Foraging in an Tidally structuren environment by Red Knots (Calidris Canutus): Ideal, but not free

Besides the ‘‘normal’’ challenge of obtaining adequate intake rates in a patchyand dangerous world, shorebirds foraging in intertidal habitats face additional environmentalhurdles. The tide forces them to commute between a roosting site and feeding grounds, twice aday. Moreover, because intertidal food patches are not all available at the same time,shorebirds should follow itineraries along the best patches available at a given time. Finally,shorebirds need additional energy stores in order to survive unpredictable periods of badweather, during which food patches are covered by extreme tides. In order to model such tidespecificdecisions, we applied stochastic dynamic programming in a spatially explicit context.Two assumptions were varied, leading to four models. First, birds had either perfect (ideal) orno (non-ideal) information about the intake rate at each site. Second, traveling between siteswas either for free or incurred time and energy costs (non-free). Predictions were generated forthree aspects of foraging: area use, foraging routines, and energy stores. In general, non-idealforagers should feed most intensely and should maintain low energy stores. If traveling forsuch birds is free, they should feed at a random site; otherwise, they should feed close to theirroost. Ideal foragers should concentrate their feeding around low tide (especially when free)and should maintain larger energy stores (especially when non-free). If traveling for such birdsis free, they should feed at the site offering the highest intake rate; otherwise, they should tradeoff travel costs and intake rate. Models were parameterized for Red Knots (Calidris canutus)living in the Dutch Wadden Sea in late summer, an area for which detailed, spatially explicitdata on prey densities and tidal heights are available. Observations of radio-marked knots(area use) and unmarked knots (foraging routines, energy stores) showed the closest matchwith the ideal/non-free model. We conclude that knots make state-dependent decisions bytrading off starvation against foraging-associated risks, including predation. Presumably,knots share public information about resource quality that enables them to behave in a moreor less ideal manner. We suggest that our modeling approach may be applicable in othersystems where resources fluctuate in space and time.