Biofilm and invertebrate consumption by western sandpipers () and dunlin ( during spring migratory stopover: insights from tissue and breath CO isotopic ( C, N) analyses.

Shorebirds use key migratory stopover habitats in spring and fall where body proteins are replenished and lipids stored as fuel for the remaining journey. The Fraser River estuary, British Columbia, Canada, is a critical spring stopover site for hundreds of thousands of migrating western sandpiper, , and dunlin, . Intertidal biofilm in spring is an important nutritional source for western sandpiper, with previous isotopic research predicting 45-59% of total diet and 50% of total energy needs. However, these studies relied on isotopic mixing models that did not consider metabolic routing of key dietary macromolecules. Complexity arises due to the mixed macromolecular composition of biofilm that is difficult to characterize isotopically. We expanded on these earlier findings by considering a protein pathway from diet to the body protein pool represented by liver tissue, using a Bayesian mixing model based on C and N. We used C measurements of adipose tissue and breath CO to provide an estimate of the carbohydrate and protein C values of microphytobenthos and used these derived values to better inform the isotopic mixing models. Our results reinforce earlier estimates of the importance of biofilm to staging shorebirds in predicting that assimilated nutrients from biofilm contribute ~35% of the protein budgets for staging western sandpipers ( = 13) and dunlin ( = 11) and at least 41% of the energy budget of western sandpiper ( = 69). Dunlin's ingestion of biofilm appeared higher than anticipated given their expected reliance on invertebrate prey compared to western sandpiper, a biofilm specialist. Isotopic analyses of bulk tissues that consider metabolic routing and that make use of breath CO and adipose lipid assays can provide new insights into avian physiology. We advocate further isotopic research to better understand biofilm use by migratory shorebirds in general and as a critical requirement for more effective conservation.