Latitude and parasite interactions drive global patterns of avian haemosporidian infection.

Infectious diseases are a powerful ecological force, yet the predictors shaping variation in parasite prevalence across wildlife remain incompletely understood. Birds provide an exceptional system to address this challenge, as their global diversity, ecological breadth, and role as pathogen reservoirs make them excellent models for studying infection dynamics. Among avian parasites, haemosporidians (Plasmodium, Haemoproteus, and Haemoproteus subgenus Parahaemoproteus) are nearly ubiquitous, affecting host physiology and survival while offering a model for testing how ecological and evolutionary factors modulate infection risk. Using a georeferenced global dataset of haemosporidian infection, we evaluated how latitude, bird host abundance, bird habitat specialization, bird species age, and parasite co-occurrence influence infection prevalence across avian communities. Bayesian phylogenetic logistic models revealed that these predictors capture key variation in infection patterns. Latitude emerged as the strongest predictor among ecological and evolutionary variables considered, showing contrasting associations across parasite groups: prevalence decreased with latitude for Plasmodium and Haemoproteus, but increased for Parahaemoproteus. Associations among parasite groups also emerged as important predictors of prevalence. Plasmodium prevalence was negatively associated with the prevalence of both Haemoproteus and Parahaemoproteus, while Parahaemoproteus prevalence was negatively associated with Plasmodium prevalence. In contrast, Haemoproteus prevalence was not significantly associated with the occurrence of either Plasmodium or Parahaemoproteus. Abundance and habitat breadth of avian hosts were not meaningfully associated with prevalence in any of the three parasite groups. Similarly, bird species age showed no supported association with parasite prevalence, suggesting that contemporary ecological and geographic factors and interactions among parasite groups are more important than host evolutionary age in explaining global variation in avian haemosporidian prevalence. Collectively, these results demonstrate that incorporating cross-parasite associations within a phylogenetic framework provides new insights into the mechanisms structuring global spatial patterns of haemosporidian prevalence.