Purpose: Avian haemosporidians are widespread parasites, occurring in many bird families and causing pathologies ranging from rather benign infections to highly virulent diseases. The state of knowledge about lineage-specific intensities of haemosporidian infection (i.e., parasitaemia) is mainly based on infection experiments conducted under laboratory conditions. The levels and range of parasitaemia in natural host–parasite associations as well as their influencing factor remain largely unexplored. Methods: Thus, we explored the parasitaemia of four songbird species (i.e., European Robins, Black and Common Redstarts and Whinchats) during migration by screening individuals upon landing on an insular passage site after extensive endurance flights to (1) describe their natural host–parasite associations, (2) quantify parasitaemia and (3) explore potential host- and parasite-related factors influencing parasitaemia. Results: We found 68% of Whinchats to be infected with haemosporidians, which is more frequent than any other of the studied host species (30–34%). Furthermore, we confirmed that parasitaemia of Haemoproteus infections was higher than average Plasmodium infections. Median parasitaemia levels were rather low (parasite cells in 0.01% of hosts’ red blood cells) and varied largely among the different parasite lineages. However, we found four individuals hosting infections with parasitaemia higher than typical chronic infections. Conclusions: Based on the known transmission areas of the respective lineages, we argue that these higher intensity infections might be relapses of consisting infections rather than acute phases of recent primary infections.
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