Data analysis
A measure we called “fuel stores” was extracted from the variables fat score and muscle score for each species using principle component analysis. Both fat and muscle scores were included, because migrating passerines depend on both fat (90-95%) and protein (5-10%) for energy during endurance flight (Jenni & Jenni-Eiermann 1998). To test if there is an effect of fuel stores on IgY levels a general linear model was constructed for each species separately. Sex was added as a covariate to the model as immune function can differ between sexes (Arriero et al. 2015). Julian day (date of capture) was added as a covariate as early migrating individuals may differ in immune function from late migrating individuals (Hegemann et al. 2022). Model selection was performed by a stepwise elimination of non-significant variables by using the drop.1 function in R. To test if there were differences in the level of immunoglobulins between the four species, a one-way ANOVA along with the Tukey test was performed. The microbial killing ability data was not normally distributed. Therefore, Spearman’s correlations were performed to test whether microbial killing ability is correlated with fuel stores and Julian day. A Kruskal-Wallis test along with a pairwise Wilcoxon test was performed to test for sex and species differences in microbial killing ability.