The apparent absence of blood parasites in some bird taxa is commonly explained by the absence or scarcity of parasite vectors in some habitats such as marine, saline, arid, open, alpine or high latitude environments. However, in addition to such extrinsic factors, immunological capabilities of the host and absence of the right host-parasite assemblage might also be important. In this paper, we suggest that extant hypotheses are not mutually exclusive and may act simultaneously at different hierarchical levels. Hence, one should consider all explanations equally to prevent inadequate generalizations. Additionally, we suggest a new hypothesis, according to which highly mobile ectoparasites might exclude the dipteran vectors of blood parasites in bird species with high prevalences and intensities of ectoparasites.
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