Flight characteristics forecast entry by eagles into rotor‐swept zones of wind turbines

Abstract

Operators of wind power facilities can mitigate wildlife mortality by slowing or stopping wind turbines (hereafter 'curtail') when birds are at an increased risk of collision. Some facility operators curtail when individual birds have flight characteristics (e.g. altitude, distance or relative bearing of a bird's flight path) that exceed some threshold value, but thresholds currently in use have not been empirically evaluated. Overly restrictive thresholds can cause turbine curtailment for birds that never enter rotorswept zones, thereby resulting in excess power loss. We evaluated the probability that birds, specifically eagles, entered the rotor-swept zone (hereafter 'entry probability') in response to their flight characteristics. We used an automated monitoring system to classify individuals as eagles or non-eagles and record flight paths of purported eagles at a wind facility in Wyoming, USA. We used logistic regression with occupancy dynamics and a distance-dependent colonization process to model entry probability. As a result, this model allowed entry probability to decrease with horizontal distance to the nearest turbine. The probability of entry varied with distance to the nearest turbine and approached zero when that distance was more than 202 m. Entry probability peaked when eagles flew 89 m above ground, corresponding to hub heights of turbines (80 m), and decreased to near-zero at altitudes of 189 m or more. Entry probabilities were greatest when flight paths were near the rotor-swept zone and when eagles flew slowly toward the nearest turbine. Compass bearing of a flight path was not associated with entry probability. Our model accurately forecasted entry probability in Wyoming (area under the curve (AUC) = 0.96) and was transferable to another facility in California, USA (AUC = 0.97); therefore, our results may be applicable across a variety of settings. Curtailment criteria can be based on flight path characteristics to forecast entry into rotor-swept zones. The use of distance and altitude thresholds when making curtailment decisions is justified. However, this analysis suggests alteration of the time to collision threshold, with curtailment initiated at greater distances as the speed of the bird decreases. Our novel modelling method and our results can inform curtailment criteria in any situation where curtailment decisions are made in real-time.

Variation in flight characteristics associated with entry by eagles into rotor‐swept zones of wind turbines

“…EAST was a continuous variable that measured whether a flight path was directed eastward (EAST = 1 as the maximum value) or westward (EAST = À1 as the minimum value). For additional details about explanatory variables, see Rolek et al (2022).…”

“…We briefly describe the aspects of Rolek et al's (2022) modelling framework that we used and below we highlight the differences between that past work and this effort. We used a hierarchical generalized linear model to evaluate the probability that flight segments entered the rotor-swept zone (Rolek et al 2022). We allowed the probability of entry to decrease as distance from the nearest turbine increased (Rolek et al 2022).…”

“…We used a hierarchical generalized linear model to evaluate the probability that flight segments entered the rotor-swept zone (Rolek et al 2022). We allowed the probability of entry to decrease as distance from the nearest turbine increased (Rolek et al 2022).…”

“…To include an effect of distance, we included the probability of entry as a Gaussian kernel that decreased monotonically as Euclidean distance (x i,tÀ1 ) increased (Clobert et al 2012) from a flight path segment to the rotor-swept zone of the nearest wind turbine (Rolek et al 2022). The y-intercept (ρ) was a parameter estimated by the model that represented the maximum probability of entry at distance equal to zero (x i,tÀ1 ¼ 0).…”

“…Our objective is to test this prediction. We modified the modelling framework presented by Rolek et al (2022) to estimate the variation among turbines in flight characteristics associated with entry by eagles into the rotor-swept zone. Such information could help further improve curtailment criteria for automated curtailment by the machine vision system using in-flight characteristics to better predict an eagle's probability of entering the rotorswept zone and potentially reduce excess curtailment.…”