In addition to the infection experiment, we examined 124 free-living great reed warblers (95 males, 12 females and 17 birds of unknown sex) over late-breeding (July 2015), post-breeding (late July/early August) and migration periods (late August). In addition to standard biometric measurements of body mass, fat score, pectoral muscle score and wing size , we determined each bird's infection status and parasitaemia by PCR and microscopy. The total haemosporidian prevalence in the sample for the seasonal effect measurement was 45.2% with Plasmodium ssp. infections accounting for 13.0%, Haemoproteus ssp. for 29.9% and mixed infections for 2.6%. In nine birds, the PCR signal was positive yet no infected cells were found by microscopy. Therefore, parasitaemia must have been lower than one infected cell in 100 microscopic fields (less than 0.003%) and we set it to 0.0015%, i.e. half of the value at microscopic detection limit. Finally, for the later analyses, we categorized birds according to their parasitaemia with zero, low (less than 0.2%) and high parasitaemia (greater than 0.2 to max. 4.0%). The ‘high-parasitaemia’ category is mainly formed by hosts with a Haemoproteus infection, although these levels are much lower than those generally associated with acute primary infections . In total, the free-living bird samples contained 54.5% non-infected, 29.2% low parasitaemia and 16.2% high-parasitaemia individuals (n = 156 birds incl. experimental birds before infection).