The highly endangered Bermuda Petrel (Pterodroma cahow) lacks blood parasite infections, but shows sexual differences in leukocyte counts

Abstract Avian malaria parasites and related haemosporidians include a highly diverse group of parasites infecting wild birds worldwide, with variable knowledge depending on bird taxa. Although comparatively less studied than terrestrial species, avian malaria infections in seabirds are generally rare, a pattern attributed to several non-mutually exclusive hypotheses, including limited exposure to vectors and differences in host life history traits. Here, we screened blood parasites by combining molecular detection and observation of blood smears in the highly endangered Bermuda Petrel (Cahow) Pterodroma cahow, sampled during the 2022 breeding season (January–April). In addition, we quantified the blood cell types to provide baseline information on the immunological status of this species. Furthermore, few blood samples of the White-tailed Tropicbird Phaethon lepturus catsbyii breeding in sympatry on the same archipelago were also analyzed for parasite presence. Our results support the absence of blood parasites of the genera Plasmodium , Haemoproteus , and Leucocytozoon as well as filaroid nematodes. The absence of blood parasite infections in both species may reflect low exposure to vectors in offshore marine and terrestrial environments, although other explanations, such as the absence of parasites able to infect these host species, are also possible. We found no significant differences in the relative abundance of monocytes among islets, although differences were observed, and a significantly higher total abundance of leucocytes in female Bermuda Petrels than males. Differential parasite exposure among islets and differential life history traits between sexes could account for these results. Negative records of parasite infections in wild birds, including endangered species, are essential for accurately characterizing global infection patterns and understanding host–parasite associations in avian communities.