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

ABSTRACT

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 expenditure 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. Contrary 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 the water at higher speeds, but do brake, i.e. dissipate mechanical energy to lose both height and speed. Braking seems 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 a water surface requires fine motor control, and high speed increases the risk of falling into the water, which would have serious energetic and survival costs for a swift.