Movie 1.

For this first track, recorded on site 1 (river), the 5 s approach to water surface can be divided in 5 main phases, based on mechanical energy profile: • From −5 to −3.5 s the swift glides at ∼13.5 m/s ground speed, while also turning right by ∼45°. Height decreases from 8.7 to 6.5 m (sink rate −1.5 m/s). During this phase, gliding is efficient and energy dissipation is low (−12.8 W/kg power, average over 1.5 s duration). • From −3.5 to −2s, the swift executes a sustained, descending right turn, diving from 6.5 to 0.9 m height (−3.7 m/s sink rate). This turn is not associated with an increase in ground speed. In fact, groundspeed decreases by 1.8 m/s (from 13.4 to 11.6 m/s), partly due to the bird turning in an upwind direction (wind speed estimation: 1.2 m/s). This descending turn manoeuver allows the bird to dissipate 77.6 J/kg (−51.7 W/kg power). This phase is an interesting example where the bird achieves losing height at a high rate without gaining speed, i.e. can sometimes dissipate potential energy without first converting it to kinetic energy. • Between −2 and −0.9 s, the bird executes a short flapping flight phase, with slight decrease in height (from 0.9 to 0.6 m), and increase in speed (11.6 to 12.6 m/s). Power is positive for this phase (+8.6 W/kg), reflecting that the bird produces muscular work. • An interesting final braking phase is observed between −0.9 and −0.3s : the bird makes a quick ∼45° left turn, followed by a striking posture with high wing dihedral and high body-and-tail incidence, clearly visible on the video extract. During this 0.6 s phase, the bird loses 4.6 m/s in speed (from 12.6 to 8.0 m/s), in addition to descending 0.5 m (from 0.6 to 0.1 m), which results in a −87.9 W/kg dissipative power. • The last 0.3 s of approach are a straight (in the horizontal plane), efficient gliding flight to water surface (power −15.2 W/kg), with reduced wing dihedral and open gape (Note that heading rate of change (ROC) peak at water contact reflects the vertical pitch up inflexion). After water contact, the swift actively flaps its wings, with an average climb rate of +2.2 m/s over the first second, while also slightly accelerating (average power +25.3 W/kg).