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. 2012;7(4):e34025.
doi: 10.1371/journal.pone.0034025. Epub 2012 Apr 25.

An Estimate of Avian Mortality at Communication Towers in the United States and Canada

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Free PMC article

An Estimate of Avian Mortality at Communication Towers in the United States and Canada

Travis Longcore et al. PLoS One. .
Free PMC article

Abstract

Avian mortality at communication towers in the continental United States and Canada is an issue of pressing conservation concern. Previous estimates of this mortality have been based on limited data and have not included Canada. We compiled a database of communication towers in the continental United States and Canada and estimated avian mortality by tower with a regression relating avian mortality to tower height. This equation was derived from 38 tower studies for which mortality data were available and corrected for sampling effort, search efficiency, and scavenging where appropriate. Although most studies document mortality at guyed towers with steady-burning lights, we accounted for lower mortality at towers without guy wires or steady-burning lights by adjusting estimates based on published studies. The resulting estimate of mortality at towers is 6.8 million birds per year in the United States and Canada. Bootstrapped subsampling indicated that the regression was robust to the choice of studies included and a comparison of multiple regression models showed that incorporating sampling, scavenging, and search efficiency adjustments improved model fit. Estimating total avian mortality is only a first step in developing an assessment of the biological significance of mortality at communication towers for individual species or groups of species. Nevertheless, our estimate can be used to evaluate this source of mortality, develop subsequent per-species mortality estimates, and motivate policy action.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Relationship of bird fatalities to free airspace at WCTV Tower, 1956–1967.
Raw data from Crawford and Engstrom (2001) were used to plot daily bird fatalities against the mean free airspace between the top of the tower and the cloud ceiling each day. Days with maximum ceiling were excluded. Daily avian mortality increases significantly as free airspace decreases (Ln(Bird Fatalities +1) = 1.443928 – 0.0016667 · Mean Free Airspace (m), R 2 = 0.17, p<0.001).
Figure 2
Figure 2. Bird Conservation Regions and locations of towers used for tower height–mortality regression.
Figure 3
Figure 3. Regression and 95% confidence intervals of annual avian fatalities by tower height.
Annual avian fatalities were adjusted for sampling effort, search efficiency, and scavenging and regressed by log-transformed tower height (Ln(Mean Annual Fatalities +1) = 3.4684 · Ln(Tower Height) – 12.86, R 2 = 0.84, p<0.0001).
Figure 4
Figure 4. Influence of study choice on tower height–mortality regression.
Distribution of counts for R 2 (adjusted), standard error, and coefficient for 5,000 iterations (subset = 18 studies, left; subset = 37 studies, right) for a linear regression between the natural logarithms of tower height (m) and mean annual fatalities.
Figure 5
Figure 5. Map of communication towers in the United States and Canada by height class.
Data acquired from Federal Communications Commission, Towermaps.com, and NAV CANADA.
Figure 6
Figure 6. Estimated annual avian mortality from communication towers by Bird Conservation Region.
High mortality estimates in Peninsular Florida and Southeastern Coastal Plain reflect the more numerous and taller communication towers in these regions.
Figure 7
Figure 7. Distribution of residuals of tower height–mortality regression for 38 towers in the United States as adjusted for sampling effort, search efficiency, and scavenging.
Contour lines indicate regions above and below the regression line. Although exhibiting a geographically variable pattern, the residuals are not significantly spatially autocorrelated.

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