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Windows and Bird Mortality

North American bird species face natural hazards year-round, hazards including severe weather, food shortages and predators. In addition to these inherent survival challenges, wild birds also confront human-related hazards throughout their summer breeding territories, along migration routes and across wintering grounds. Ubiquitous threats such as habitat destruction, secondary poisoning from pesticides and herbicides, outdoor pet cats and feral cats, oil spills, high tension wires, wind turbines, communica-tion towers and vehicles along our roadways pose lethal dangers for common and abundant bird species and for rare, threatened and endangered species.

Surprisingly, of all the human-related bird mortality factors, bird deaths from striking windows ranks one step from the top of the list – second only to the degrading and loss of habitat. Each year, a staggering number of birds are killed in the United States because they collide with windows on homes and buildings. Scientists estimate that between 100 million and one billion birds perish annually as a result of win-dow strikes in our country alone. More than 272 bird species have been documented dying in window strikes, which is nearly one-third of the bird species found in the United States. Since Pennsylvania has a mix of houses and good bird habitat, it is important that birders know about this mortality factor, especially since we may be inadvertently adding to the dangers to birds on our property. At the fate of natural hazards, weak and unhealthy birds perish more often than strong and healthy birds. It is nature's way of preserving the fittest, healthiest and genetically well-suited birds of a species for breeding. Window strikes, however, kill all types, those in their prime along with the sick or weak. Windows indiscrimi-nately claim the lives of young birds, aging birds, males and females.

All buildings with reflective and transparent sheet glass or plastic hold the potential to kill birds, and the more glass on a structure, the greater the hazard. Research shows a single skyscraper can kill as many as 200 birds in one day. The average home is responsible for one to 10 fatal strikes per year; a mere fraction when compared to skyscraper death tolls, until you consider there are more than 100 million houses across the United States. Many Pennsylvania homes are placed in good bird habitat, like woods or fields, so the homes of birdwatchers who live in the country may be particularly dangerous for birds because we choose to live in good bird habitat. Appropriately, our state has been important to the research about glass-caused bird mortality. Dr. Daniel Klem of Muhlenberg College has been a pioneer in this research.

Birds fail to recognize a window as a solid surface, especially the large expanses of glass in picture win-dows and glass doors. They see only the reflected image which is most often sky or landscape. To a bird, the mirrored scene of a cloudy sky, a foliage covered branch or water source looks authentic and attractive. The reflectivity of glass may vary with the material and with time of day, weather and angle of view, but many windows, especially those with a mirrored finish, pose a constant danger to birds.

With transparent glass situations, a bird may see through a window to the space inside, unaware of the barrier between itself and an indoor plant, potential perch site or escape cover. Two transparent windows situated parallel at opposite sides of a structure or perpendicular as on the corners of a building hold an added danger by creating the illusion of a passageway into familiar habitat. Buildings and structures har-boring this type of deadly flight path take a high toll on birds.

Too often, birds make the fatal mistake of trying to fly through a passage or into a reflected habitat or appealing inside space while foraging or trying to elude a predator. The odds are grim, as one out of two window strikes results in death. Flight angle and momentum determine the outcome of a collision and birds may be stunned, injured, or killed outright. Many birds die instantly but a large number of victims fly away, seemingly okay, only to die soon after from head trauma or internal injuries.

Stunned and injured birds frequently fall prey to dogs, cats, raccoons, skunks, opossums and crows. In rural areas, throughout suburban neighborhoods and across cities, opportunistic predators, instinctively aware of an easy meal, have learned to routinely forage the area below collision prone windows. They wait in ambush for a crash to occur or patrol a known site looking for stunned or injured birds and bird carcasses.

Birds are undoubtedly drawn to properties with diverse habitats and bird-friendly landscapes, as well as houses with bird feeders nearby. This increase in bird traffic and activity increases the danger to birds and the risk of deadly window strikes. To alleviate the danger our homes and businesses present, it is important to identify problem areas and keep birds away from the façade of our homes and buildings. We can reduce and eliminate window collision dangers by implementing practical solutions or a combination of solutions to keep birds safe while maintaining unobstructed views and keeping the aesthetic value of our windows.

Possible Solutions

New construction and renovation:
In the planning stage of home building or remodeling, consider window designs that utilize smaller lights with muntin bars or multiple windows to fill a space. These choices will break up the solid expanses of reflective glass on a home. For example: a five-light bow window may pose less of a bird strike risk than a three-light bow window and three casement windows with thick mullions pose less of a danger than one large picture window. Surface textures on many designer glass products reduce the overall reflective surface of a window and in addition to being more bird-friendly than a plain glass window, these widely available style choices add curb appeal to a home.

Glass blocks and stained glass windows lend a unique design element to homes and buildings and do not cause bird collisions. Consider incorporating glass blocks and stained glass window panes into the architecture of new buildings and renovation projects.

