Towerkill Mechanisms
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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, haze, smoke, or foggy conditions, lights on a tower refract off water or other 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, they have often 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 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.

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 phototactic 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 (see Literature). In particular, see the Evans et al. article in North American Birds for a recent in depth discussion on bird aggregation in artificial light
   
 


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