Biofilm and invertebrate consumption by western sandpipers (Calidris mauri) and dunlin (Calidris alpina)during spring migratory stopover: insights from tissue and breath CO2 isotopic (δ13C,δ15N) analyses

Abstract

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, Calidris mauri, and dunlin, Calidris alpina. 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 δ13C and δ15N. We used δ13C measurements of adipose tissue and breath CO2 to provide an estimate of the carbohydrate and protein δ13C 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 (n = 13) and dunlin (n = 11) and at least 41% of the energy budget of western sandpiper (n = 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 CO2 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.

Shorebird Abundance is Associated with Nutritional Quality of Intertidal Biofilm on the Fraser River Estuary

“…Invertebrates can constitute a large fraction (~ 20-50%) of the diets of western sandpipers and dunlin (Kuwae et al 2008;Jardine et al 2015;Hobson et al 2022), and their abundance, biomass, and diversity might explain the observed differences in shorebird counts between Brunswick Point and Iona Foreshore. Shorebird distribution and abundance has been related to the abundance of benthic invertebrates (Colwell and Landrum 1993) but not always (Horn et al 2020).…”

“…Smallbodied shorebirds consume a variety of prey, including marine invertebrates and intertidal biofilm, a thin layer of microalgae, bacteria, and other microorganisms embedded in a mucilaginous matrix composed of extracellular polymeric substances (EPS; Decho 2000;Stal 2010). Intertidal biofilm can account for 45-59% of the shorebird total diet or 50% of their daily energy budget (Kuwae et al 2012;Hobson et al 2022). On the Fraser River Estuary, British Columbia, Canada, Jardine et al (2015) investigated biofilm consumption by Western Sandpiper Calidris mauri and found that sites with the highest proportion of intertidal biofilm in the diet supported larger numbers of birds, suggesting that site use was related to biofilm consumption.…”