Digestive bottleneck affects foraging decisions in red knots Calidris canutus. I. Prey choice

1. Rate-maximizing foragers that only divide their time between searching and handling prey should, according to the classical contingency model (CM), only select those prey whose energy content per unit handling time (i.e. profitability) exceeds or equals long-term average energy intake rate. 2. However, if digestively constrained foragers were to follow this so-called 'zero-one rule', they would need to take digestive breaks and their energy intake over total time would not be maximized. They should, according to the digestive rate model (DRM), also consider the rate at which a prey type is digested (i.e. digestive quality), such that time lost to digestive breaks is minimized. 3. In three different contexts, we tested these competing models in a mollusc-eating shorebird, the red knot (Calidris canutus), that is often digestively constrained due to its habit of ingesting its bulky prey whole. Measurements on gizzard size (using ultrasonography) and prey-characteristics confirmed that in each test the birds were digestively bottlenecked and should thus follow the DRM in order to maximize long-term energy intake. 4. In the first experiment, knots were offered a choice between two fully exposed prey, and tended to select prey by the criterion of digestive quality rather than profitability. 5. In the second experiment, knots were offered two buried prey types and preferred the highest quality prey to the most profitable prey. 6. In the wild, knots mainly fed on high quality Mya and largely ignored poor quality, but equally profitable, Cerastoderma. 7. Thus, each test verified the predictions of the DRM and rejected those of the CM. Given that many species face digestion constraints, we expect that the DRM is likely to explain diet composition in many more studies.