Western Sandpiper ( Calidris mauri ) use of intertidal biofilm in freshwater and marine influenced mudflats during migratory stopover

Migrating Western Sandpipers (Calidris mauri) often stop to refuel at mudflats where intertidal biofilm forms a major component of their diet. Western Sandpipers may increase intake of this food source at sites where biofilm diatoms are high in Omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFAs), which have been hypothesized to improve shorebird physiological condition and migration performance. At mudflats in the Fraser River Delta (FRD) of British Columbia (BC), Canada, outflowing freshwater from the spring snowmelt is believed to induce n-3 LC-PUFA proliferation in biofilm diatoms, potentially making this refuelling site a distinctly important source of intertidal biofilm. We used Bayesian mixing models to analyse δ13C and δ15N of Western Sandpiper blood plasma to compare biofilm consumption at FRD areas to another highly frequented stopover site, the relatively marine-influenced Wah-nah-jus Hilth-hoo-is mudflat system at Tofino, BC. While past studies have estimated biofilm as a significant portion of Western Sandpiper diet in the FRD, the assumption that this refuelling area provides a distinct advantage had not been tested by comparing proportional dietary estimates of biofilm at the FRD to other stopover mudflat systems. Our results confirm the assumption that biofilm consumption during the northward, pre-breeding migration at the FRD (∼31 % of diet) is greater than at another commonly used refuelling location (Tofino, BC; ∼16 % of diet). Seasonal effects were also measured between migratory periods, revealing an apparent, but insignificant, decrease in biofilm consumption at the FRD during post-breeding migration. Biofilm consumption appeared to be marginally higher in juveniles (i.e. hatch-year) than in adults (i.e. after-hatch-year) at the FRD but not at Tofino, and no significant differences were detected by sex class at either mudflat system. These findings can inform land use decisions that may impact essential shorebird habitat, particularly at the Roberts Bank mudflat area of the FRD.