Blood parasites are often considered as indicators of immunity in birds, and data on parasite prevalence and intensity of infections are essential to reveal information about the condition of both individuals and populations. We prevented parasite vectors from biting and infecting nestling great tit Parus major by using insect repellent inside nest boxes. We found that in the absence of blood parasites, great tit nestlings had higher concentrations of haemoglobin, and they survived at significantly higher rates through the nestling phase and also during the first weeks of their fledgling period. This is the first demonstration so far of the impact these parasites have on haemoglobin levels of the hosts, which reveals one mechanism of adverse impact by blood parasites. This study shows that the effects of blood parasites can be assessed without using anti-malaria drugs, which can cause additional risk of oxidative stress.