Avian haemosporidians are a diverse group of parasites that infect birds worldwide and have been a major focus of research for decades. Yet, few studies have identified the drivers of infection at the intraspecific host level. We aimed to study the drivers of prevalence and diversity of haemosporidian parasites infecting a common North American songbird species, the American Robin (Turdus migratorius), which breeds across most of the continent. We found little seasonal variation in haemosporidian prevalence in robins, although prevalence of Leucocytozoon increases with breeding latitude in this host species. We also detected a significantly positive relationship between the robin breeding latitude and coinfection with different haemosporidian parasite lineages. Additionally, robins infected with Plasmodium had a significantly better body condition than those of uninfected robins, which could be due to migratory culling. When matched against known haemosporidian lineages, our results suggest that robins harbor a higher diversity of haemosporidian parasites than previously known. The results of this study suggest that comparisons of common, widespread bird species such as robins across their range could help unveil novel aspects of the haemosporidian–host relationship and how such a relationship may change under current and future rapid environmental change.

Biodiversity varies across the world and is influenced by multiple factors, such as environmental stability and past historical events (e.g., Panama Isthmus). At same time, organisms with unique life-histories (e.g., parasites) are subject to unique selection pressures that structure their diversity patterns. Parasites represent one the most successful life-strategies, impacting directly and indirectly the ecosystem by cascading effects on host fitness and survival. Here, I focused on a highly diverse, prevalent, and cosmopolitan group of parasites (avian haemosporidians) to investigate the main drivers of regional parasite diversity on a global scale. To do so, I compiled data from four global datasets on (i) avian haemosporidian (malaria and malaria-like) parasites, (ii) bird species richness, (iii) avian functional traits, and (iv) climate data. Then, using generalized mixed models, I evaluated potential drivers of haemosporidian diversity. I found that haemosporidian diversity is driven by both host regional diversity and functional traits, and by environmental conditions. In other words, parasite diversity increased with increasing host richness and higher numbers of resident and territorial birds. Further, greater temperature seasonality was also positively correlated with parasite diversity. Hence, regions harboring the greatest resident/territorial avian diversity (e.g., neotropics) and/or higher temperature seasonality (e.g., North America) generally harbor the highest diversity of haemosporidian parasites. Overall, I demonstrated that haemosporidian parasite diversity is intrinsically associated with their hosts’ diversity and functional traits.