Shorebirds, mud snails, and Corophium volutator in the upper Bay of Fundy, Canada: predicting bird activity on intertidal mud flats

The upper Bay of Fundy is a critical staging area for migrating semipalmated sandpipers (Calidris pusilla), which feed extensively on the amphipod Corophium volutator. Recent changes in the distributions of birds around the bay have generated interest in understanding the factors that influence shorebirds' habitat use. To examine some of these factors, we surveyed four mud flats during summer 2000. Abundance of shorebirds and density of C. volutator and mud snails (Ilyanassa obsoleta) were assessed in two focused areas and on two broad-scale transects per mud flat. Shorebird abundance varied widely across sites, and was positively correlated with C. volutator availability, though the relationship had little predictive ability. A strong negative relationship was identified between shorebirds and mud snails counted before birds arrived, especially in the focused areas. This relationship probably stems from a negative effect of mud snails on C. volutator. Snails present in July may reduce recruitment of C. volutator, and therefore reduce the prey base for shorebirds arriving later in the summer. By considering mud snail abundance, we have identified a simple and effective method of predicting shorebird habitat use several weeks before birds arrive. This will enhance our ability to study these birds in future, and will contribute to our understanding of shorebird habitat use and movements and the importance of conserving particular mud flats.