Movie 3.

This track recorded on site 3 (large pond) shows an example of high, sustained mechanical energy dissipation. From an energy level of 396 J/kg at −5 s, to only 43 J/kg at water contact, the bird produces an average dissipative power of −70.6 W/kg over 5s (minimal value in Fig. 3F). The approach can be divided in 2 main phases, based on energy profile : • From −5 to −2.5s, the bird steeply dives from 26.0 to 3.6 m (−9.0 m/s sink rate), and converts this large amount of potential energy into very high speed, accelerating from 16.8 to 24.7 m/s. This conversion is efficient, with a typical gliding power of −21.6 W/kg power over this first phase. • On the contrary, in the last 2.5s before water contact, the swift brakes from 24.7 to 9.3 m/s, while still losing 3.6 m in height. Over this second phase, mechanical energy is dissipated at a rate of −119.4 W/kg. Looking at bird heading rate of change, and bird flight posture in video extract, braking appears to be associated with short alternating banking manoeuvers, with short peaks in heading change. These rapid “zigzag” manoeuvers are barely visible on the track top view, as they are not sustained turns, and the swift maintains an overall upwind heading. In the last second before water contact, complementary manoeuvers, such as high-incidence body-and-tail posture and leg trailing (from around −0.4 s) are also visible on this video extract. Unfortunately, our tracking data was too noisy (and hence required significant smoothing) to precisely assess the contribution of each of these transient manoeuvers to braking. After water contact, the swift actively climbs at a rate of +3.4 m/s over the first second, while also slightly accelerating (average power +37.1 W/kg).