Symbiotic microbes play a role more important than preen gland in avian pheromone production--A review

The uropygial gland or preen gland is a complex holocrine structure present only in birds, and plays an important role in avian communication and reproduction. This gland produces preen oil, which helps birds maintain intact plumage, plumage colorage, but also possesses antibacterial and anti-predator properties, and the evidence for these claims is still in infancy. Preen gland harbour a large number of microbiota among which as many as 110 are bacterial genera dominated by Firmicutes, Proteobacteria, Bacteroidetes, etc., families, and the roles of these microbes are largely unknown. However, these microbes are believed to maintain symbiotic relationship with the host and exert positive effects in the host's physiology and behaviour. Many studies have proven that these microbes produce chemical cues as metabolic by-products that modulate the host's behavior. In birds, these symbiotic microbes are needed for normal growth, development and even reproduction. Earlier findings about preen gland microbiota of birds connect it to good feather condition, recent evidences connect it to antifungal and antimicrobial activities. Although preen gland plays a major role in bird's development, symbiotic microbes of preen gland seem to play a crucial role in reproduction and pheromonal communication. Here, we review the role of microbes present in avian preen gland in production of chemical signals and document the relationship between the microbes and preen gland in chemical communication.