Blood parasites prevalence of migrating passerines increases over the spring passage period

Whether long‐distance animal migration facilitates or hampers pathogen transmission depends on how infections affect the routes and timing of migrating hosts. If an infection directly or indirectly impedes migratory flight capacity, infected individuals lag behind their uninfected conspecifics. Although such temporal segregation can limit parasite transmission and thus play an important role for host–parasite interactions, empirical evidence remains scarce. Here, we investigated haemosporidians – blood parasites commonly infecting birds – in four passerine species on spring passage and linked infection status to passage date. As a step towards identifying the mechanisms behind infection‐related delays, we incorporated sets of individual, energetic, haematological and biometric variables into the analysis. Haemosporidian prevalence virtually doubled between birds sampled at the beginning of the passage period with those sampled 1 month later. This indicates that infected individuals arrived later than uninfected individuals. Both the average prevalence and its increase over time varied among host species. In addition, the leucocyte counts of infected birds were elevated, suggesting that immune response may require resources which could otherwise be allocated to migratory flights. However, infection status was not related to any other variable such as body mass, energy stores, sex, age and feather length. Yet regardless of the underlying mechanisms, infection‐related differential timing might influence transmission and affect pathogen prevalence in wildlife populations year‐round.