During incubation, ground-breeding sandpipers such as Red Knots (Calidris canutus) create a warm, humid microclimate in the nest, conditions that favor the growth of feather-degrading bacteria present in their plumage. Just before incubation, the composition of waxes secreted by the uropygial gland of Red Knots and other sandpipers changes quickly and completely from a mixture of only monoesters to a mixture of only diesters. We hypothesized that the change in composition of the preen wax helps protect the plumage against feather-degrading bacteria. We tested the hypothesis by studying growth of the feather-degrading bacterium Bacillus licheniformis, which we found in the plumage of Red Knots. The removal of preen waxes from feathers resulted in faster bacterial degradation, confirming earlier findings that preen wax inhibits growth of feather-degrading bacteria. However, the degradation rate of feathers with preen wax based on diesters did not differ from that of feathers with preen wax based on monoesters. We suggest that preen waxes protect feathers by forming a physical barrier to microbes rather than through chemical properties of the waxes.