Towerkill is a phenomenon in which large numbers of birds are killed by antenna towers

Radio masts and towers

Radio masts and towers are, typically, tall structures designed to support antennas for telecommunications and broadcasting, including television. They are among the tallest man-made structures...

. The U.S. Fish and Wildlife Service estimates that between 5 and 50 million birds are killed in the U.S. each year by towerkill. Window crashes, by comparison, kill between 100 and 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 phenomenon 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. In at least one instance, several thousand birds were killed at a single tower in one night. Additionally, the unnatural lights on communication towers disrupt bird migration patterns in ways that are still not fully understood. At least 231 species have been affected, with neotropical migrants making up a large proportion of all species killed.
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 (60.7 m), or on shorter towers if they are near airports. In 2008 it is estimated there were roughly 125,000 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 feet (307.8 m) tower at Tall Timbers Research Station near Tallahassee, Florida

Tallahassee, Florida

Tallahassee is the capital of the U.S. state of Florida. It is the county seat and only incorporated municipality in Leon County, and is the 128th largest city in the United States. Tallahassee became the capital of Florida, then the Florida Territory, in 1824. In 2010, the population recorded by...

, 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.

Wind turbines

Wind turbines represent a much smaller threat to birds, due to being much lower in number and lacking guy-wires. It is estimated wind turbines kill 10,000 to 40,000 birds a year, which is very small compared to communication towers, which kill 40 to 60 million a year. Overall, wind turbines kill about one-tenth of a percent of all unnatural bird deaths in the United States each year.

External links

• Towerkill.com - database of towers and research bibliography
• Avian Mortality at Communications Towers - workshop proceedings
• Bird Kills at Towers and Other Human-Made Structures - a partial annotated bibliography (1960-1998) (via archive.org)
• PUTTING WIND POWER'S EFFECT ON BIRDS IN PERSPECTIVE