Response of Golden Eagle Flight Altitude to Topographic Variation in California and Implications for Potential for Wind-Wildlife Interactions

Planning renewable-energy development to minimize effects to sensitive species of wildlife requires knowledge of how those species use the environment. In the case of wind-wildlife interactions and volant species, it is... [ view full abstract ]

Planning renewable-energy development to minimize effects to sensitive species of wildlife requires knowledge of how those species use the environment. In the case of wind-wildlife interactions and volant species, it is helpful to understand how flying animals select the altitude above ground level (AGL) at which they fly. To aid understanding of interactions between wind energy and golden eagles, we investigated how flight altitude of 97 GPS-tagged golden eagles changed relative to topography in each of 5 Bird Conservation Regions (BCRs) within California. We associated flight locations from these data with four measures of the topography directly below each point. The first, a topographic position index, was categorized along a gradient that included valleys, gentle slopes, steep slopes, and ridges. The second, a topographic roughness index, was categorized into 5 classes that included smooth or flat areas, and areas with slight, low, moderate and high roughness. We also included measures of slope and aspect of the terrain. Unpublished results suggest that flight altitude above ground level (AGL) for golden eagles differed by BCR and, in each region, responded distinctly to topographic position, topographic roughness, slope and northing (north-south component of aspect). In contrast, response to topographic roughness and easting (east-west component of aspect) did not vary among BCRs. Empirical estimates of eagle flight AGL were highest over the Sonoran and Mojave Desert BCRs (250 ± 112 m; grand mean by bird ± SE); intermediate over the Sierra Nevada (234 ± 178), Coastal California (227 ± 150) and Great Basin BCRs (210 ± 147) and lowest over Northern Pacific Rainforest BCR (161 ± 104 m). Model results projected eagle flight AGL was highest over valleys and lower over other topographic positions. However, flight AGL increased on gentle slopes in Coastal California and increased on gentle and steep slopes in both Northern Pacific Rainforests and the Sierra Nevadas. For all BCRs, flight AGL decreased as the terrain became rougher. Flight AGL also decreased as slopes increased in steepness, with the strongest effects in Coastal California and the Sonoran and Mojave Deserts. The compass direction of slope also influenced flight altitude, such that flight AGL always was higher on east-facing slopes and varied by region when slopes faced other directions. Eagle-turbine interactions are most risky when flight is within the rotor swept zone of turbines, generally ~20 – 130 m. Although eagles may have flown within the rotor-swept zone in all settings, our data suggests that this occurred most frequently over steep slopes and ridges, and over terrain that had moderate to high roughness. Developers may be able to use this information to target relatively less risky areas for turbine siting. Likewise, in cases where turbines have already been built, managers may implement cost-effective mitigation strategies targeted at relatively more risky areas to minimize turbine blade-eagle collisions.