'Why are wind farms so deadly for bats,' scientists ask

Scientists Paul Cryan and Robert Barclay tackle a perplexing question in the newest issue of the Journal of Mammalogy. "Why are wind farms so deadly for bats?" Their article, a synthesis of the research on the topic should be required reading for anyone working on bat conservation, wind development, or for those with a strong interest in either topic.

A surprising but unfortunate consequence of the clean energy boom is that wind turbines are killing bats in large numbers potentially threatening some populations. Scientists believe that wind turbines primarily kill bats in two ways:

1) The blades hit bats causing severe external injuries that are too gory for me to summarize here; and 

2) The spinning blades lead to rapid pressure change (barotrauma), damaging the blood vessels in the lungs of nearby bats, causing bleeding in the thoracic cavity.
 
These impacts, along with those to birds, have created tension in the environmental community by pitting two important objectives - clean energy development and the conservation of biodiversity - against one another. It is critical that we figure why wind farms are so deadly for bats so that we can make clean energy more bat-frieldly.

One of the themes to emerge from past research on this topic is that bat mortality in wind farms is likely not random. For example, certain types of bats are killed more often; certain times of the year see more deaths; etc.

Cryan and Barclay synthesize the literature on the topic and present around 15 different hypotheses that may explain this non-randomness. One group of explanations, which they term "Coincidental Hypotheses," contends that something inherent in the behavior of bats puts certain populations at greater risk.

For example, migration patterns may make certain bats more susceptible to wind farm mortality. Past studies have found that long-distance migratory bats make up a disproportionately large percentage of those that are killed. Alternatively, bats may be more at risk when they increase foraging activity for insects in the late summer to prepare for overwintering.

They present another group of hypotheses based on the idea that wind turbines themselves may attract certain types of bats. For example, certain bats may be attracted to insects that congregate around wind turbines. Bats may be attracted to turbines as mating or gathering sites.

Cryan and Barclay stop short from endorsing any single explanation. They point out some of the supporting data from past research, but at this point there are too many holes in our current understanding to say definitively what is happening.

One of the really cool things about Cryan and Barclay's review article is that for each hypothesis, they layout predictions of what we would observe if it were true. For example, if bats are attracted to turbines as roosts, as one hypothesis contends, then Cryan and Barclay predict that: a) highly visible turbines will kill more bats, b) more fatalities will occur on moonlit nights, c) more fatalities will occur with experimentally lit turbines, etc.

In this sense, their review article outlines literally dozens of potential studies that enterprising graduate students could do to advance our knowledge of this topic. Wind energy companies would be wise to help fund such research. The findings from these studies could potentially make efforts to reduce the impacts of wind turbines on bats more effective and less expensive. The authors write,

"If the ultimate cause or causes of bat fatalities at wind turbines are not established, it may never be possible to accurately assess risk to bats before turbines are built, measure the true impacts of turbines on affected populations, or come up with the most efficient ways to avoid or minimize fatalities. Although mitigation techniques such as operational curtailment are likely to be an effective way of minimizing fatalities at existing turbines land managers, conservationists, and wind energy developers also want to know in advance, before turbines are built, how they can avoid bat fatalities." 

--Reviewed by Rob Goldstein

Cryan, P., & Barclay, R. (2009). Causes of Bat Fatalities at Wind Turbines: Hypotheses and Predictions Journal of Mammalogy, 90 (6), 1330-1340 DOI: 10.1644/09-MAMM-S-076R1.1