Human-related activities account for a significant number of bird deaths. Habitat destruction is clearly the most important but the next most important factor is collisions with clear and reflective sheet glass. Such collisions account for at least a billion bird deaths each year in the United States. These deaths are spread over at least 225 species. Compare this mortality with deaths from other human-related factors: 120 million from hunting, 60 million from collisions with moving vehicles, 400,000 from wind turbines and potentially hundreds of millions by cats.

Dan Klem, a professor of biology at Muhlenberg College, has been studying bird kills from glass collisions for the past 35 years. Much of today’s column comes from Klem’s extensive research.

Why do so many birds die from flying into windows? There is little evidence to indicate that birds that hit windows have faulty eyesight. Reduced vision in fog and smoke also plays only a minor role in explaining window kills. The chief explanation seems to be that birds perceive clear windows as open space and attempt to fly through. The likelihood of window kills is increased if there are windows on both sides of a building, producing a see-through effect. Windows with reflective surfaces act as mirrors, fooling birds into thinking that flight is possible through the window.

Bird feeding increases window kills by attracting birds to buildings. The high concentrations of birds at feeders, particularly in the winter, attract bird predators like Sharp-shinned Hawks or Cooper’s Hawks. The appearance of such a predator, loud noises or sudden movements, may cause a panic flight of birds from a feeder with the possibility of fatal window strikes. A short flight of only one meter can produce a fatal collision.

Klem examined 300 fatalities and 31 survivors to determine the types of injury, causes of death, and recovery from collisions. Every fatality showed intracranial bleeding, suggesting that death was caused by ruptured blood vessels in the head and brain damage. Survivors often sustained intracranial bleeding as well and may have died later on from the injury. Bone fractures were relatively rare, indicating the surprising strength of the light bones of birds.

If you find a stunned bird after a window collision, place the bird in a protected enclosure (I use a paper bag), keep the enclosure warm, and provide the bird with food and water.

What can homeowners do to minimize window strikes by birds? The best solution, to eliminate windows altogether, is not acceptable. Covering windows with netting can be expensive and affects the aesthetics of a building. Making windows obvious as obstacles that should be avoided can reduce window kills. Spiders use this strategy to protect their webs. The webs of many spiders have thick, highly visible strands. Flying birds easily see these strands and avoid flying through the webs. Klem’s research has shown that placing vertical strips of tape on windows cuts down on window strikes dramatically, particularly if the strips are separated by four inches or less. Horizontal strips are less effective. The black silhouettes of hawks and falcons that can be purchased for placement on windows are not effective unless they are stacked closely together. These silhouettes do not provide enough contrast for birds to realize that windows are obstacles. In new or remodeled homes, architects are encouraged to install windows at a slight angle so that they reflect the ground rather than trees or the sky. Birds are not likely to try to fly into a reflection of the ground. In addition, birds are likely to hit an angled window at an angle and thus soften the force from the collision with the glass.

The placement of bird feeders can also curtail window kills. Klem’s studies have shown that placing feeders very close to a window reduces fatal window strikes during panic flights. Although birds may hit the window after they fly up in response to a loud noise or a predator, the birds will not be flying fast enough to suffer harm or death from the collision. An alternative solution is to place your feeders well away from the nearest window. So, the best advice is to either place your feeders within three feet of a window or at least 30 feet away from the window.

One of Klem’s recent publications involved the assistance of 30 trained volunteers from the New York City Audubon Society. The purpose of the study was to determine building and landscape characteristics associated with higher risk of collisions with migrating birds in an urban environment. The research was done in Manhattan. The volunteers monitored 73 facades of buildings throughout the island borough.

The team recorded 475 bird collisions in the fall of 2006 and 74 collisions were in the spring of 2007. Their analyses provide good advice for architectural and landscape designs to reduce collisions: reducing the number of reflective panes in windows, keeping trees away from walls, and reducing ground cover.

[First published September 19, 2009]