Several hypotheses have been proposed to explain why female birds either copulate repeatedly with a single mate or copulate with multiple partners even though only a single copulation may be sufficient to fertilize an entire clutch. We hypothesize that females may directly benefit from high frequencies of copulation and multiple copulation partners if they receive a cloacal inoculation of beneficial sexually transmitted microbes (STMs) that can either protect them against future encounters with pathogens and/or serve as therapy against present infections. Experiments in domestic animal production, wildlife rehabilitation, and clinical medicine indicate that inoculations of beneficial microbes derived from the indigenous microflora of hosts can lead to nutritional benefits, resistance to colonization by pathogens, the elimination of infection, and improved immune system functioning in recipients. Our hypothesis predicts greater copulatory rates when the probability of the transmission of beneficial microbes exceeds that of pathogens and when the positive effects of beneficial microbes on host fitness exceed the negative effects of pathogens. Patterns of copulatory behavior in birds suggest the potential utility of our hypothesis. We discuss our hypothesis in the context of observed patterns of copulation in birds and propose some ways to directly test our hypothesis. Information on the probabilities of transmission during copulation of beneficial and pathogenic microbes and their relative potencies in birds are needed to directly test the predictions of our hypothesis.