Bulk stable isotope and fatty acids analysis
We used bulk tissue analyses of δ13C fin clips and potential prey to estimate diet composition of experimental individuals. Freeze-dried and homogenized samples were analysed in WasserCluster Lunz, Austria. Isotope ratios are reported relative to the international Vienna PeeDee Belemnite carbonate standard. δ13C values were mathematically corrected for lipid content following methods as described by Post et al. (2007). The baseline 13C value was further corrected to account for variability in basal resources across the sampling sites (Olsson et al. 2009; Musseau et al. 2020) using the following equation:
\begin{equation} \delta^{13}C_{\text{corr}i}=\ \frac{\delta^{13}C_{i}-\delta^{13}C_{\text{inv}}}{\text{CR}_{\text{inv}}}\nonumber \\ \end{equation}
where δ13Ccorri is the corrected carbon isotopic ratio for individual i , δ13Ci is the carbon isotopic ratio for individual i , δ13 Cinv is the average carbon isotope ratio of macroinvertebrates and CRinv is the carbon range (δ13Cmax − δ13Cmin ) of macroinvertebrates. δ13Ccorri is hereafter used as an indicator of reliance on terrestrial prey (i.e., increasing value suggests increasing reliance of an individual on terrestrial prey).
Fatty acids were extracted and analysed from freeze-dried samples (3–10 mg dry mass) that were homogenized, sonicated and vortexed (4 times) in a chloroform-methanol (2:1) mixture, following Böhm et al. (2014). Total lipid mass ratios were determined via gravimetry. Fatty acids were derivatized to obtain fatty acid methyl esters (FAME) using toluene and sulfuric acid-methanol-solution (incubated at 16 h at 50 C). FAME were identified using a gas chromatograph (Thermo Scientific TRACE GC Ultra) equipped with a flame ionization detector (FID) and a Supelco SP-2560 column (100 m, 25 mm i.d., 0.2 µm film thickness). Quantification of fatty acids were performed by comparison with a known concentration of the internal standard using Excalibur 1.4 (Thermo Electron Corporation).