Topography drives migratory flight altitude of golden eagles: implications for on‐shore wind energy development

Abstract

Summary 1.Wind power is a fast-growing industry with broad potential to impact volant wildlife. Flight altitude is a key determinant of the risk to wildlife from modern horizontal-axis wind turbines, which typically have a rotor-swept zone of 50-150 m above the ground. 2. We used altitudinal GPS data collected from golden eagles Aquila chrysaetos tracked using satellite telemetry to evaluate the potential impacts of wind turbines on eagles and other raptors along migratory routes. Eagle movements during migration were classified as local (1-5 km h À1 ) or migratory (>10 km h À1 ) and were characterized based on the type of terrain over which each bird was flying, and the bird's distance from wind resources preferred for energy development.3. Birds engaged in local movements turned more frequently and flew at lower altitude than during active migration. This flight behaviour potentially exposes them to greater risk of collision with turbines than when engaged in longer-distance movements. 4. Eagles flew at relatively lower altitude over steep slopes and cliffs (sites where orographic lift can develop) than over flats and gentle slopes (sites where thermal lift is more likely). 5. Eagles predominantly flew near to wind resources preferred by energy developers, and locally moving eagles flew closer to those wind resources with greater frequency than eagles in active migration. 6. Synthesis and applications. Our research outlines the general effects of topography on raptor flight altitude and demonstrates how topography can interact with raptor migration behaviour to drive a potential human-wildlife conflict resulting from wind energy development. Management of risk to migratory species from industrial-scale wind turbines should consider the behavioural differences between both locally moving and actively migrating individuals. Additionally, risk assessment for wind energy-wildlife interactions should incorporate the consequences of topography on the flight altitude of potentially impacted wildlife.

Use of multiple modes of flight subsidy by a soaring terrestrial bird, the golden eagleAquila chrysaetos, when on migration

“…Interface 12: 20150530 flight they used. Previous work indicates that eagles demonstrate a flight altitude response to changes in topography [36] and to variation in wind speed [22]. We also know that flight speed of eagles using a thermal-glide strategy is faster than that of eagles using exclusively orographic updraft [19], that eagles preferentially migrate when weather conditions favour availability of that thermal updraft [21], and that adult and pre-adult eagles face different temporal and energetic pressures when migrating [21].…”

“…This complexity is reflected in the fact that golden eagles respond to the variable local-scale meteorological events they encounter by switching flight behaviour to take advantage of at least two, and probably several other, types of subsidy over short temporal intervals. Use of flight subsidy is driven by interactions not only with weather but also with topography [36]. Thus, hawk counts are conducted at locations where topography diverts and concentrates flight paths of migrating raptors.…”

Flight Paths of Migrating Golden Eagles and the Risk Associated with Wind Energy Development in the Rocky Mountains

“…First, it has the capacity to document a large number of flight tracks, similar to radar, but has the added potential to document movement both above and below radar-detection elevations, as well as to provide information on species and age of targeted migrants. Although the errors in distance and height estimation are likely slightly greater than those reported with satellite tracking (e.g., Katzner et al 2012, the potential to document a larger number of flight paths and lack of requirements to catch birds make our technique a valuable additional tool for assessing collision risk potential at proposed installations.…”

High passage rates and different seasonal migration strategies of birds along the lower Texas coast

“…Consequently, these type of situations may increase the number of migrants susceptible to impacts with wind turbines along the lower Texas coast. The rotor swept zone area in which migrants may be at risk is generally from 50 to 150 m above ground level (Katzner et al, 2012). Thus, the minimum number of migrants at risk across a 1-km front in our study is between 5 to 10% at Site 1 and 13 to 19% at Site 2 (Tables 6 and 7).…”