Theory strongly suggests that beneficial symbiotic bacteria could be common within birds. Our argument is based on the existence of within-host competition for resources between bacteria (i.e. bacterial interference), and on the differential effect that host fitness (i.e., reproductive success and probability of survival) has on fitness of different bacteria. If reproductive success of hosts is positively related to that of a first bacterium, and negatively related to that of a second bacterium, it would be of selective advantage for the former to develop chemicals that prevent host infection by the later pathogenic bacterium. Furthermore, we exemplify the possibility that hosts use antibiotic producing bacteria to prevent infections in different body parts (i.e., replacement therapy) or environment (i.e. nest sanitation). We review the up to now few available results suggesting associations of birds with antibiotic producing bacteria that result in fitness advantages to hosts. Evidence for such beneficial associations, however, has been very scarce so far, and an important research effort testing predictions of that relationship in different contexts is needed for a generalization of the hypothesis. Keywords: Egg incubation, embryo infection, mutualism, nest sanitation, nest lining material, symbiosis, uropygial gland.