Abstract

Background: Introduced predators at seabird colonies and parasites may have lethal and/or sub-lethal consequences for bird populations. We assessed the potential sub-lethal negative effects of these stressors in a native seabird listed as vulnerable in its south-eastern pacific distribution. This study was conducted in two red-tailed tropicbird (Phaethon rubricauda) colonies, one located in Rapa Nui Island, which is threatened by the presence of introduced predators, and the other located in Salas Gomez Island, which has no introduced predators, but birds are infested by ectoparasitic louse flies. Methods: The effects on physiological stress traits of both, predation risk on different nest types (protected and exposed) on Rapa Nui, and different levels of louse flies' parasitic loads on Salas Gomez were studied. Three variables were analyzed: the heterophil/lymphocyte (H/L) ratio, the transcriptional levels of mRNA HSP70 in blood, and the body condition. These stress indicators and leukocyte counts were compared between colonies. Results: No significant differences were found in any stress indicator between different nest types within Rapa Nui, showing that the effect of predator's presence was the same for adults nesting in both, protected and exposed nests. No significant correlation was found between louse flies' parasitic loads and any stress indicators in the birds of Salas Gomez. Also, there was no difference in any stress indicator between islands. However, a significant opposite trend between islands was found in the eosinophil, lymphocyte, and monocyte counts when related to body condition. Conclusions: We found a lack of significant differentiation in all the stress level indicators assessed within and between islands. The presence of louse flies in Salas Gomez vs. the absence of this parasite at Rapa Nui may be the cause for the significant difference in the trend of eosinophil, lymphocyte and monocyte counts between the islands. However, further studies are necessary to elucidate the reason for this difference and to better investigate the lethal effects of introduced predators on the Rapa Nui colony to evaluate appropriate conservation measures for this native seabird.