Aromatic plants, nest bacterial diversity, and nestling condition in Corsican blue tits

According to the ‘nest protection hypothesis', some passerines incorporate fresh aromatic plants into their nests which reduce pathogens that can negatively affect nestlings. We experimentally evaluated the effect of five aromatic plant species on the nest bacterial microbiota of Corsican blue tits Cyanistes caeruleus . The experimental addition of aromatic plants decreased bacterial diversity in nests collected post‐hatching. We also detected a weak effect of aromatic plants on nest bacterial composition. In the observational approach, we tested the effect of these plants on the bacterial microbiota diversity and composition of eggshells and nests, and on nestling condition and behaviour. Bacterial diversity decreased with the quantity of aromatic plants in nests containing nestlings and on eggshells during incubation, but only in one of the three studied populations. Again, there was a weak effect of aromatic plants on bacterial composition in nests and no effect on eggshell bacterial composition. Finally, nests with a high quantity of aromatic plants tended to have bigger and taller nestlings in two out of three populations. The quantity of aromatic plants was not associated with speed of feather development and with docility. Our results support the ‘nest protection hypothesis', while highlighting differences in the effect of aromatic plants among populations. To our knowledge, our study is the first to reveal a correlation between the presence of aromatic plants and the bacterial diversity of nests and eggshells in a natural bird population, and to demonstrate experimentally the direct effect of five aromatic plant species on bacterial diversity in nest material.