Parasitic infections can have extremely negative impacts on host individuals and may scale up to influence the population-level dynamics and life history evolution of host species. Host immune responses to parasitism can contribute to these detrimental impacts by inducing direct costs through an increase in resource use, as well as indirect costs resulting from trade-offs with life history traits. In order to improve our understanding of the costs that parasites may impose on hosts via host immune activation, this study assessed the immune response to avian haemosporidian infections in wild birds (genera: Plasmodium, Haemoproteus, and Leucocytozoon). Immune response was quantified through calculation of H:L ratios and WBC counts and compared between infected and uninfected individuals (parasite presence) as well as between individuals infected with either Plasmodium, Haemoproteus, or Leucocytozoon parasites (parasite genus). We found that neither parasite presence nor parasite genus had a significant effect on the H:L ratios or WBC counts of host individuals. This suggests that immune-related costs of avian malaria infection are relatively low, perhaps due to the long-term coevolutionary relationships between host and parasite species. However, the dynamics of parasitic infections in wild environments are complex and can be influenced by a multitude of different factors, such as parasite species, host species, host age, parasite burden, and environmental conditions, making it difficult to accurately assess infection costs. Therefore, in order to assess the validity of our results and improve our understanding of the true costs of parasitism, further research is needed into the ways in which these variables may shape infection dynamics and regulate immune-related costs in host individuals.
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