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First published online October 5, 2006
Journal of Experimental Biology 209, 3974-3983 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02482

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Regulation of stroke pattern and swim speed across a range of current velocities: diving by common eiders wintering in polynyas in the Canadian Arctic

Joel P. Heath^1,*, H. Grant Gilchrist² and Ronald C. Ydenberg¹

¹ Centre for Wildlife Ecology / Behavioural Ecology Research Group, Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada
² National Wildlife Research Centre, Canadian Wildlife Service, 1125 Colonel by Drive, Raven Road, Carleton University, Ottawa, Ontario, K1A 0H3, Canada

View larger version (13K): [in a new window]   Fig. 1. A schematic illustrating the measured descent velocity of an eider diving into currents at the edge of a polynya. Eiders always dived into currents and ended upstream of their surface departure point. This schematic illustrates the dive angle and vectors used to calculate effective swim velocity, relative to the moving fluid, as described in the text.

View larger version (211K): [in a new window]   Fig. 2. Various stages of the wing and foot stoke cycle illustrated from 1/30th of a second video frames of common eiders during descent, from four different angles (rows). Each stroke cycle illustrated was 0.43 s and so frame numbers from 1 to 13 are used to describe the various stages of the stroke cycle in the text, and correspond with stoke cycle stages indicated in Fig. 3.

View larger version (7K): [in a new window]   Fig. 3. Timing of various stages of the wing and foot stroke cycle. Wing (black circles) and foot (white circles) stokes are divided into three stages, which are illustrated in Fig. 2 and described in the text. Thrust from foot propulsion ubiquitously corresponded with the transition between the upstroke and downstroke of the wings, when drag is probably greatest because of the large angle of attack of the wings. Correspondence in timing between power and recover phases of wing and foot propulsion could therefore be important in maintaining steady speed, minimizing the cost of drag during diving.

View larger version (12K): [in a new window]   Fig. 4. Descent duration (A) and number of wing stroke cycles (B) to descend to depth (11.3 m) increased non-linearly with increasing current velocity (m s–1). Note that each wing stroke cycle also included a foot stroke.

View larger version (5K): [in a new window]   Fig. 5. Average stroke cycle frequency per dive did not vary with respect to current velocity.

View larger version (8K): [in a new window]   Fig. 6. Regression equation of vertical descent speed (relative to the bottom; solid line) and effective swim speed relative to the fluid (calculated using vector geometry; see Materials and methods), over a range of current speeds. The dashed line is regression of effective swim speed at a dive angle of 10° with 95% confidence intervals. The shaded area indicates regression equations from sensitivity analysis of dive angle from 0° (lower edge of shaded area) to 20° (upper edge of shaded area). Effective swim speed was regulated across currents at a relatively constant value of 1.24±0.14 m s–1 while vertical descent velocity decreased non-linearly. Regression equations are presented in Table 1.