Timing of Shorebird Migration in Relation to Prey Depletion

Abstract The southward departure of migratory shorebirds (Charadrii) before the collapse of food supplies on the breeding grounds would seem to disprove the hypothesis that migratory timing is a trait actively maintained by selective pressures created by seasonal production of food supplies. In this study we looked at seasonal changes in the food supplies of shorebirds at a migratory stopover to see if dwindling food supplies here might explain early arrival from Arctic breeding grounds. For the three earliest migrants through Plymouth, Massachusetts we found substantial reductions of prey densities (7-90%) during migration (July-September). Reductions in prey density were not substantial before September for a later migrant. Caging experiments showed that the reductions in density of infaunal prey were due to predation. If these reductions are sufficient to cause energetic deficits among late arrivals, then competition may be an important selective agent in shaping migratory patterns.