US wind farm data shows when bird and bat fatalities most likely

A US study has revealed bat deaths at wind farms are probably seasonal, and curtailing wind turbine movements in line with seasonal patterns could reduce bat deaths.

Bat fatalities at US wind farms peak from mid-July to early September, according to analysis of post construction bird and bat fatality data across nearly a third of installed US wind capacity  published in PLOS One.

Peak fatalities of woodland birds correspond to seasonal movements in May and September. 

Ryan Butryn, from the Renewable Energy Wildlife Institute (REWI) and a co-author of the paper says seasonal timing of bird and bat collisions is significant in better understanding patterns at a regional and national level and gaining insights into the ecological mechanisms that lead bats and birds to be susceptible to wind turbine collisions.

“Improved understanding of when collisions are most likely to happen will allow efforts to curtail turbines to avoid collisions to be most effective,” Butryn adds.

Curtailment, a measure which involves stopping wind turbines spinning during limited periods of low wind and high bat activity, has been shown to be effective at limiting bat deaths in Europe, the US and Australia.

The REWI analysis – drawn from 114 wind farm post-construction monitoring studies – outlines seasonal patterns in bird and bat fatalities in eight different North American ecoregions.

The timing of peak bat fatalities ranged from 5 August in the Southern Texas Plains through to 5 September in the South Central Semiarid Prairies, for example. Peak fatalities for individual species tend to coincide with when those species are present in a region.

“At a basic level, this study indicates that species and ecoregion have significantly different timing patterns and that there is not a ‘one-size-fits-all’ approach to curtailment,” Butryn says. 

“Curtailment is effective at reducing collisions, but it results in reduced electricity production, so the study will be helpful at understanding the pattern in collision timing and refining curtailment prescriptions to match the time of greatest collision risk.”

The study analyses bird and bat fatalities at US wind farms between 2009 – 2021. The analysis draws on data from the American Wind Wildlife Information Center database. The database, developed by REWI, its partners and friends, is the most detailed and geographically extensive dataset on bird and bat fatalities. It contains post-construction monitoring from 248 operating wind facilities, or nearly 30% of installed wind capacity in the US.

By gathering data on bird and bat collisions across multiple facilities and regions, REWI aims to understand the scope of the issue and how it might be affecting the entire populations of these species.

“The data we have in the US indicates that some of the most common bat and bird species to collide with turbines are migratory and travel long distances seasonally,” he says. 

Multiple studies in the US and Canada have observed that three species of bats, Hoary Bat, Eastern Red Bat, and Silver-haired bat, account for 70-80% of all bat fatalities, Butryn says.

“Unfortunately, the life history of these species makes them difficult to study so there is not as much known about their behaviour or population status. Analyses like this one can help shed some light on the ecology of these species and be useful in reducing their collision risk,” he says. 

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Eastern red bat / Credit: Flick user Cataloging Nature licensed under CC BY 2.0

In Australia, data science company Symbolix undertook similar regional-scale analysis of post-construction bird and bat monitoring data from Victoria in late 2020. The study included data from surveys between 2014 and 2019.

Elizabeth Stark, is a mathematician and data analyst and the Managing Director of Symbolix. She says analysing bird and bat collision data gathered across multiple wind farms can help with understanding cumulative impacts for different species on a larger landscape scale.

Wind farm impacts on birds and bats are still a rare statistical event, she says. “Six out of seven [wind] turbines searched have no finds.”

One thing that stood out from the Victorian study was that, bats are quite impacted by wind farms, more so than birds, Stark says.

“It’s the little bats that we need to be having a conversation about,” she says, especially given the other habitat pressures bats are facing.

The Symbolix analysis found bat mortality rates in Victoria, ranged between 7 and 10.8 bats per turbine, per year. Whereas bird fatalities ranged between 3.4 – 4.1 for smaller wind turbines, and 5 – 6.7 for larger wind turbines.

Particular species such as the White-striped freetail bat are impacted more “by an order of magnitude”, she says.

“In sheer numbers, in Victoria, we’re losing by a factor of 10, more White-striped freetail bats, than the nearest other bat, which is the Gould’s wattled bat.”

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Gould’s wattled bat / Credit: Flickr user Department of Environment and Primary Industries Victoria licensed under CC BY-NC 2.0

She says there would be value in undertaking similar analysis on a national scale – like the US study – given some bird and bat species migrate across landscapes and recent growth in renewable energy development in states like Queensland and New South Wales.

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