Immunosuppressive effects of testosterone lead to a prediction of male-biased parasitism. To test this prediction, prevalences of blood parasites were compared between male and female birds using statistically correct vote counts of data from 33 studies. We found no overall difference in prevalence between males and females, in either breeding or non-breeding birds. However, infections by Haemoproteus (the most common genus of blood parasite found) were significantly more common among breeding females than breeding males. Restricting the analysis to breeding birds of polygynous species, females again were more likely than males to be infected by blood parasites; this result held for an intra-family comparison that controlled for phylogenetic effects. In comparison, measures of sexual size dimorphism did not relate to sex biases in parasitism as predicted, after controlling for phylogeny using independent comparisons. Because testosterone is often implicated in suppressing the immune system, female biases in parasitism are unexpected. Female biases in parasitism by blood parasites could result from differential exposure of the sexes to vectors, or from oestrogen-based effects on immunity.

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Journal Oikos
McCurdy, D.G. (Dean G.), Shutler, D. (Dave), Mullie, A. (Adele), & Forbes, M. (1998). Sex-biased parasitism of avian hosts: Relations to blood parasite taxon and mating system. Oikos, 82(2), 303–312.