Towerkill

Towerkill is a phenomenon in which 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 by tower kill. Window crashes, by comparison, kill between 100 and 900 million birds a year, and are still by far the largest human-related cause of wild 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 lose their stellar cues for nocturnal migration in such conditions. In addition, they often lose 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 (61 m), or on shorter towers if they are near airports. In 2008, it was estimated there were roughly 125,000 lit towers in the U.S. and more than 7,000 new towers are constructed each year.

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