In birds, microparasites found in both the reproductive and the digestive tracts may be transmitted through copulations via cloacal contact (male-to-female and vice versa) and/or through the seminal fluid (mainly male-to-female). Most importantly, such cloacal microparasites are affected by and may in turn affect sexual selection processes and the evolution of mating systems. Here I provide preliminary comparative evidence that at least some cloacal microparasites tend to be distributed in hosts according to the host's mating system and as broadly expected from predictions of sexual selection theory. The patterns, however, are more suggestive than conclusive. There is a non-significant trend for polygamy to be associated with higher richness of cloacal microparasite taxa; with body size, however, also having a positive association with both polygamy and parasite richness. Although increased sexual plumage dichromatism tends to be associated with decreased cloacal microparasite richness, indicating that secondary sexual traits may be used by sexual partners to discriminate between infected and uninfected individuals, qualitative trends also suggest that non-mating periods of the year tend to be associated with slightly higher levels of prevalence and richness of cloacal microparasites. Given this variability of results, it is suggested that future studies should focus on specialist sexually transmitted microbes, to be compared with more generalist one.
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