Saturday, 10 September 2011

Power line markers reduce the likelihood of bird collisions

Researchers have estimated that as many as 1 billion birds die each year as a result of collisions with power lines and associated structures.  Large, heavy, low-flying birds and those with narrow fields of vision are particularly likely to be affected; thus, species such as swans, geese, bustards, storks, and cranes are the most common casualties of power line encounters. For some endangered species, power lines are the biggest source of mortality--in South Africa, for instance, 30% of Denham's bustard (Neotis denhami) deaths each year result from collisions with power lines.

(Denham's bustard, Neotis denhami, a "near threatened" species that can be found throughout much of sub-Saharan Africa.)

The hazard of these structures has been recognized since the beginning of the last century, though mitigation measures were not introduced until the 1970's. Managers have used a variety of techniques to scare or warn birds away from the lines--placement of raptor decoys on posts, replacing static wire with lighting arrestors, and attachment of spirals, plates, swivels, and spheres (collectively called bird flight diverters) on static wire. Presence of these markers generally appears to cause the birds to fly higher (rather than changing direction), thus avoiding collisions. Installation of the warning features can be quite costly, and some even reduce power flow and cause line damage. 

Surprisingly, despite the obvious need for rigorous research investigating the utility of mitigation tactics and comparing the efficacy of different techniques, there is a relative dearth of informative studies in the scientific literature. This prompted collaborators from the Museo Nacional de Ciencias Naturales to perform a meta-analysis of existing datasets in order to look for broad trends uncovered by work on different species exposed to different types of flight diverters in different habitats. The researchers looked for relevant publications by scouring four scientific databases and Google, and directly contacting researchers, environmental agencies, electrical companies, and conservation organizations. This allowed them to include not data not only from papers published in scientific journals, but also data from unpublished reports and datasets.

 (Example of bird flight diverters produced by Energy Equipment. This company also makes a variety of diverter that emits UV light, theoretically making it even more obvious to flying birds.)

All told, the researchers found 21 studies featuring 52 separate experiments that counted bird carcasses near marked and unmarked power lines. They pooled data from all the experiments and looked for overarching trends. They also performed a second analysis in which their pooled dataset only included counts that looked not at absolute numbers of carcasses, but at proportions of dead birds out of the entire number of birds known to fly near the lines within a certain time period. 

Overall, there were significantly fewer collisions at marked wires. Collision rates at marked wires were 0.05/1000 bird crossings, while at unmarked wires, this number rose to 0.21/1000--over four times as high. Wire marking appeared to reduce collision-related mortality by anywhere from 55-94%; this wide range reflects the high amount of variation among the datasets and suggests that other variables (for instance, type of vegetation at, or topography of, the habitats where the studies were performed) may also have contributed to how visible wires were to flying birds. 

 (Bird flight converters along powerlines owned by the REN Group, which collaborates with the Nature Conservation Institute to minimize the negative effects of its energy delivery structures.)

The authors point out that, even though wire marking does reduce the number of bird casualties at power lines, collision risks were fairly low at unmarked lines to begin with. They also emphasize that, despite the statistical significance of their findings, there was still a fair amount of variation in their dataset because of the small number of studies that they could conclude in their analyses. Thus, they encourage more researchers to investigate the efficacy of bird flight diverters. 

However, they stress that future studies should improve on those that have been done in the past. For instance, scientists should survey marked and unmarked wires for consistent lengths of time, at the same time of year, and in areas with similar vegetation and topography. When they search for carcasses, they should do so over similar spatial scales and at consistent time points--taking into consideration the fact that scavengers are likely to eat the carcasses of small birds before they eat those of large birds, which may influence study results. Finally, in order to make practical recommendations to managers, the researchers stress the necessity of comparing the effectiveness of the different types, colors, shapes, sizes, and spacing patterns of wire markers. Given that the presence of power lines is increasing at a rate of 5% each year, the sooner the new studies are started, the better.

Barrientos, R., Alonso, J.C., Ponce, C., and Palacín. 2011. Meta-analysis of the effectiveness of marked wire in reducing avian collisionswith power lines. Conservation Biology 25(5):893-903.  

Thanks to the following websites for providing the images used in this post:

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