The push for new wind power facilities across the country is driving research to understand the various ecological impacts wind turbines may have. One such study may play a role in the siting of new wind farms by providing new insight into bat migration patterns.
Erin Baerwald, a doctoral student at the University of Calgary, monitored bat activity and fatalities near wind turbines located across a 200 kilometer stretch of geographically varied terrain in southern Alberta. She used acoustic monitoring devices placed at ground level, 30 meters off the ground, and on top of 67-meter tall turbines. Baerwald said that bat fatalities at wind facilities are a phenomenon that takes place almost exclusively during fall migration. Her study was aimed at determining which species move through her study area at different periods during the fall and whether they are funneled into narrow migratory corridors or migrate in a dispersed way across a broad area.
“There were some areas where there were lots of bats flying through, and other areas where we detected hardly any,” she said, “so it seems that the bats are funneled into particular routes that are related to trees. Almost all of the bats that migrate are tree bats, so if you’re a bat who relies on trees, it makes sense that you follow trees.”
As she expected, very few bats were detected near turbines placed on prairie habitat. In forested regions, Baerwald said that migratory bats appear to follow linear features like ridge tops or river valleys, somewhat like birds do.
The four most abundant species of migratory bats in the U.S. are silver-haired bats, hoary bats, Eastern red bats and Perimyotis (formerly called Eastern pipistrelle). Baerwald detected migratory bats at higher altitudes than the nonmigratory species, which seemed to stay at ground level.
Baerwald is concerned about the impact that increasing numbers of wind turbines will have on migratory bat populations. She said that bat fatalities at wind facilities outnumber bird fatalities by 10 to 1, which is troubling for animals that may live 20-30 years and reproduce very slowly. “The impacts on birds and bats are very different,” she said. “If you’re a small bird that reproduces quickly, your population can rebound quickly. But bats will have a much harder time recovering.”
The biggest surprise she found was how the different bat species responded to weather conditions. According to Baerwald, smaller bat species are more likely to suspend migration during windy conditions than larger bats. “That means that we can’t lump all bats together when we think about mitigation strategies. We may need to do different things for different species,” she said.