Gliding for a free lunch: biomechanics of foraging flight in common swifts (Apus apus)

Abstract

Although the biomechanics of animal flight have been well studied in laboratory apparatus such as wind tunnels for many years, the applicability of these data to natural flight behaviour has been examined in few instances and mostly in the context of long-distance migration. Here, we used rotational stereo-videography to record the free-flight trajectories of foraging common swifts. We found that, despite their exquisite manoeuvring capabilities, the swifts only rarely performed high-acceleration turns. More surprisingly, we also found that despite feeding on tiny insects probably moving with ambient flow, the birds adjust their air speed to optimize cost of transport over distance. Finally, swifts spent only 25% of their time flapping; the majority of their time (71%) was spent in extended wing gliding, during which the average power expended for changes in speed or elevation was 0.84 W kg −1 and not significantly different from 0. Thus, gliding swifts extracted sufficient environmental energy to pay the cost of flight during foraging. Kinematic power P (W kg −1 ) (mean±s.d.) 6.85±5.71 P<0.0001 −0.84±4.00 P=0.0775 P<0.0001 Number of tracks with relevant data, out of 73 72 73 -P-values in columns are the result of t-tests for that cell only compared with the null hypothesis; the paired t-test column is a comparison of flapping and gliding results. 6

How Turkey vultures tune their airspeed to environmental and behavioral factors

“…We showed that Turkey vultures respond to a fundamental environmental gradient that could impact their flight performance, the decrease of air density at high elevation by increasing their flight speed and adjust their flight speed to optimize their gliding flight. The tool that we used was developed for field studies of biomechanics [13,14,25,38], but this study also demonstrates that tools from the biomechanics toolchest can be successfully applied to ecological questions.…”

“…airspeed adjusted for air density), and 𝑘 2 is the induced power coefficient. See Hedrick et al (2018) for further details on this calculation. Literature values of 0.456 m 2 and 2.18 kg [31] were used for 𝑆 (wing area) and 𝑚 (body mass), respectively.…”

Compound-V formations in shorebird flocks

“…This approach is similar in principle to that used to estimate local wind speed from the drift in the ground reference frame position of circling vultures (Weinzierl et al, 2016), and shares the important assumption that airspeed is independent of wind direction. However, birds are theoretically expected and empirically known to vary airspeed with wind speed when flying in order to reach a destination efficiently (Hedrick et al, 2018; Shamoun-Baranes et al, 2007). Whether this is the case for shorebird flocks (making shorter flocks around the stopover point) is unknown, so we did not attempt to model this possible effect.…”

Bird Velocity Optimization as Inspiration for Unmanned Aerial Vehicles in Urban Environments

“…1c). COT is calculated for a given distance and so could be considered in the inertial frame [31]; however, in environmental harvesting strategies, it is the airspeed in relation to the wind which should be optimized [30]. This study considers the wind vectors in the air and inertial frame as side and crosswinds, respectively.…”