Fig. 3. Typical changes of heave (solid line) and surge (dotted line) acceleration of a common guillemot (COGU) and a little penguin (LIPE) during descent at depths of 2 m and 20 m and during the bottom phase of the dive. Crosses along the line for surge indicate samples at 64 Hz. Downstrokes were assumed to occur during periods of positive heave, as indicated by bold horizontal bars. For COGU, the second small peak of heave was observed at the end of the downstroke at 2 m and 20 m. The horizontal dotted bar in the panel for horizontal swimming during the bottom phase by COGU indicates a glide when there was little heave or surge. Circles show peaks of surge acceleration, indicating forward thrust during upstroke (filled circles) and downstroke (open circles).