The uropygial gland of hoopoe nestlings and nesting females hosts bacterial symbionts that
cause changes in the characteristics of its secretion, including an increase of its antimicrobial activity.
These changes occur only in nesting individuals during the breeding season, possibly associated
with the high infection risk experienced during the stay in the hole-nests. However, the knowledge
on hoopoes uropygial gland microbial community dynamics is quite limited and based so far on
culture-dependent and molecular fingerprinting studies. In this work, we sampled wild and captive
hoopoes of different sex, age, and reproductive status, and studied their microbiota using quantitative
polymerase chain reaction (qPCR), fluorescence in situ hybridization (FISH) and pyrosequencing.
Surprisingly, we found a complex bacterial community in all individuals (including non-nesting
ones) during the breeding season. Nevertheless, dark secretions from nesting hoopoes harbored
significantly higher bacterial density than white secretions from breeding males and both sexes in
winter. We hypothesize that bacterial proliferation may be host-regulated in phases of high infection
risk (i.e., nesting). We also highlight the importance of specific antimicrobial-producing bacteria
present only in dark secretions that may be key in this defensive symbiosis. Finally, we discuss the
possible role of environmental conditions in shaping the uropygial microbiota, based on differences
found between wild and captive hoopoes.This work was supported by the Spanish Ministry of Economy and Competitiveness and European
(FEDER) funds (CGL2013-48193-C3-1-P/BOS, CGL2013-48193-C3-2-P/BOS, CGL2017-83103-P), and the Junta de
Andalucía (RNM 339, RNM 340). S. M. Rodríguez-Ruano received a grant from the Spanish Ministry of Science
and Innovation (FPI program)