Slime, safety and shorebirds: Biofilm production and grazing by migrating western sandpipers (Calidris mauri)

The quality of stopover sites for migrant shorebirds is thought to be determined by food availability and safety from predators. This thesis investigates this interaction on an estuarine mudflat in British Columbia, where migrant western sandpipers graze biofilm. I measured biofilm concentration and grazing intensity on transects across the mudflat. I found that the concentration of biofilm rose 4.1 mg m-2 hr-1 during tidal emersion periods, with total accumulation matching that removed by sandpipers during grazing visits. During the higher-intensity (10 – 100 fold, based on daily sandpiper counts) northward migration, biofilm concentration increased and grazing decreased with proximity to the shoreline. In contrast, during southward migration biofilm was uniformly high. A danger manipulation experiment supported a trade-off with biofilm concentration: grazing declines with danger, but less so where biofilm is higher. Together the results indicate that dynamic trophic interactions between danger, sandpipers and biofilm create spatial patterns in biofilm concentration.