We derived our experimental results from a well-established host–parasite pair: the P. relictum lineage pGRW04 is a host generalist (recorded in 75 host species of 22 bird families around the globe, http://mbio-serv2.mbioekol.lu.se/Malavi/ (accessed 14 September 2017)) and has been frequently found in great reed warbler-breeding populations in Europe  as well as non-breeding populations in Africa . We are confident that the results of our study are robust because both the experimental manipulation as well as the observations on wild, naturally infected adults with higher parasitaemia by Plasmodium and Haemoproteus showed the same pattern in metabolic rates (see below).