Comparison of daily avian mortality characteristics at two television towers in western New York, 1970-1999.

Publication: Wilson Bulletin

Publication Date: 01-MAR-05

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COPYRIGHT 2005 Wilson Ornithological Society

ABSTRACT.--Recent increases in the demand for communication towers have renewed interest in the impact of these towers on birds, particularly during migration. The objective of this study was to investigate avian mortality at two television towers (WGRZ and WKBW) in western New York from 1970 through 1999. Daily mortality totals ranged from 1 to 1,089 birds. The majority of the kill events were small, involving 10 or fewer birds; however, the majority of birds died in larger kill events. Both kill events and the numbers of individuals salvaged peaked in September. Patterns in avian mortality at the towers that we studied were consistent with normal migration events, during which the number of birds migrating varies substantially between nights. The two towers differed significantly in kill characteristics. At the WGRZ tower, median daily mortality generally ranged from 1 to 10 birds and was usually lower than at the WKBW tower. The size of kill events varied across the 3 decades, with no very large kill events (>500 birds) occurring in the 1990s. Because most birds salvaged in the 1970s and 1980s were killed in medium and large kill events, the absence of any very large kill events in the 1990s could explain the previously published decline in birds salvaged at these towers. Received 24 May 2004, accepted 7 February 2005.

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Although the Migratory Bird Treaty Act of 1918 prohibits human interference with migratory birds, lighted man-made structures, such as communication towers, have been responsible for the deaths of many nocturnal migrants (Avery et al. 1980). Avian mortality at communication towers (tower kill) results from collisions of birds with the towers themselves or their supporting guy cables. In the United States, there are more than 83,000 towers (Federal Communications Commission 2004), and the number of tower kills is likely to increase as tower construction continues. Many factors affect the number of nocturnal migrants colliding with towers, including density of migrants aloft, weather, tower location and elevation, tower height, number and location of guy cables, and lighting. Northerly winds spur bird migration in the fall, and overcast conditions may disorient birds, resulting in their gravitation toward lighted towers (Clark 1973).

Only a few long-term studies have documented the effects of specific towers on avian mortality. Crawford and Engstrom (2001) noted that the pattern of avian mortality they witnessed at a northern Florida television tower was "distinctly seasonal," with most of the kill events occurring during fall migration. In a study of tower mortality in western New York and Ohio, Morris et al. (2003) documented a significant decline in the number of salvaged birds over a 30-year period. This decrease could result from fewer kill events, smaller kill events, or both.

The goal of our study was to examine patterns of avian mortality at two television towers in western New York. Specifically, we documented temporal patterns in the kill events, both within the autumnal migratory period and among the last 3 decades.

METHODS

We examined fall avian mortality at two television towers in southern Erie County during the 30-year period from 1970 to 1999. The WGRZ tower is located in Wales, New York (78[degrees] 33' N, 42[degrees] 43' W); it is 261 m tall, not including a 32-m antenna, and was erected in 1968. This tower...

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