The study of feather-degrading microorganisms in avian plumage is steadily growing, but it is still a poorly understood field. Feather-degrading microorganisms were first isolated from chicken feathers in 1992, and their presence in wild birds was first described in 1997. The bulk of research in this area has focused on the possible selection pressures generated by these organisms. Yet we still lack detailed knowledge about the pattern of distribution among species of birds, ecological associations of the birds and feather-degrading microbes, and the effects of these bacteria and fungi on live wild birds. We sampled 3,548 birds representing 154 species for a group of 3 closely related bacilli that are well known to degrade feathers. We found these bacilli to be widespread among birds, occurring in the plumage of 39% of sampled individuals. Furthermore, these bacteria occur in most, if not all, avian taxa at similar frequencies, though variation exists. We found that ground-foraging species had a higher prevalence of feather-degrading bacilli, and tree-probing and nectivorous species had a lower prevalence. Additionally, fly-catching and foliage-gleaning birds were more likely to have feather-degrading bacilli than tree-probing species. Furthermore, the presence of feather-degrading bacilli, but not the abundance of bacteria in general, was correlated with our measure of plumage condition. A correlation cannot separate cause from effect, but it suggests that the presence of these bacteria is related to degradation of feathers in wild birds. This relationship implies that these bacteria may indeed constitute an important selection pressure that broadly influences the evolution of color, timing of molt, and behaviors such as preening and other maintenance activities in birds.