No clear links between haemosporidian parasite infection, host condition, and pairing success in a multi-year study of a population of Blue-black Grassquits

Abstract Pathogens can shape host ecology and evolution by influencing survival, reproduction, and the expression of sexually selected traits. Among avian pathogens, haemosporidian parasites ( Plasmodium , Haemoproteus , and Leucocytozoon ) are widespread and frequently assessed for their impacts on host–parasite interactions. Susceptibility to infection is expected to vary with host sex, age, temporal dynamics, genetic diversity, body condition, and sexually selected traits, yet empirical evidence from longitudinal studies in wild populations remains limited. Here, we studied haemosporidian infections in Blue-black Grassquits ( Volatinia jacarina ), a Neotropical passerine characterized by sexual dichromatism and conspicuous courtship displays in males. Across three breeding seasons in central Brazil, we sampled 98 males and 53 females, combining morphological, behavioral, and genetic data with molecular screening of parasites. Parasite lineages were identified and assigned to the genera Plasmodium and Leucocytozoon using phylogenetic inference. Some detected lineages extend the previously known geographic distributions of these parasites. Parasite occurrence did not differ between sexes or years. Longitudinal sampling revealed that, among individuals that acquired infections, some became PCR negative after an initial positive result, whereas others remained PCR positive across breeding seasons, indicating that infections are dynamic rather than uniformly chronic. Contrary to expectations, parasite occurrence did not correlate with body condition, molt progression, heterozygosity, display rate, or pairing success. These results indicate no detectable associations between parasite occurrence and the measured indices of body condition, ornamentation, display behavior, or pairing success. Our findings highlight the value of publishing longitudinal results, which temper assumptions of universally high parasite costs and thus foster a more nuanced understanding of host–parasite dynamics in natural populations.