AUTHOR BLOG: Fly like an eagle? Topography tells us how high Golden Eagles soar

Adam Duerr

Linked paper: Topographic drivers of flight altitude over large spatial and temporal scales by A.E. Duerr, T.A. Miller, L. Dunn, D.A. Bell, P.H. Bloom, R.N. Fisher, J.A. Tracey, and T.E. Katzner, The Auk: Ornithological Advances.

A Golden Eagle soars over a line of ridgetop wind turbines. Photo by Dave Brandes.

Like many people, I am fascinated by bird flight. Unlike most people, I get to study flight of Golden Eagles for a living. These large birds move through the landscape primarily by soaring—a style of flying where they hold their wings outward and rarely flap, saving them considerable energy. Instead of flapping, they rely on rising air currents to gain altitude.

Two types of rising air currents provide most lift for soaring eagles. The first, thermal updrafts, form when energy from the sun heats air at the Earth’s surface and causes it to rise. Eagles circle within these columns of rising air to gain great altitude and then glide out of the thermals to move across the landscape. The second, orographic updrafts, form when winds are deflected upward by structures such as ridges or hills. Eagles can then soar at relatively low altitude above and along these structures.

Although updraft formation depends on the interaction of weather and topography, our goal for this study was to determine if topography alone can explain how high eagles soar. To do this, we used telemetry systems that we placed on the backs of 91 Golden Eagles in California, which recorded the eagles’ locations every 15 minutes. For each of the almost 180,000 locations we recorded of eagles in flight, we compared the eagle’s altitude with the characteristics of the topography below.

We found a strong relationship between topography and flight altitude for the Golden Eagles in our study. In places where the topography made the formation of orographic updrafts likely, eagles were more likely to fly at lower altitudes, while in places where the topography made the formation of thermal updrafts likely, eagles were more likely to fly at higher altitudes. We also found that the effects of some topographic features depended on their region within California, which may be due to regional differences in weather patterns, land cover, or a variety of other factors that we did not include in our analysis. Our topography-based model of flight altitude is much simpler than other models of avian flight altitude, thanks to the fact that it lets us ignore weather conditions, which are constantly changing. Instead, we can simply estimate how high a Golden Eagle is likely to be flying as it crosses over any point of interest in California. Wildlife managers can use this type of information to predict where eagles may collide with wind turbines and power lines; therefore, making these predictions in an accurate and straightforward way is critical for Golden Eagle conservation.