Aerobic performance, haemoglobin concentration and body mass of free-living non-infected and chronically infected great reed warblers during sequential stages of the annual cycle. Infected birds were subdivided into low- and high-parasitaemia birds. Infection status had no effect on aerobic performance, haemoglobin content or period of exercise endurance during any stage we sampled. Thus, birds with chronic low and high parasitaemia were as aerobically capable as their non-infected counterparts, even during the migration phase. Data are medians ± 25/75%.