Drink safely: common swifts ( Apus apus ) dissipate mechanical energy to decrease flight speed before touch-and-go drinking

• Published in: Journal of Experimental Biology
• Main Authors: Ruaux, Geoffrey, Monmasson, Kyra, Hedrick, Tyson, Lumineau, Sophie, de Margerie, Emmanuel
• Other Authors: Ethologie animale et humaine (EthoS), Université de Caen Normandie (UNICAEN), Normandie Université (NU)-Normandie Université (NU)-Université de Rennes (UR)-Centre National de la Recherche Scientifique (CNRS), University of North Carolina Chapel Hill (UNC), University of North Carolina System (UNC), Centre National de la Recherche Scientifique (CNRS), Research on bird flight supervised by E. de Margerie was supported by a grant from theMission for Transversal and Interdisciplinary Initiatives at the CNRS in 2018, and an Emergingscientific challenge grant from the Rennes University in 2020, which made it possible to acquiresome of the material used in this study.
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2023
• Subjects: air-brake; wind; kinematics; [SDV]Life Sciences [q-bio]; Apus apus
• Online Access: https://cnrs.hal.science/hal-04016167; https://cnrs.hal.science/hal-04016167/document; https://cnrs.hal.science/hal-04016167/file/Swift_drinking_Manuscript_REV1_Preprint_HAL.pdf; https://doi.org/10.1242/jeb.244961

International audience Flight is an efficient way of transport over a unit of distance, but it can be very costly over each unit of time, and reducing flight energy expenditures is a major selective pressure in birds. The common swift (Apus apus) is one of the most aerial bird species, performing most behaviours in flight: foraging, sleeping, and also drinking by regularly descending to various waterbodies and skimming over the surface. An energy-saving way to perform such touch-and-go drinking would be to strive to conserve mechanical energy, by transforming potential energy to kinetic energy during the gliding descent, touching water at high speed, and regaining height with minimal muscular work. Using 3D optical tracking, we recorded 163 swift drinking trajectories, over three waterbodies near Rennes, France. Contrarily to the energy conservation hypothesis, we show that swifts approaching a waterbody with a higher mechanical energy (higher height and/or speed 5 s before contact) do not reach water at higher speeds, but do brake, i.e. dissipate mechanical energy to lose both height and speed. Braking seemed to be linked with sharp turns and the use of headwind to some extent, but finer turns and postural adjustments, beyond the resolving power of our tracking data, could also be involved. We hypothesize that this surprisingly costly behaviour results from a trade-off between energy expenditure and safety, because approaching water surface requires fine motor control, and high speed increases the risk of falling into water, which would have serious energetic and survival costs for a swift.