Towerkill

Guy wires poise a fatal threat to birds, who often circle the tower lights at night.

Towerkill is a phenomena where large numbers of birds are killed by antenna towers. The U.S. Fish and Wildlife Service estimates that between 5 and 50 million birds are killed in the U.S. each year after by towerkill. Window crashes, by comparison, kill between 100 to 900 million birds a year, and are still by far the largest human related cause of bird death. The effect on overall bird populations by towerkill may be small, but the phenomena is of considerable concern to ornithologists because many endangered bird species are being killed, and because so many birds are killed in such a small area of land. Also, the unnatural lights on communication towers disrupt bird migration patterns in ways that are still being researched.

There are two mechanisms of bird death due to communications towers. The first is the "blind kill" where birds flying in poor visibility do not see the guy wires in time to avoid them. This is more of a threat for faster flying birds such as waterfowl or shorebirds. Slower and more agile birds, such as songbirds, are not as likely to succumb to blind collision.

Communications towers that are lighted at night for aviation safety may help reduce bird collisions caused by poor visibility, but they bring about a second, even more deadly mechanism for mortality. When there is a low cloud ceiling, hazy or foggy conditions, lights on a tower reflect off water or other particles in the air creating an illuminated area around the tower. Migrating birds loose their stellar cues for nocturnal migration in such conditions. In addition, they often loose any broad orienting perspective they might have had on the landscape. When passing the lighted area, it may be that the increased visibility around the tower becomes the strongest cue the birds have for navigation, and thus they tend to remain in the lighted space near the tower, afraid to leave. Mortality occurs when they run into the structure and its guy wires, or even other migrating birds as more and more passing birds aggregate in the relatively small, lighted space. It is important to clarify that the lights are not documented to attract birds from afar, but appear to hold birds that fly into the illuminated vicinity. Lights are required by the FCC on any tower taller than 199 feet, or on shorter towers if they are near airports. There are roughly 80,000 lit towers in the US and more than 7,000 new towers are constructed each year.

In a 25-year study of bird mortality at the 1010-foot tower at Tall Timbers Research Station near Tallahassee, Florida, kills occurred nearly every night from mid-August through mid-November. Moderate numbers of migrants were killed under perfectly clear skies, but the toll increased markedly with overcast conditions. Researchers believe the attraction to lighted regions results in most towerkill,and numerous studies have been conducted to further understand the phenomena.

External links

• Towerkill.com-database of towers and research bibliography
• Avian Mortality at Communications Towers-Workshop Proceedings