We have examined the metabolic consequences of blood parasite infections over three consecutive stages of the annual cycle in great reed warblers (Acrocephalus arundinaceus), which are long-distance Palaearctic migrants with non-breeding sites in tropical Africa . We expect a haemosporidian infection will decrease a host's aerobic capacity, and, in turn, reduce both its MMR and its exercise endurance. Moreover, we expect that adverse effects of parasite infection to be particularly apparent during the migration stage, with its associated requirement for high aerobic capacity.