Feather bacterial load affects plumage condition, iridescent color, and investment in preening in pigeons

Feathers are inhabited by numerous bacteria, some of them being able to degrade feathers, and thus potentially alter thermoregulation and visual communication. To limit the negative effects of feather bacteria on fitness, birds have therefore evolved antimicrobial defense mechanisms, including preening feathers with secretions of the preen gland. However, whether feather bacteria can alter feather condition and color signaling in vivo, and thus whether birds adjust their investment in preening according to feather bacterial load, has barely been investigated. Here, we experimentally decreased and increased feather bacterial load on captive feral pigeons Columba livia and investigated the effects on plumage characteristics and investment in preening. We found that birds of both sexes had a plumage in higher condition and invested less in preen secretion quantity and preening behavior when feather bacterial load was lower. It suggests that preen secretions may be used by pigeons to limit feather degradation by bacteria, but as they are probably costly to produce, their quantity is adjusted depending on feather bacteria load. Birds with lower bacteria load on feathers had brighter iridescent neck feathers, suggesting that feather bacteria may play an important role in the evolution of the signaling function of iridescent color in pigeons. Altogether, our study provides the first experimental evidence for in vivo effects of feather bacteria on plumage degradation and coloration and suggests that preening is an inducible antibacterial defense.