Antimicrobial capacity is related to body colouration and reproductive success in female spotless starlings

Pathogenic microorganisms select for a plethora of defensive mechanisms on
their hosts. In males of some species, flashy traits might signal
antimicrobial capacity and, thus, they might be favoured in scenarios of
sexual selection. Antimicrobial capacity of individuals may predict
reproductive success in males, and it could be adapted to changing
environments. However, evidence for these associations is still scarce in
females. Here, we evaluated antimicrobial capacity of spotless starling
(Sturnus unicolor) females during the mating and nestling-provisioning
phase. We did this by measuring (i) the blood plasma inhibition capacity
against 12 bacterial strains (antagonistic index), (ii) the constitutive
innate humoral immunity (lysis and agglutination capacity, a non-specific
first barrier of protection of hosts against microbial parasites), and
(iii) the uropygial gland size and volume of secretion produced, relevant
traits in the protection against, among others, feather-degrading
bacteria. We also measured colouration of throat and back feathers, and of
leg and beak integuments. This information was collected during the
pre-laying and nestling stages to compare values from these two periods.
We found an increase in the plasma antagonistic index from the pre-laying
to the nestling period, while a decrease on the plasma capacity of
agglutination of foreign antigens. Both plasma antagonistic index and
humoral immune response measured before breeding were positively related
to future female reproductive success. In addition, the level of
antimicrobial capacity was related to colouration of leg skin and beak
integuments and of back feathers. These associations suggest that, similar
to males, females might show through their physical appearance their
capacity to fight microbial infections; information that could be
evaluated by their potential partners and by female competitors.