Groups of Common Eiders Somateria mollissima sedentaria winter in polynyas around the Belcher Islands, Nunavut. As part of a program investigating the response of eider populations to environmental change in Hudson Bay, I studied how abiotic and physiological processes interact to affect foraging strategies of eiders during midwinter. I used a video camera to record their underwater activities, and radio telemetry to follow individuals continuously. Polynyas are open water sea ice habitats maintained by strong tidal currents. As currents slackened with changing tides, eiders required less time and work to dive to the polynya bottom (11m), and spent more foraging time at depth. Total dive times were constant (58.47±5.32 s), and corresponded with estimated aerobic capacity. Pauses between dives did not change with current speed, and were longer than necessary for recovery from diving, suggesting other factors were important. Energetic modeling indicated that the profitability of dives fell non-linearly with increasing current speed, so that diving becomes unprofitable at about 1.2 m/s; eiders then rest on the ice edge. When tidal amplitude was large and current profiles strong (i.e., spring tide), eiders foraged most intensely at the beginning and end of foraging periods, rather than during mid-period slack currents, when dives would be most profitable. To understand this counter-intuitive result, I considered that digestion of benthic prey limits intake rate. A dynamic state variable (DSV) model incorporating a digestive processing constraint predicted that foraging patterns like those observed maximize energy gain over an entire tidal cycle, because eiders load up the gullet and gizzard at the start and end of the foraging period. The DSV model also correctly predicted different foraging patterns observed during weaker current profiles. During weak current profiles, diving occurred in well-defined bouts, but bout structure was less evident under strong profiles. I hypothesized bout structure is generated by interactions between processes on several time scales, including the profitability of dives, digestive rate, and the tidal and lunar cycles. A multi-scale approach is necessary to understand the dynamic range of behavioural patterns that allow eiders to balance energy budgets during mid-winter in the Arctic.
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