Movie 2.

In this track recorded on site 2 (small pond), the approach can be divided in 4 phases: • From −5 to −4s, the bird climbs 2.2 m, while decelerating from 13.6 to 10 m/s. The bird also produces 4 wing strokes. However, power over this phase is negative at −19 W/kg. • From −4 to −1.4s, the swift dives from 22.6 to 6.4m height (−6.2 m/s average sink rate). Simultaneously, conversion from potential energy to kinetic energy occurs and the bird accelerates from 10.0 to 17.3 m/s. The −23.0 W/kg power over this phase suggests that the conversion is quite efficient, as energy dissipation rate remains similar to a typical gliding flight (∼ −20 W/kg). Note that a ∼90° turn into a ∼1.7 m/s wind occurs during this phase, without a clear effect in energy dissipation. • Most interestingly, between −1.4 and −0.7s, energy dissipation occurs in a very abrupt way: the swift loses another 5.5 m in height (from 6.4 to 0.9 m, 7.9 m/s sink rate), but simultaneously violently decelerates from 17.3 to 12 m/s. Overall, mass-specific mechanical energy drops from 213 to 81 J/kg, which corresponds to a −189 W/kg power, more than 9 times the typical gliding power. Looking at the video extract (of limited image quality because of heavy croping in compressed 4K frames), it appears the bird uses high body-and-tail incidence, followed by high wing dihedral during braking. Note that, in the present case, braking occurs without any significant horizontal heading change (see track top view). • During the last 0.7 s before contact, the swift descends the last 0.9 m to the water surface (sink rate −1.2 m/s), with a small decrease in speed (from 12.0 to 11.5 m/s). The power is back to a standard gliding power of −21.7 W/kg. After water contact, this swift actively climbs at an average +2.8 m/s (power +26.6 W/kg).