Small Clutch Sizes as a Possible Adaptation against Ectoparasitism: A Comparative Analysis

Abstract

Clutch size is expected to decrease with an increased rate of ectoparasite infestation. In order to test this hypothesis, I carried out comparative analyses using Australian and North American passerines. The allometry method showed a trend for passerine families which have higher degrees of ectoparasitism to also have smaller clutches. In the North American sample, however, the trend was confirmed for mite prevalence but was not for fly relative density. I also analyzed both samples using a method of independent comparisons. Results for Australian passerines suggest that an evolutionary change of clutch size in one direction is associated with an evolutionary change of relative density of ectoparasites in the opposite direction, after controlling for the effects of evolutionary changes in body size. The method of independent comparisons produced non-significant results for the North American sample. The hypothesis was also supported by a direct comparison of Australian and North American passerine families: Australian passerines lay smaller clutches than their North American counterparts and are infested more with contagious ectoparasites.