Parasites can impose substantial fitness costs on their hosts. At energetically demanding life-history stages (e.g., during reproduction), their effects may be particularly taxing. In species with parental care, offspring rearing can be very costly for parents, and especially so, if they are burdened by infections. Vector-borne haemosporidians are some of the most widespread endoparasites. Haemosporidians can affect their hosts in several ways, including the ability of parents to care for their young. Costs of infections are, however, not consistent across species, with correlational and experimental studies showing contrasting effects of infections on parental care behavior. Most research has, however, been conducted in systems with chronically low infection levels. Here, we present the results of a study we conducted in a free-ranging population of red-winged blackbirds (Agelaius phoeniceus) naturally experiencing unusually high haemosporidian infection levels. At our field site in south-eastern Ontario, >90% of individuals are parasitized with one or more haemosporidian genus. To assess effects of infection on mothers’ abilities to incubate and provision their offspring, we caught adult red-winged blackbird females before onset of egg-laying and experimentally manipulated their parasite levels by administering either an anti-malarial medication or a control solution. Our results provide experimental evidence that reduced haemosporidian burdens lead to increased maternal care behavior, supporting the idea that parasitic infections reduce fitness even in populations with chronically high infection levels.