PASSERINE POLYGYNY - A ROLE FOR PARASITES

Using sister-taxon comparisons to control for the effects of phylogeny, I show for European and North American passerine birds that the proportion of individuals infected with blood parasites is significantly lower in polygynous species than it is monogamous species. This remains so even after controlling for the effects of body length, sampling effort, latitude, migratory behavior, habitat, nest type, dispersion, and height, diet, and male parental effort in nest building, incubation, and feeding offspring. Several potential explanations of this association are discussed. One possibility is that polygynous species are, on average, more resistant to hematozoa infection. This could arise if there is heritable variation in resistance and if polygyny results in resistant males obtaining more mates. Females in polygynous species might settle with already-mated resistant males rather than unmated, less resistant males if they choose males resistant to parasites currently invading the population or if their decision to settle is influenced by the outcome of male-male competition, which might in part be affected by the health of the combatants. These and other possible explanations (which invoke resistance for which there need not be heritable variation or differences in risk of exposure) cannot be easily distinguished by interspecific analyses but are amenable to intraspecific tests. The patterns reported here are striking and suggest that parasitic infection should be considered as a factor influencing passerine mating systems whichever way the causal arrow goes.