Influence of rotation speed and frequency on the decision of Columba livia domestica to cross the rotor-swept area of paper blades mimicking a wind turbine

To reduce bird collisions with wind turbines, automatic detection systems
have been developed to slow the blades down when a bird is approaching. We
experimentally tested whether blade rotational speed (i.e., number of
rotations per min) and frequency (i.e., number of times a blade passes a
point per min) affected the decision time (i.e., time to take-off), path
choice (i.e., the position in the aviary), and decision to cross the
rotor-swept area in Columba livia domestica (rock dove [domestic variety];
aka homing pigeon; hereafter, pigeon). We used a homemade device with
paper blades, mimicking the movement of wind turbine blades. We adjusted
the paper blade dimensions and achromatic contrast with the background to
match the visual capabilities of pigeons, increasing the probability of
detection. Pigeons were less likely to cross the rotor-swept area at
higher speeds and frequencies, independent of their decision time. When
pigeons crossed the rotor-swept area (43 out of 160 trials), 63% collided
with the blades, regardless of blade speed or frequency. Pigeons chose to
avoid the rotor-swept area after they had traveled half the distance to
the wind turbine. Pigeons were not better able to avoid the rotor-swept
area when blades were rotating at low speed and/or frequency and often
collided with the blades. Thus, slowing blades to a low rotational speed
may not reduce collisions with some species and a complete turbine
shutdown may be necessary. The feasibility and economic costs of regular
complete shutdowns after the deceleration triggered by the automatic
detection systems need further investigation.