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Bird kills at tall communications towers:
brief historical overview

People began documenting bird kills at tall communications towers in North America during the late 1940s - such towers were then being constructed on the continent to broadcast the emerging television medium. Thousands of migrant songbirds killed in a night at a single 1000 foot high television tower was news to ornithologists. Though bird kills at lighthouses had been noted for centuries, it is unlikely that anyone anticipated the staggering number of songbirds that would be killed at tall TV towers which were lighted at night for aviation safety. Like the lighthouses, on foggy or low cloud ceiling nights, migrating birds appeared to become attracted to the lights of the towers and mill about them for lack of stronger navigational cues. The large mortality at these towers was chiefly attributed to collisions with the many relatively invisible guy wires used to support the towers.

Though seen as tragic, these large kills appeared to be relatively rare, and there is not much evidence that anyone thought songbirds were declining - they seemed abundant. Nonetheless, the kills were appalling to bird lovers and tower kill studies began at a number of tall towers across the continent. Most ornithologists and a small portion of the public became aware of the periodic bird kills. In the 1960s and 1970s, the shock over songbird tower kills appears to have begun transformation into mostly an attitude of acceptance, and the notion of salvaging kills for scientific study may have diffused concern over the matter. For whatever reason, a decline in the number of tower kill studies and attention to the issue occurred during the 1980s and 1990s. Indeed, there are only a few studies on the continent that have been ongoing for more than twenty years, and there are only a handful of studies which have attempted to understand the mechanism of the tower kills.

But this situation is likely to change for two reasons. One, because concurrently during the 1980s and 1990s, the compilations of the first long-term studies on North American bird populations revealed declining populations for many species of migrant songbirds. And two, the 1990s have seen the emergence of deregulation in the communications industry, along with the advent of cellular telephones and digital television. These communications developments have brought on a recent rapid proliferation of new communications towers, a trend which is expected to continue well into the next century and based on ample evidence (Avery 1980, Weir 1976), will kill additional millions of songbirds each year.

Towerkill Mechanisms

Two independent mechanisms of bird mortality occur at communications towers. The first is when birds flying in poor visibility do not see the structure in time to avoid it (i.e., blind collision). This is more of a threat for faster flying birds such as waterfowl or shorebirds; variables in bird vision and flight agility are factors - slower, more agile flying birds, such as songbirds, are not as likely to succumb to blind collision. This mechanism can occur during the day when the tower is obscured by fog, or at night, theoretically more often with unlighted towers.

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 mechanism for mortality. When there is a low cloud ceiling or foggy conditions, lights on a tower refract off water particles in the air creating an illuminated area around the tower. Migrating birds have lost their stellar cues for nocturnal migration in these weather conditions. In addition, because they are flying beneath a relatively low cloud ceiling, they have lost 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 by the tower. Mortality occurs when they run into the structure and its guy wires, or even other migrating birds as more and more passing birds cram into the relatively small, lighted space. It is important to clarify that the lights apparently do not attract birds from afar, but rather tend to hold birds that pass within a certain illuminated vicinity.

In the 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. Theoretically the small kills on clear nights were not from birds drawn to the tower lights but from birds that happened to be flying near the tower and didn't see a guy wire - blind collision. The bulk of the kills on overcast nights likely involved the refracted light mechanism. The majority of mortality from communications towers is thought to result from this mechanism, and thus a number of studies have been conducted to further understand it.

Source: Towerkill.com