Digestive bottleneck affects foraging decisions in red knots Calidris canutus. II. Patch choice and length of working day

1. When prey occur at high densities, energy assimilation rates are generally constrained by rates of digestion rather than by rates of collection (i.e. search and handle). As predators usually select patches containing high prey densities, rates of digestion will play an important role in the foraging ecology of a species. 2. The red knot Calidris canutus shows tremendous inter- and intra-individual variation in maximum rates of digestion due to variation in the size of the processing machinery (gizzard and intestine), which makes it a suitable species to study the effects of digestive processing rate on foraging decisions. 3. Here we report on patch use, prey choice, and daily foraging times as a function of gizzard size in free-ranging, radio-marked, red knots. As knots crush their bulky bivalve prey in their gizzard, the size of this organ, which we measured using ultrasonography, determines digestive processing rate. 4. Using the digestive rate model, we a priori modelled patch use, prey choice, and daily foraging times as a function of gizzard mass. Focusing on two contrasting patches, birds with small gizzards were expected to feed on high-quality (soft-bodied) prey found in low densities in the one patch, while birds with large gizzards were expected to feed on low-quality (hard-shelled) prey found in high densities in the other patch. Assuming that red knots aim to balance their energy budget on a daily basis, we expected daily foraging time to decline with gizzard mass. 5. Observed patch and prey choices were in quantitative agreement with these theoretical predictions. Observed daily foraging times were only in qualitative agreement: they declined with gizzard mass but less steeply than predicted. 6. We discuss that red knots might be aiming for a slightly positive energy budget in order to (i) refuel their stores depleted during migration, and (ii) to insure against unpredictability in supply and demand during winter. Red knots arriving from their breeding grounds with small gizzards are only able to realize this aim when densities of soft-bodied prey are high, which is the case in late July and early August. Rapidly declining soft-bodied prey densities throughout late summer pose a major penalty for individuals arriving late at their wintering grounds. 7. The long daily foraging periods required by knots with small gizzards are only feasible through ‘tide-extension’. In our study area, birds can and do raise the daily low tide period from 12 h to almost 17 h by moving along with the tide in an easterly direction, subsequently flying back to their starting point at the high tide roost.