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To maximize fitness, animals must respond to a variety of processes that operate at different rates or timescales. Appropriate decisions could therefore involve complex interactions among these processes. For example, eiders wintering in the arctic sea ice must consider locomotion and physiology of diving for benthic invertebrates, digestive processing rate and a nonlinear decrease in profitability of diving as currents increase over the tidal cycle. Using a multi-scale dynamic modelling approach and continuous field observations of individuals, we demonstrate that the strategy that maximizes long-term energy gain involves resting during the most profitable foraging period (slack currents). These counterintuitive foraging patterns are an adaptive trade-off between multiple overlapping rate processes and cannot be explained by classical rate-maximizing optimization theory, which only considers a single timescale and predicts a constant rate of foraging. By reducing foraging and instead digesting during slack currents, eiders structure their activity in order to maximize long-term energetic gain over an entire tide cycle. This study reveals how counterintuitive patterns and a complex functional response can result from a simple trade-off among several overlapping rate processes, emphasizing the necessity of a multi-scale approach for understanding adaptive routines in the wild and evaluating mechanisms in ecological time series.

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 stroking can increase dive costs due to drag. While most diving birds have been considered either foot or wing propelled, eider ducks used both wing and foot propulsion during descent. Our observations indicate that the power phase of foot strokes coincides with the transition between upstroke and downstroke of the wings, when drag is greatest. Coordinated timing between foot and wing propulsion could therefore serve to maintain a steadier speed during descent and decrease the costs of diving. Despite tight regulation of stroke and swim speed patterns, descent duration and total number of foot and wing strokes during descent increase non-linearly with increasing current velocity, suggesting an increase in energetic costs of diving.