Golden eagles exploit turbulence intermittency

We examine the statistics of the accelerations of six golden eagles in natural flight and find that extreme vertical accelerations are consistent with the selective amplification of small-scale turbulent updrafts. The finding appears to be the first observation of wildlife exploiting turbulent fluctuations, and extends what we know about how wildlife uses relatively stationary flows to stay aloft, such as thermal updrafts and wind shear, by showing how fully unsteady flows, or gusts, can also be beneficial. The evidence in favor of our interpretation includes a probability distribution of vertical acceleration differences that exhibits long tails consistent with turbulence intermittency and inconsistent with gust suppression, or with the mitigation of extreme events. Furthermore, the breadth of the tails, measured by the acceleration difference flatness, increases toward short timescales, thereby breaking self-similarity in the same way as does turbulence intermittency. Finally, the acceleration difference flatness blows up toward large values on timescales shorter than a few seconds in a way predicted by a simple nonlinear model of the eagles' response to turbulence. The model breaks up-down symmetry in favor of upward gusts, and is consistent with the eagles' interest in staying aloft while minimizing energy expenditure.