The phrase "bat mortality" refers to the number of bats that have died. It is used to describe the situation when a significant number of bats are dying, which can have negative effects on bat populations and ecosystems. Full definition
The wind industry is actively engaged in leading - edge research to mitigate bat mortality at wind farms. (canwea.ca)
In tests at the Casselman Wind Power Project in Pennsylvania, small changes to turbine operations reduced bat mortality significantly. (scientificamerican.com)
In the US, a conservative estimate of bat mortality indicates that at least 4 million bats have been killed by wind turbines since 2012. (savetheeaglesinternational.org)