Turbulence causes kinematic and behavioural adjustments in a flapping flier

Abstract

Turbulence is a widespread phenomenon in the natural world, but its influence on flapping fliers remains little studied. We assessed how freestream turbulence affected the kinematics, flight effort and track properties of homing pigeons ( Columba livia ), using the fine-scale variations in flight height as a proxy for turbulence levels. Birds showed a small increase in their wingbeat amplitude with increasing turbulence (similar to laboratory studies), but this was accompanied by a reduction in mean wingbeat frequency, such that their flapping wing speed remained the same. Mean kinematic responses to turbulence may therefore enable birds to increase their stability without a reduction in propulsive efficiency. Nonetheless, the most marked response to turbulence was an increase in the variability of wingbeat frequency and amplitude. These stroke-to-stroke changes in kinematics provide instantaneous compensation for turbulence. They will also increase flight costs. Yet pigeons only made small adjustments to their flight altitude, likely resulting in little change in exposure to strong convective turbulence. Responses to turbulence were therefore distinct from responses to wind, with the costs of high turbulence being levied through an increase in the variability of their kinematics and airspeed. This highlights the value of investigating the variability in flight parameters in free-living animals.

Generating controlled gust perturbations using vortex rings

“…The deceleration of flies due to gust in the present study is consistent with observation in bumble bees [ 15 , 18 ], indicating gust as a limiting parameter on the forward speed of insects [ 4 ]. On the other hand, recently, researchers have found a increase in airspeed of pigeon when flying in turbulence [ 35 ], and golden eagles exploiting gust to accelerate [ 36 ], suggesting that birds might enhance their airspeed by harnessing energy from the surroundings.…”