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Swim speeds and stroke patterns in wing-propelled divers: a comparison among alcids and a penguin
Watanuki, Y., Wanless, S., Harris, M.P., Lovvorn, J.R., Miyazaki, M., Tanaka, H. & Sato, K. (2006) J. Exp. Biol. 209(7):1217-1230
In diving birds, the volume and resulting buoyancy of air spaces changes with dive depth, and hydrodynamic drag varies with swim speed. These factors are important in the dive patterns and locomotion of aclids that use their wings both for aerial flight and underwater swimming. Using small data-loggers on free-ranging birds diving to 20-30m depth, we measured depth at 1Hz and surge and heave accelerations at 32-64Hz of four species of alcids (0.6-1.0kg mass) and the smallest penguin species (1.2kg). Low - and high-frequency components of the fluctuation of acceleration yielded estimates of body angles and stroke frequencies, respectively. Swim speed was estimated from body angle and rate of depth change. Brünnich's (Uria lomvia) and Common (Uria aalge) Guillemots descended almost vertically, whereas descent of Razorbills (Alca torda), Rhinoceros Auklets (Cerorhinca monocerata) and Little Penguins (Eudyptula minor) was more oblique. For all species, swim speed during descent was within a relatively narrow range. Above depths of 20-30m, where they were all positively buoyant, all species ascended without wing stroking. During descent, Little Penguins made forward accelerations on both the upstroke and down stroke regardless of dive depth. By contrast, descending alcids produced forward accelerations on depths of <10m but mainly on the down stroke at greater depths; this change seemed to correspond to the decrease in buoyancy with increasing depth. The magnitude of surge (forward) acceleration during down strokes was smaller, and that during upstrokes greater, in Little Penguins than in alcids. This pattern presumably reflected the proportionally greater mass of upstroke muscles in penguins compared with alcids and may allow Little Penguins to swim at less variable instantaneous speeds.

Key words: acceleration, buoyancy, data-logger, swimming mode, wing stroke


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