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Regulation of stroke and glide in a foot-propelled avian diver

SUMMARY Bottom-feeding, breath-hold divers would be expected to minimize transit time between the surface and foraging depth, thus maximizing the opportunities for prey capture during the bottom phase of the dive. To achieve this they can potentially adjust a variety of dive parameters, including dive angle and swim speed. However, because of predictable changes in buoyancy with depth, individuals would also be expected to adjust dive behavior according to dive depth. To test these predictions we deployed miniature, dorsally attached data-loggers that recorded surge and heave accelerations at 64 Hz to obtain the first detailed measurements of a foot-propelled diving bird, the European shag Phalacrocorax aristotelis, in the wild. The results were used to investigate biomechanical changes during the descent, ascent and bottom phases for dives varying between 7 m and 43 m deep. Shags descended and ascended almost vertically (60–90° relative to the sea surface). During descent, swim speed varied between 1.2–1.8 m s–1 and the frequency of the foot stroke used for propulsion decreased significantly with depth, mainly due to a fivefold increase in the duration of the glide between strokes. Birds appeared to maintain the duration and the maximum strength of power stroke and thus optimize muscle contraction efficiency.