Optimal orientation in flows: providing a benchmark for animal movement strategies

Abstract

Animal movements in air and water can be strongly affected by experienced flow. While various flow-orientation strategies have been proposed and observed, their performance in variable flow conditions remains unclear. We apply control theory to establish a benchmark for time-minimizing (optimal) orientation. We then define optimal orientation for movement in steady flow patterns and, using dynamic wind data, for short-distance mass movements of thrushes (Turdus sp.) and 6000 km non-stop migratory flights by great snipes, Gallinago media. Relative to the optimal benchmark, we assess the efficiency (travel speed) and reliability (success rate) of three generic orientation strategies: full compensation for lateral drift, vector orientation (single-heading movement) and goal orientation (continually heading towards the goal). Optimal orientation is characterized by detours to regions of high flow support, especially when flow speeds approach and exceed the animal's self-propelled speed. In strong predictable flow (short distance thrush flights), vector orientation adjusted to flow on departure is nearly optimal, whereas for unpredictable flow (inter-continental snipe flights), only goal orientation was near-optimally reliable and efficient. Optimal orientation provides a benchmark for assessing efficiency of responses to complex flow conditions, thereby offering insight into adaptive flow-orientation across taxa in the light of flow strength, predictability and navigation capacity.

Airplane tracking documents the fastest flight speeds recorded for bats

“…The adjustments of air speed in response to wind support that we document (figure 2 b and figure 3 a ) are similar to those previously observed for birds, insects and other bats [33], and are consistent with models for optimal orientation to winds [34]. As a result of these adjustments, our analyses indicate that bats maintain similar ground speeds regardless of the strength of wind support (figure 2 a,b ) [33].…”

Atmospheric conditions create freeways, detours and tailbacks for migrating birds

“…Regardless of what form of energy is used to power flight, a trade-off exists between a bird’s airspeed and the rate at which energy is consumed. Optimal migration theory incorporating the effect of wind or thermal convection has been used to model optimal airspeeds for minimizing flight time or energy expenditure under different atmospheric conditions (Liechti et al 1994; Pennycuick 2008; McLaren et al 2014) and often used as a theoretical benchmark for comparison with field observations. For example, when flying at maximum range airspeed, the airspeed at which energy expenditure per unit distance travelled is minimized, a bird should reduce its airspeed in tailwinds and increase its airspeed in side- and headwinds, depending also on the extent to which it compensates for wind drift (Liechti et al 1994) (see section “How to fly: adjusting orientation strategies in response to horizontal and vertical flows”).…”

Flap or soar? How a flight generalist responds to its aerial environment

“…Yet soaring flight, whether supported by thermal convection, orographic lift (air that rises after hitting a topographic obstruction) or vertical wind shear, will be constrained by the availability of appropriate atmospheric conditions [25,[32][33][34]. Horizontal rather than vertical air flow will strongly influence flapping flight costs in the form of time and energy [16,18,29,35].…”

“…As the aerosphere is heterogeneous in space and in time, animals may alter their flight behaviour to take advantage of profitable conditions or avoid adverse ones [6]. Animals respond to atmospheric conditions by selecting when [7,8] or where to fly in three-dimensional space [9][10][11][12][13], by adjusting flight kinematics [14,15] or by altering airspeeds or flight directions [16][17][18]. Several species of birds and bats reduce flight costs by altering their mode of flight from continuous flapping to intermittent or soaring flight [19][20][21][22].…”