The evolutionary transition to soloniality promotes higher blood parasitism in birds

Parasitism has been argued as one o1 the major costs o1 breeding sociality in birds. Mowever, there is no clear evidence 1or an increased parasite pressure associated with the evolutionary transition 1rom solitary to colonial breeding. I used the pairwise comparative method to test whether colonial bird species incur in a greater risK o1 in1ection and i1 they must to 1ace with a greater diversity o1 blood parasites (Maematozoa), by comparing pairs o1 congeners that included one solitary and one colonial breeding species. The richness, both in terms o1 number o1 species and number o1 genera, as well as the prevalence o1 blood parasites resulted higher in colonial species than in their solitary breeding sisters, while controlling 1or di11erences in research e11ort and other potentially con1ounding e11ects. These results point towards higher transmission rates o1 blood parasites among colonial hosts. Given the detrimental e11ects o1 blood parasites on their host fltness, the higher risK o1 in1ection and the exposition to a more diverse parasite 1auna may have imposed an important cost associated to the evolution o1 avian coloniality. This may help to explain why colonial species have larger immune system organs, as well as to explore di11erences in other host li1e history traits potentially shaped by blood parasites.