Figure 2: Scatter plots of immune function and Julian day in four bird species during spring migration. Panels 1-4 show the relationship between microbial killing ability (% E.coli killed) and Julian day and panels 5-8 show the relationship between IgY levels (mOD/min) and Julian day. Dashed lines indicate non-significant correlations. Significant relationships are depicted in bold lines in the panels 3, 4 and 7 respectively. Please note that the x-axis is differently scaled among the figure panels

Microbial killing ability was significantly different between the four species (Figure 3, chi-squared= 20.43, df= 3, P<0.001). A pairwise Wilcoxon test showed significantly higher microbial killing ability in Dunnocks when compared to Chaffinches (P=0.004) and Wheatears (P=0.009). Song thrushes showed a significantly higher microbial killing ability than Chaffinches (P=0.004) and Wheatears (P=0.009). There was no significant difference in microbial killing ability between Wheatears and Chaffinches (P=0.59), and Dunnocks and Song thrushes (P=0.59). Similarly, immunoglobulin levels were significantly different between the four species (f=11.25, df= 3, P< 0.001, Figure 3). A Tukey test revealed that the IgY levels in Dunnocks were significantly lower than IgY levels in Chaffinches (P=0.001) and Song Thrushes (P<=0.001). The IgY levels in Wheatears were significantly lower than that in Song thrushes (P=0.01). There was no difference in levels of IgY between Song thrushes and Chaffinches (P=0.19), Wheatears and Chaffinches (P=0.68) or Wheatears and Dunnocks (P=0.07).