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

• Published in: Journal of Experimental Biology
• Main Authors: Heath, J.P., Gilchrist, H.G., Ydenberg, R.C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2006
• Subjects: acceleration; birds; body shape; brunnichs guillemots; cost; drag; flight; frequency; glide; size; Arctic; Nunavut; Hudson Bay; Canada; Hudson; Belcher; Belcher Islands; Sea ice; Somateria mollissima
• Online Access: https://research.wur.nl/en/publications/regulation-of-stroke-pattern-and-swim-speed-across-a-range-of-cur; https://doi.org/10.1242/jeb.02482

Swim speed during diving has important energetic consequences. Not only do costs increase as drag rises non-linearly with increasing speed, but speed also affects travel time to foraging patches and therefore time and energy budgets over the entire dive cycle. However, diving behaviour has rarely been considered in relation to current velocity. Strong tidal currents around the Belcher Islands, Nunavut, Canada, produce polynyas, persistent areas of open water in the sea ice which are important habitats for wildlife wintering in Hudson Bay. Some populations of common eiders Somateria mollissima sedentaria remain in polynyas through the winter where they dive to forage on benthic invertebrates. Strong tidal currents keep polynyas from freezing, but current velocity can exceed 1.5 m s¿1 and could influence time and energy costs of diving and foraging. Polynyas therefore provide naturally occurring flume tanks allowing investigation of diving strategies of free ranging birds in relation to current velocity. We used a custom designed sub-sea ice camera to non-invasively investigate over 150 dives to a depth of 11.3 m by a population of approximately 100 common eiders at Ulutsatuq polynya during February and March of 2002 and 2003. Current speed during recorded dives ranged from 0 to 1 m s¿1. As currents increased, vertical descent speed of eiders decreased, while descent duration and the number of wing strokes and foot strokes during descent to the bottom increased. However, nearly simultaneous strokes of wings and feet, and swim speed relative to the moving water, were maintained within a narrow range (2.28±0.23 Hz; 1.25±0.14 m s¿1, respectively). This close regulation of swim speed over a range in current speed of 1.0 m s¿1 might correspond to efficient muscle contraction rates, and probably reduces work rates by avoiding rapidly increasing drag at greater speeds; however, it also increases travel time to benthic foraging patches. Despite regulation of average swim speed, high instantaneous speeds during oscillatory ...