Haemosporidian infection does not alter aerobic performance in <i>Junco hyemalis</i> (Dark-eyed Junco)

Abstract Avian haemosporidia are blood parasites that can have dramatic fitness consequences on their hosts, including large-scale population declines when introduced to naïve hosts. Yet the physiological effects that accompany haemosporidian infection and underlie these fitness decrements are poorly characterized in most wild birds. Because haemosporidia destroy host red blood cells and consume host hemoglobin, they are predicted to have detrimental impacts on avian blood-oxygen transport and, as a result, reduce aerobic performance. However, the documented effects of infection on avian hematological traits vary across species and no effects have been demonstrated on avian aerobic performance to date. Here we quantified the physiological effects of haemosporidian infections on wild Junco hyemalis (Dark-eyed Junco) breeding in northwestern Wyoming, USA. We assayed hematological traits (hemoglobin concentration and hematocrit) and aerobic performance (resting and summit metabolic rates, thermogenic endurance, and aerobic scope), then screened individuals for haemosporidian infection by quantitative polymerase chain reaction post hoc (n = 110 adult juncos). We found that infection status and infection intensity did not significantly correlate with any of the physiological indices that we measured, suggesting there is little cost of haemosporidian infection on either junco aerobic performance or energy budgets. The fact that our results contradict predicted outcomes highlights the need for more studies of haemosporidia infections in a broader range of species and in a wider array of environmental contexts.