Smithsonian.com

A European hoopoe (via Wikimedia Commons)

Birds produce special chemicals in their preen gland that they spread over their skin and feathers to protect themselves from pathogens and parasites. The secretions of European hoopoes (Upupa epops) and green woodhoopoes (Phoeniculus purpureus), however, are different from those of other birds. The substance they produce is brown in color instead of white and it is especially pungent.

Scientists in Spain, reporting in Proceedings of the Royal Society B, have discovered that the hoopoes are not entirely responsible for the smelly secretions. The preen glands of these two species harbor symbiotic Enterococcus bacteria that produce helpful chemicals that the birds use in defense against pathogens like the Bacillus licheniformus, a bacterium that degrades feathers.

How do the biologists know the bacteria were responsible? They injected nestling hoopoes with antibiotics that prevented the Enterococcus bacteria from taking up residence in their preen glands. Birds that grew up without the bacteria lacked most of the helpful chemicals.