Immunoglobulin (IgY) levels
To measure the constitutive part of acquired immune function we measured immunoglobulin levels. We used the enzyme-linked immunosorbent assay described by Sköld Chiriac et al. (2014) to determine the total level of immunoglobulins (antibodies) in plasma. For the standard curve, a plasma pool from adult jackdaws (Corvus monedula ) was used. We used a seven-step serial dilution (1000, 250, 62.5, 15.62, 3.90, 0.97, 0.24) for the standard curve. Plasma samples of the focal species was diluted 1:600 and run in triplicate. A BioTeK ELx50 plate washer was used to do the plate washes during the assay and the mean absorbance was quantified using a BioTek EL808 plate reader (measured in 10^3 optical density per minute [mOD/min]). Antibody levels are calculated as the mean of the triplicates of each sample minus the mean value of the blanks and corrected for variation between plates according to the standard curves.