Functional response of staging semipalmated sandpipers feeding on burrowing amphipods

Despite its fundamental relevance to many ecological processes in predator-prey relationships, the functional response, which relates predator intake rate to prey density, remains difficult to document in the field. Here, I document the functional response of semipalmated sandpipers (Calidris pusilla) foraging on a burrowing amphipod Corophium volutator during three field seasons at the peak of fall migration in the upper Bay of Fundy (New Brunswick, Canada). I gathered data during the ebbing tide when all sandpipers are highly motivated to feed after a lengthy hide-tide fast. As birds follow the receding tideline, foragers encounter prey at different densities and do not aggregate in the richest food patches. Results show that intake rate increased at a decreasing rate with Corophium density, yielding a type II functional response typical of many shorebird species. Intake rate decreased in the later stages of migration stopover at a time where preferred prey items have been shown to occur at lower densities due to prior depletion. At this period of lower prey availability, intake rate also decreased with sandpiper density providing evidence for interference at low prey density. The results illustrate the fact that the functional response may not be unique but instead vary as a function of the type of competitive relationship among foragers.