Limitations of acoustic monitoring at wind turbines to evaluate fatality risk of bats

Wind turbines (WTs) frequently kill bats worldwide. During environmental
impact assessments, consultant ecologists often use automated ultrasonic
detectors (AUDs) to estimate the activity and identity of bats in the zone
of highest mortality risk at WTs in order to formulate mitigation schemes,
such as increased curtailment speeds to prevent casualties. While
acknowledging the potential of acoustic monitoring, we evaluate the
limitations of AUDs for monitoring bats at WTs and highlight directions
for future research. We show that geometric and atmospheric attenuation of
ultrasonic echolocation calls, in conjunction with limited sensitivity of
ultrasonic microphones, severely constrain detection distances of bats at
WTs. Taking into account the acoustic shadow produced by the nacelle, AUDs
cover only approximately 23% of the risk zone for a bat calling at 20 kHz
and 4% for a bat calling at 40 kHz, assuming a 60 m blade length. This
percentage will further decrease with increasing blade lengths in modern
WTs. Additionally, the directionality of echolocation calls and the
dynamic flight behaviour of bats constrain the detectability of bats. If a
call can be detected, the low interspecific and high intraspecific
variation of echolocation call characteristics may impair species
identification, limiting the power to predict population-level effects of
fatalities. We conclude that technical, physical, and biological factors
severely constrain acoustic monitoring in its current form. We suggest the
use of several AUDs, installed at complementary sites at WTs, and the
testing of other techniques, such as radar, cameras, and thermal imaging,
to inform stakeholders on the mortality risk of bats at WTs.