BackgroundParasites exert important selective pressures on host life history traits. In birds, feathers are inhabited by numerous microorganisms, some of them being able to degrade feathers or lead to infections. Preening feathers with secretions of the uropygial gland has been found to act as an antimicrobial defence mechanism, expected to regulate feather microbial communities and thus limit feather abrasion and infections. Here, we used an experimental approach to test whether Great tits (Parus major) modify their investment in the uropygial gland in response to differences in environmental microorganisms.ResultsWe found that males, but not females, modified the size of their gland when exposed to higher bacterial densities on feathers. We also identified 16 wax esters in the uropygial gland secretions. The relative abundance of some of these esters changed in males and females, while the relative abundance of others changed only in females when exposed to greater bacterial loads on feathers.ConclusionBirds live in a bacterial world composed of commensal and pathogenic microorganisms. This study provides the first experimental evidence for modifications of investment in the defensive trait that is the uropygial gland in response to environmental microorganisms in a wild bird.

A single pdf document containing: Table S1. Model set for the analyses on the effect of the proportion of neighbours from the opposite food-treatment on laying date. The random effects is a random intercept for pair ID. Table S2. Model set for the analyses on the effect of the proportion of neighbours from the opposite food-treatment on egg mass. The random effect is a random intercept for pair ID. S3. Model set for the analyses on the effect of the proportion of neighbours from the opposite food-treatment on baseline corticosterone levels (log-transformed) among unsupplemented birds. The random effects is a random intercept for bird ID.  Table Table S4. Model set for the analyses on the effect of the proportion of neighbours from the opposite food-treatment on baseline corticosterone levels (log-transformed) among foodsupplemented birds. The random effects is a random intercept for bird ID. Table S5. AICc model selection table for the models explaining laying date. 'AICc' is the AIC corrected for finite sample size; 'ΔAICc' is the difference between AICc of a given model to that of the best model; 'ωAICc' is the probability of each model being the best model given the data and the model set. The null model is given for comparison purposes. Laying date was centred to improve model convergence. RE: random effect (1|PairID). 3 Table S6. AICc model selection table for the models explaining laying date in the subset of individuals that were captured. 'AICc' is the AIC corrected for finite sample size; 'ΔAICc' is the difference between AICc of a given model to that of the best model; 'ωAICc' is the probability of each model being the best model given the data and the model set. Laying date was centred to improve model convergence. RE: random effect (1|PairID). Table S7. Model-averaged estimates of the variables explaining variation in laying date, in the subset of individuals that were captured (i.e. with body mass data). Binary and continuous explanatory variables were standardised to facilitate model comparisons. CI = confidence interval. Estimates with a CI not overlapping zero are in bold. Table S8. AICc model selection table for the models explaining egg mass. 'AICc' is the AIC corrected for finite sample size; 'ΔAICc' is the difference between AICc of a given model to that of the best model; 'ωAICc' is the probability of each model being the best model given the data and the model set. The null model is given for comparison purposes. RE: random effect (1|PairID). Table S9. AICc model selection table for the models explaining baseline corticosterone (logtransformed) among unsupplemented birds. 'AICc' is the AIC corrected for finite sample size; 'ΔAICc' is the difference between AICc of a given model to that of the best model; 'ωAICc' is the probability of each model being the best model given the data and the model set. RE: random effect (1|BirdID). Table S10. AICc model selection table for the models explaining baseline corticosterone (logtransformed) among food-supplemented birds. 'AICc' is the AIC co...