Similar to the lack of effect of haemosporidian infection on aerobic performance, haemoglobin concentration did not differ between infected and non-infected captive birds (t = −1.21; figure 2), indicating minimal damage to haemoglobin at the low-parasitaemia levels that Plasmodium strain pGRW04 elicited in great reed warblers. Similarly, haemoglobin concentration did not differ between non-infected free-living birds and those with either low or high parasitaemia over the course of our study (both p > 0.95; figure 3). By contrast, haemoglobin concentration increased significantly from late-breeding to subsequent periods (p < 0.05 and 0.001 for post-breeding and migration periods, respectively), reaching levels about 11% higher during migration compared with late-breeding stages (figure 3).