Fig. 4. Changes in acceleration parallel to the body fuselage (surge) throughout single swimming strokes by a Brünnich's guillemot (BRGU 13) during descent (A–C) and horizontal swimming at the bottom of a dive (D). Curves for depths >2 m during descent and at the bottom represent pooled groups of strokes with similar curves (numbers of pooled strokes in parentheses). Plots are of deviations from the mean acceleration during an entire stroke (including upstroke and downstroke), based on regression equations fitted to accelerometer recordings at 0.03125 s intervals (32 Hz). The first peak is for the upstroke, and the second for the downstroke. The first two entire strokes encompass a change in body angle from horizontal to vertical, perhaps confounding surge measurements.