Fig. 5. Velocity () and acceleration () in the x (A) and y (B) directions of the puffin in the sequence used in Figs 2–Fig. 5 and in the x (C) and y (D) directions of a bird in a sequence showing positive acceleration in the x direction during the upstroke. The upstroke of the wrist point (Wrist) is from the start of the light grey area to the end of the dark grey area. The upstroke of the wing tip (Tip) is from the start of the dark grey area to the end of the light grey area. The values are means of the velocity and acceleration of three digitised points on the body (Beak, Eye and Breast) and error bars are ± S.E.M. (see text for definitions).