Fitness impacts of Plasmodium vary by host age and sex in a North American songbird

Birds infected with the malaria-causing parasite Plasmodium comprise a popular model in evolutionary ecology, yet Plasmodium’s impact on host survival and reproduction remains unclear in wild bird populations where Plasmodium is historically endemic. Recent research in endemic host populations shows that Plasmodium reduces survival in juveniles, but not adults. Additional research is needed to understand 1) age-based variation in the impact of Plasmodium on host reproduction, and 2) sex-based variation in the impact of Plasmodium on survival and reproduction. To this end, we leveraged a long-term dataset from a dark-eyed junco ( Junco hyemalis carolinensis ) population with a high prevalence of Plasmodium . Juvenile males had lower probability of Plasmodium infection than juvenile females, indicating either elevated resistance or mortality in young males before the juvenile stage. Second year females had a lower probability of infection than females of other age classes, which suggests elevated mortality in infected yearling females after their first breeding season. Model comparison did not identify a direct relationship between Plasmodium infection and adult return (a proxy of survival), yet surprisingly, females paired with infected males in the previous breeding season had a higher probability of return. There was no relationship between Plasmodium infection and juvenile return, but returners with relatively higher parasite loads as juveniles had shorter lifespans than those without juvenile infections. Among adults, Plasmodium infection did not predict fledging success or fledgling number. However, returning juncos with relatively heavier bodies and higher parasite loads as juveniles had an increased probability of breeding in their first adult year. Combined with the shorter lifespans observed in infected juveniles, this suggests a terminal investment strategy for juncos contracting Plasmodium early in life. Our results indicate that the impact of Plasmodium on host fitness likely varies by host age at first infection and may exacerbate physiologically stressful life stages.