Occurrence of Feather-Degrading Bacilli in the Plumage of Birds

We sampled bacteria from the plumage of 1,588 individuals of 83 species of birds. Feather-degrading bacteria, those able to extract energy and nutrients by breaking up β-keratin, were isolated from 134 individuals in 32 species. Nine of 11 samples of feather-degrading (keratinolytic) bacteria were identified as Bacillus licheniformis, one as B. pumilus, and one as a Bacillus of undetermined species. A strong correlation between occurrence of keratinolytic bacilli and the number of birds sampled per species suggests that feather-degrading bacilli are widespread among birds. The bacillus occurred on 6.7 to 10.7% of birds and showed little annual variation. The incidence of birds with feather-degrading bacilli was highest in late fall and winter and lowest in early spring and late summer. The bacilli occurred most frequently on the venter and less commonly on the dorsum and tail. They occurred most frequently on ground-foraging species and least frequently on aerial-foraging species. Regardless of avian species, time of year, or area of the bird from which the bacilli were isolated, the rate at which bacilli degraded feathers was similar. Because bacilli are active only when conditions are warm and humid, we suggest that they degrade feathers during the summer when the bird becomes wet, for example during thunderstorms. Such feather degradation may contribute to the deterioration of feathers and be a selective force in the evolution and timing of molt.