Building styles with recessed windows and doors reflect much less of the surrounding habitat than standard flush-mounted windows. They also limit the angles at which a bird can fly from. This type of design is more prevalent on historic buildings, especially stone build-ings, and in buildings with complex architectural ele-ments.

Existing Windows
One of the most effective methods of eliminating bird strikes at existing problem windows is to string para-chute cord on four inch centers across a window. This treatment has been found to reduce strikes by 97% and does not pose an inconvenience for routine window cleaning.

Although a bit more involved, another solution is to install exterior mesh netting across window panes. A fine, fabric netting, also called fruit tree netting or crop netting, nearly eliminates a window's reflection and serves as a safety net if a bird flies into it. When it is installed properly, birds usually bounce off it unharmed.

The net must be two to three inches away from the window and be stretched taut across the surface. Several bird screen products provide complete mounting materials and installation instructions.

Research has shown that most birds will not attempt to fly through narrow horizontal spaces of less than two inches high and vertical spaces of four inches or less. Using this 2- x 4-inch rule in a pattern, apply a tape to the outside surface of problem windows. Bird tape is an inexpensive and effective solution for prob-lem windows.

Decorative window films created for privacy, sun shading and design accent can make windows safer for birds if the pattern intervals match the 2- by 4-inch rule and if the films are placed on the exterior surface of the window. These aesthetically pleasing films are affordable and easy to install.

Window decals, when patterned to uniformly cover the surface using the 2- by 4-inch rule, significantly reduce bird strikes. All reflected spaces between decals should not exceed four inches apart when arranged vertically and two inches apart when arranged horizontally.

Consider reglazing or modifying windows with sandblasting or acid etching to create fritted, etched, frosted or translucent panes.

Relocate bird feeders to within three feet of the house. At close range, a bird usually does not build enough momentum for a fatal window strike.

Keep birds away from the structure by spacing trees and shrubs farther away from the building.

Draw shades during the day when possible. Light colored shades, blinds and curtain panels reduce the transparency and exterior reflection better than dark colors.

For problem transparent windows, relocate a house plant known to attract birds. Or, conceal the plant behind window blinds or a curtain panel.

Display artwork at the glass surface so it is clearly visible at all angles when looking in. For large expanses of glass, hang multiple works of art.

Consider leaving summer window screens in double-hung windows year-round. These screens cover half of the re-flective surface thus reducing the danger of each window by half.

Put off cleaning exterior problem windows as a layer of dust can cut down on the reflection. Sparkling clean windows are particularly hazardous.

While shade does not eliminate window strikes, awnings, louvered shutters, sunshades, extended roof overhangs or a porch roof all reduce window reflection and are safer than windows in full sun.

Dangers in the Dark

Birds that migrate at night are at an increased risk of collisions with windows and buildings because of the light pollution engulfing our cities and towns. At night, manmade lights and sky-glow lure migrating birds off flight courses and into urban settings, especially when visibility is hampered by the fog, clouds or precipitation of bad weather. Birds, often large flocks of birds, become disoriented and even trapped by artificial light which includes light escaping from inside a structure and exterior beams of light. The lights and reflections of brightly lit skyscrapers and buildings, communication towers, lighted activity centers and other sources can be fatal to many birds, but particularly those migrating in spring and fall.

At night, urban settings are unsuitable and hostile places for migrating birds. Birds disoriented by city lights fly in bewildering circles within the unnatural light. They fly haphazardly and often succumb to exhaustion. Many birds in this situation collide with other birds, with building windows or plummet to their death from exhaustion. Those that survive the night face new challenges at dawn in this unfamiliar setting, such as navigating safely through myriad reflective buildings and windows, finding food sources to replenish vital energy stores needed to continue their migration and evade predators.

We can implement a few simple strategies around our homes and workplaces to reduce light pollution and make nighttime environments less dangerous to birds. Implementing these strategies during bird migration periods is particularly important.

Interior Lighting Considerations:
  • Pull curtains, shades and blinds after dark to minimize your home's night-glow.
  • Develop the habit of using work space task lights instead of lighting an entire room.
  • Use automatic timers to turn indoor night lighting off between hours when lighting is not necessary.
Interior Lighting Considerations:
  • Install motion-detecting light fixtures and automatic timers when lighting areas such as driveways, walkways and entrances where appropriate.
  • Pull curtains, shades and blinds after dark to minimize your home's night-glow.
  • Develop the habit of using work space task lights instead of lighting an entire room.
  • Only purchase outdoor lights with efficient designs that direct all light downward.
  • Replace existing fixtures that spill light beyond the needed space.
  • Carefully consider exterior lighting and eliminate all unnecessary lights.
  • Use concentrated guidance lighting at low heights along paths, edges and walkways.

For additional information, try the following online references:
Acopian Center for Ornithology, Muhlenberg College
Birds and Buildings Forum
International Dark Skies Association
American Bird Conservancy
Fatal Light Awareness Program

No portlets in this column.
Pennsylvania Game Commission, 2001 Elmerton Ave, Harrisburg Pennsylvania 17110-9797