Feather micro‐organisms and uropygial antimicrobial defences in a colonial passerine bird

Summary 1. The relationship between the composition of communities of micro‐organisms and their hosts remains poorly understood. We conducted extensive field studies of feather‐degrading bacteria, other cultivable bacteria, and fungi on the plumage of a migratory bird, the barn swallowHirundo rusticaLinnaeus, to understand the association between micro‐organisms, host sociality and host antimicrobial defences, as reflected by the size of the uropygial gland. 2. The abundance of feather‐degrading bacteria, but not other cultivable bacteria or fungi, decreased with increasing size of the uropygial gland. 3. Females had more feather‐degrading bacteria than males. 4. Barn swallows living in larger colonies had more feather‐degrading bacteria than less social conspecifics. 5. These findings suggest that the uropygial gland plays a specific role in regulating the abundance of feather‐degrading bacteria that furthermore depends on the social environment of the host.