Life-history theory predicts that hosts may adjust the costs of parasites by altering their reproductive effort. Haemosporidian parasites can affect the reproductive output of wild birds in multiple ways. Thorn-tailed Rayaditos Aphrastura spinicauda breeding in Navarino Island, southern Chile (55°-40°S), experience a high prevalence of the haemosporidian Leucocytozoon spp., which opens the possibility of exploring how these parasites may affect reproductive output in a Neotropical bird species. We compared several variables describing reproductive output (laying date, clutch size, incubation period, brood size, nestling body condition and early-life telomere length) of infected and non-infected parents (individually and as breeding pairs). We found that infected females and breeding pairs with both parents infected showed significantly shorter incubation periods than uninfected Thorn-tailed Rayaditos. Furthermore, breeding pairs where both parents were infected raised nestlings with higher body condition than nestlings for which infection was present in only one or in neither of the parents. Our results suggest that the higher the parental investment, the higher the risk of relapse of chronic infection by Leucocytozoon spp. Thorn-tailed Rayaditos that decrease their incubation period pay the cost of infection to take advantage of early breeding through a greater availability of resources, producing nestlings with higher body condition.