Soaring in Turbulence intermittency

Continually turbulent and gusty wind conditions characterize the atmosphere in which birds fly, and yet there is no consensus on how turbulence affects their flight. Here, we present an analysis of the acceleration of six wild golden eagles and six wild bald eagles obtained using an onboard accelerometer that shows a direct relationship between their flight and atmospheric turbulence over a range of time intervals. The histograms of acceleration increments on these time intervals are scale-dependent and exhibit long tails of violent activity (intermittency), as also observed for fluid and inertial particles in a classical turbulent flow. In particular, the flatness of the acceleration increment increases monotonically with a decrement in time interval, with a logarithmic slope close to that reported for fluid tracers in the inertial range. Besides, scaling exponents for higher-order structure functions agree well with those expected for fluid particles in turbulence. This suggests a linear relationship between eagle flight and atmospheric turbulence and implies that turbulence may not be a disturbance to mitigate but rather a source of energy for avian flight, and inspires the design of artificial fliers that could harness turbulent energy.