Flight Response to Topographic, Vegetative, and Temporal Correlates Predicts Risk from Wind Turbines to an Obligate-Soaring Bird, the California Condor

Wind power is a fast-growing energy source in the United States, and the state of California is a national leader in wind energy development. However, the flight behavior of soaring birds may place them at risk of collision... [ view full abstract ]

Wind power is a fast-growing energy source in the United States, and the state of California is a national leader in wind energy development. However, the flight behavior of soaring birds may place them at risk of collision with these structures. The objectives of our research were to evaluate patterns in individual-specific flight responses of critically-endangered California Condors (Gymnogyps californianus) to topographic, vegetative, and temporal variation in their environment, and to place these flight responses in the perspective of potential risk from collision with wind turbines. We hypothesized that condors would vary their flight altitude with spatially-, temporally-, and sex-specific responses to topography and land cover, and that risk from wind energy development would vary seasonally. We analyzed altitudinal data from GPS telemetry collected between December 2013 and November 2015 from 24 condors in southern California. We examined the types of terrain and land cover over which condors flew and daily and seasonal patterns in flight behaviors. We also measured the distance from each flight location to the nearest commercially-valuable winds and calculated the proportion of flight locations that were within the rotor-swept zone of wind turbines. We evaluated multivariate relationships within our data with linear mixed-effects models. Our as yet unpublished results indicate that condor flight behavior was strongly influenced by topography and land cover, such that birds flew at lower altitudes when over ridge lines and steep slopes and over forested and grassland cover types. Condor flight behavior also was strongly cyclical, such that birds flew lower during early morning and evening hours and during the winter months, when thermal updrafts were weakest. Although condors infrequently flew at altitudes placing them in the rotor-swept zone of modern horizontal-axis wind turbines, they regularly did fly near or within wind resources preferred by energy developers. The strong response of condors to variation in the spatial and temporal updraft environment they experience provides insight into risk management for this species. Our analyses indicate that this risk should vary seasonally and may be greatest when condors fly over areas with high topographic relief and from turbines placed near locations where they fly at lower altitudes, such as near their nocturnal roosting sites. In contrast, risk should be relatively lower when condors fly over less rough areas and from turbines placed in habitat they use during daytime soaring. Although the condors we studied were from the southern California population, our results should be applicable throughout their range. Development planners can incorporate knowledge about the flight behaviors of condors to reduce the potential for wind-wildlife conflicts.