Fig. 7. Alula mechanics during gliding perching manoeuvre. Time interval between frames: 80 ms. (A) Alula is flush with the wing during the early stages of the gliding approach. (B) Alula peels from the tip towards the end of the gliding approach. (C) Hand-wing begins to sweep back, causing peeling alula to protrude further from wing surface. (D) Alula actively protracts. An animation of the complete sequence at 500 frames s^–1 showing the entire bird is provided in supplementary material Movie 7.