Digestively constrained predators evade the cost of interference competition

1. Models of functional and aggregative responses generally assume that rates of prey encounter and handling times limit a predator's intake rate (Holling's disc equation). Two different lines of approach build upon this fundamental foraging concept. In the first, mutual interference further constrains intake rate, while in the second, intake rate may be constrained by rate of digestion. By combining both approaches, we come up with four competing models that differ in whether predators interfere and whether they face a digestive constraint.2. The functional responses expected by these four models are tested experimentally in a medium-sized shorebird, the red knot (Calidris canutus), fed a shelled prey, the blue mussel (Mytilus edulis). The experimental results suggest that intake rate is constrained by rate of digestion at low bird densities and by interference at high bird densities.3. Using the experimentally obtained parameters, we predicted aggregative responses for each of the four models, which we verified by using field observations. We found evidence that the combination of interference and digestive constraints similarly governed the aggregative responses of red knots. Compared to the expectations of the models that do not include digestive constraints, red knots fed in lower and more variable prey densities and were generally aggregated in denser flocks. In addition, they were packed twice as densely when feeding on hard-shelled prey than when feeding on soft-bodied prey.4. We suggest that digestive constraints allow red knots to live in dense flocks: if digestion proceeds during interference interactions, the time-cost of interference may be negligible.