2.3 Stable isotope analysis
Analyses of δ13C and δ15N values in western sandpiper plasma, invertebrates, biofilm, and microphytobenthos were conducted at the Port and Airport Research Institute (Nagase, Yokosuka, Japan). Plasma samples were homogenized with a microspatula, and invertebrate, biofilm, and microphytobenthos samples were homogenized with a mortar and pestle. Samples were weighed and packed into tin capsules. Isotope ratios were measured with a Thermo Electron Delta Plus Advantage gas isotope ratio mass spectrometer (Bremen, Germany) interfaced with a Thermo Electron FlashEA 1112 elemental analyzer (Bremen, Germany). The long-term analytical precision was < 2 ‰ for δ13C and δ15N. Vienna Pee Dee Belemnite (VPDB) and air (AIR) were used as standards for δ13C and δ15N, respectively. L‐Histidine and L‐Alanine (Shoko Science Co., Ltd., Yokohama, Kanagawa, Japan) standards and blanks were included with each run to correct for drift. Isotope ratios are reported in parts per thousand (‰) using delta notation with δhN = (Rsample /Rstandard -1) x 1000, where R is the ratio of enriched to depleted isotopes for the sample or standard, N is the element of interest, andh is the mass of the enriched isotope. Most samples hadC:N ratios >4. Therefore, we normalizedδ13C values to account for variable lipid content according to the following equation from Post et al. (2007):
\(\delta^{13}C_{\text{normalized\ }}=\ \delta^{13}C_{\text{original\ }}-3.32+0.99\ \times C:N\).
Normalized δ13C values were used in all subsequent analyses.
Plasma was selected for stable isotope analysis because of its rapid turnover rate. Turnover rates for carbon and nitrogen isotopes in western sandpiper plasma are unknown; however, δ13C and δ15N values reached equilibrium in dunlin (Calidris alpina ) plasma after 10 days following a diet switch. The dunlin is a close relative of the western sandpiper, and it is likely that isotopic turnover occurs more quickly in western sandpiper plasma because western sandpipers have 50% less body mass and faster metabolic rates than dunlin (Lourenço, Granadeiro, Guilherme, & Catry, 2015). Western sandpipers captured in SF Bay in April could have wintered in SF Bay or migrated to SF Bay from wintering areas to the south. Migrants typically arrive in mid-March and early-April with an average duration of stay in SF Bay of 12 days (Iverson et al., 1996). The average departure date from SF Bay for western sandpipers is 28 April (range = 20 April - 9 May; Bishop et al., 2006). Our captures from 19-23 April coincided with the onset of western sandpiper departures from SF Bay, at which time most birds would have plasma at isotopic equilibrium with SF Bay prey.