Optimization of dynamic soaring in a flap-gliding seabird affects its large-scale distribution at sea

Abstract

Dynamic soaring harvests energy from a spatiotemporal wind gradient, allowing albatrosses to glide over vast distances. However, its use is challenging to demonstrate empirically and has yet to be confirmed in other seabirds. Here, we investigate how flap-gliding Manx shearwaters optimize their flight for dynamic soaring. We do so by deriving a new metric, the horizontal wind effectiveness, that quantifies how effectively flight harvests energy from a shear layer. We evaluate this metric empirically for fine-scale trajectories reconstructed from bird-borne video data using a simplified flight dynamics model. We find that the birds’ undulations are phased with their horizontal turning to optimize energy harvesting. We also assess the opportunity for energy harvesting in long-range, GPS-logged foraging trajectories and find that Manx shearwaters optimize their flight to increase the opportunity for dynamic soaring during favorable wind conditions. Our results show how small-scale dynamic soaring affects large-scale Manx shearwater distribution at sea.

Opportunistic soaring by birds suggests new opportunities for atmospheric energy harvesting by flying robots

“…While gradient soaring and gust soaring exploit distinct opportunities, they are energetically equivalent (equation (2.1)). Hence, as the wind field is rarely constant and never homogeneous in the atmospheric boundary layer, we should typically expect to find mixed exploitation of both kinds of Because aerodynamic kinetic energy can be obtained through dynamic soaring in powered as well as unpowered flight, pelagic birds that engage in dynamic soaring when the wind is strong will often flap their wings when the wind is weak [19,[102][103][104]. Nevertheless, the great majority of research on avian dynamic soaring has focused on albatrosses [77,78,[105][106][107][108][109][110], which are capable of flying immense distances without flapping, and whose flight morphology is highly specialized for this function [111].…”

“…Nevertheless, the great majority of research on avian dynamic soaring has focused on albatrosses [77,78,[105][106][107][108][109][110], which are capable of flying immense distances without flapping, and whose flight morphology is highly specialized for this function [111]. Moreover, because it is challenging to demonstrate empirically which sources of atmospheric energy are being employed in situations where a bird uses flap-gliding flight, or where gradients and updrafts occur together (as they do whenever waves are present), the use of gradient soaring has only been confirmed in albatrosses and (more recently) shearwaters [19]. Nevertheless, for the reasons discussed below, dynamic soaring can be assumed to be prevalent across a much wider range of species than this.…”

“…where γ is the aerodynamic flight path angle defined as the elevation angle of the bird's air velocity V with respect to the horizontal, and where η is the heading-to-wind angle defined as the angle between the horizontal wind vector and the horizontal component of the bird's air velocity [19].…”

“…It is clear by inspection that the rate of energy harvesting is proportional to the quantity e h ¼ Àsgng cos h, called the horizontal wind effectiveness [19], where the signum function sgng ¼ +1 is positive when ascending and negative when descending. The rate of energy harvesting is therefore maximized by aligning ascent and descent through the shear royalsocietypublishing.org/journal/rsif J. R. Soc.…”

“…The mechanisms by which birds tolerate gusts depend on sensing flow disturbances and responding appropriately [16], or mitigating gusts by having a flexible, morphing airframe of appropriate geometry [17][18][19][20]. Birds that use gust or gradient soaring to loiter close to the ground tend to have very low wing-loading.…”