Swim speeds and stroke patterns in wing-propelled divers: a comparison among alcids and a penguin

Abstract

SUMMARY 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 alcids that use their wings both for aerial flight and underwater swimming and of penguins that use their wings only for swimming. Using small data-loggers on free-ranging birds diving to 20–30 m depth, we measured depth at 1 Hz and surge and heave accelerations at 32–64 Hz of four species of alcids (0.6–1.0 kg mass) and the smallest penguin species (1.2 kg). 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 20–30 m, where they were all positively buoyant, all species ascended without wing stroking. During descent, little penguins made forward accelerations on both the upstroke and downstroke regardless of dive depth. By contrast, descending alcids produced forward accelerations on both upstroke and downstroke at depths of <10 m but mainly on the downstroke at greater depths; this change seemed to correspond to the decrease of buoyancy with increasing depth. The magnitude of surge (forward)acceleration during downstrokes 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.

High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins

“…1B). Although mechanical costs during deep dives have been precisely measured and increase approximately linearly with dive depth (30)(31)(32), actual metabolic costs measured in the field decelerated as dive depth increased (Table 1). We suggest that physiological processes during the dive, such as oxygen store management and thermoregulation, are the dominant processes determining costs in wing-propelled divers diving to depths where buoyancy costs are minimal (22,25,27).…”

Modeling profitability for the smallest marine endotherms: auklets foraging within pelagic prey patches

“…Mean speeds and angles (relative to horizontal) of swimming during descent were based on micrologger measurements for 4 alcids -TBMU, COMU, razorbill Alca torda (RAZO), and RHAU -and little penguins (LIPE) (Watanuki et al 2006). Extrapolating from regression of swim speed vs. body mass yielded estimated mean speeds during descent of 0.88 m s -1 for CAAU and 0.80 m s -1 for LEAU (Fig.…”

“…Descent angle (relative to horizontal) for both CAAU and LEAU was assumed to be the same as the mean for RHAU (0.57 kg) diving to about 20 m depth: -47° (Fig. 5 in Watanuki et al 2006). Hydrodynamic drag D (N) was based on tow-tank measurements at different speeds U (m s -1 ) for frozen birds (Lovvorn et al 2001b).…”

“…Based on the maximum circumference and length of frozen auklets, we used an added mass coefficient of 0.075 (the fractional increase in body mass due to entrained water to yield M v , Kochin et al 1964). This work against buoyancy W B during time t was then calculated as W B = B × X, and added to work against drag during horizontal swimming at swim distance increments of 0.3 m. For 4 alcid species equipped with logging TDRaccelerometers (TBMU, COMU, RAZO, RHAU), stroking ceased during ascent at depths shallower than 20 m (Watanuki et al 2006). Accordingly, we assumed passive ascent above that depth; however, strokes to begin ascent may increase the subsequent passive speed over that imparted by buoyancy alone.…”

“…Such loggers include time-depth recorders (TDRs), accelerometers, and heart rate loggers (Kato et al 2003, Tremblay et al 2003, Butler et al 2004, Watanuki et al 2006, Harding et al 2009). At the time of writing this paper, minimum sizes of loggers used in published studies were about 8 × 16 × 27 mm and 5 g for TDRs, 50 × 15 mm and 14 g for cylindrical accelerometer-TDRs (Kato et al 2003), and 40 × 30 × 13 mm and 20 g for heart rate loggers (Butler et al 2004).…”