed on film applied to the glass surface. The individual markers are elements of an overall pattern uniformly covering the window surface. The elements can be any shape (dots, lines, diamonds, etc.), but must be separated by 5 cm (2 in) when oriented in vertical columns or horizontal rows, the 2 x 2 Rule.

Another essential recommendation: place the markers on the outside surface, what architects call Surface #1. The reason: almost all glass is installed such that it covers a dark interior space of a building. Even a perfectly clear piece of glass acts as a perfect mirror when viewed from outside when covering a dark interior. If markers to prevent BWCs are placed on an inner surface of a multi-paned window, the reflection off the outside Surface #1 will hide the very elements being used to prevent a bird strike. There are exceptions, so-called fly-through illusions, such as those created where clear panes are placed on either side of corridors.  It’s also essential to treat sheet glass to make it bird safe to a minimum height of 16 metres (52.5 ft) above grade, or higher if mature vegetation is reflected in the building façade.

You have been studying bird deterrence for over 40 years. With the recent legal measures throughout North America and the newly adopted CSA A460 standard, what is the future for bird deterrence legislation? Do you think A460 will influence future development?

Our federal bird protection treaties and laws address the unintended killing of birds at sheet glass, but they are mired in confusion about their relevance and use based on differing court judgments. The growing trend is for regional and municipal governments to enact bird-safe legislation, such as in Toronto, a model of what can be done to protect birds from the fatal effects of glass. I predict the CSA A460:19 will be a like model, guiding regional and municipal laws, ordinances, zoning regulations and other legislation to make the human-built environment safe for birds.



The Fatal Light Awareness Program (FLAP) Canada, a non-profit  organization, was created in 1993 to address the issue of migratory birds hitting lit office towers during their nocturnal migration.  It soon became apparent that daytime collisions with sheet glass, whether in large commercial buildings or modest houses, was a much greater cause of bird mortality. FLAP expanded its mandate to cover both day and night strikes. Collaborations with glass and window film manufacturers, architects, builders, building owners and managers, environmental groups, governments and biologists, such as Dr. Klem, followed to find long-term, affordable solutions to the problem.   FLAP’s mission:  Safeguarding migratory birds in the built environment through education, policy development, research, rescue and rehabilitation.

Irene Fedun is the editor of FLAP’s biannual newsletter, Touching Down.  Visit and for details on how to prevent bird collisions with buildings.