Stroke patterns and regulation of swim speed and energy cost in free-ranging Brunnich's guillemots

Loggers were attached to free-ranging Brunnich's guillemots Uria lomvia during dives, to measure swim speeds, body angles, stroke rates, stroke and glide durations, and acceleration patterns within strokes, and the data were used to model the mechanical costs of propelling the body fuselage (head and trunk excluding wings). During vertical dives to 102-135 m, guillemots regulated their speed during descent and much of ascent to about 1.6 +/- 0.2 m s(-1). Stroke rate declined very gradually with depth, with little or no gliding between strokes. Entire strokes from 2 m to 20 m depth had similar forward thrust on upstroke vs downstroke, whereas at deeper depths and during horizontal swimming there was much greater thrust on the downstroke. Despite this distinct transition, these differences had small effect (<6%) on our estimates of mechanical cost to propel the body fuselage, which did not include drag of the wings. Work stroke(-1) was quite high as speed increased dramatically in the first 5 m of descent against high buoyancy. Thereafter, speed and associated drag increased gradually as buoyancy slowly declined, so that mechancal work stroke(-1) during the rest of descent stayed relatively constant. Similar work stroke(-1) was maintained during non-pursuit swimming at the bottom, and during powered ascent to the depth of neutral buoyancy (about 71 m). Even with adjustments in respiratory air volume of +/- 60%, modeled work against buoyancy was important mainly in the top 15 m of descent, after which almost all work was against drag. Drag was in fact underestimated, as our values did not include enhancement of drag by altered flow around active swimmers. With increasing buoyancy during ascent above 71 m, stroke rate, glide periods, stroke acceleration patterns, body angle and work stroke(-1) were far more variable than during descent; however, mean speed remained fairly constant until buoyancy increased rapidly near the surface. For dives to depths >20 m, drag is by far the main component of mechanical work for these diving birds, and speed may be regulated to keep work against drag within a relatively narrow range.