We studied the effect of sunlight (280–750 nm) on Bacillus licheniformis, a feather-degrading bacterium that commonly occurs in the plumage of birds. Colony-forming units (a measure of bacterial abundance) of B. licheniformis were numerous on feathers inoculated with B. licheniformis and shielded from all sunlight, whereas colony-forming units were significantly less common on inoculated feathers exposed to full spectrum sunlight and sunlight from which the shorter ultra-violet wavelengths were blocked. Sunlight appears to inhibit the growth of feather-degrading bacilli. Given that many avian species sun themselves and that feather-degrading bacilli occur commonly in avian plumage, we suggest that regulation of potentially harmful plumage microorganisms through exposure to sunlight could be one reason that birds sunbathe.