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

P>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 swallow Hirundo rustica Linnaeus, 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.