The skin represents the first barrier between an animal and its environment and it is densely populated with bacteria. Recent technologies that are independent of bacteria cultivation have revealed that the skin microbiome is very diverse and variable. Although the physiological skin flora is known for its functions in maintaining the health status (humans: , animals: ), not much is known about the composition of the physiological flora, particularly concerning the skin of birds.
So far, most studies investigating the microbiome of birds have focused on gut microbiota, e.g.  and only a few have addressed the physiological flora on the skin, such as the facial skin , brood patch , neck region  and uropygial gland [5–7]. However, feathers and skin act as barriers between the bird and its environment and are thus particularly important body sites for investigation. Feathers have been shown to carry a substantial bacterial load . Until now, most studies have focused on keratinolytic bacteria such as the feather degrading Bacillus licheniformis and investigated their impact on feather coloration and body condition [9, 10]. While our knowledge on the bacterial skin flora in birds is increasing [5, 6, 11], we are still lacking knowledge about how birds acquire their skin flora, how it is usually composed and which functions these microbial communities fulfil.
In this study, we examined the skin microbiome of wild zebra finches, in order to complement the incomplete picture that we have of the composition and function of the skin microbiota in wild birds. Therefore, we used 16S rRNA gene sequencing to characterize the skin microbial communities of different zebra finch family members. Samples comprised females, males and their offspring (family groups), from different breeding clusters (social groups) so that we were able to investigate the skin microbiome at different social organisational levels and also of different sexes and two age classes, i.e. adults and nestlings.
Age effects on the bacterial flora are well supported for the gut microbiome in birds , but it has yet to be demonstrated for the bacterial communities of the skin. Similar to most altricial songbirds, zebra finches hatch sparsely covered with down feathers and develop rapidly into adults with closed plumage. Therefore, it seems plausible that nestlings and adults differ in their bacterial communities. Another factor in shaping the skin microbiome may be the bird’s sex. It could be considered the most important genetic effect as it implies changes in the level of hormones, especially during the reproductive periods which could be influential on the skin microbiome. Some sources hint for gender-specific gut microbiota in broiler chicks, e.g. bacterial communities differ between male and female chicks’ ileum  and caeca . Investigations on the skin microbiome are scarce, but for the sea bird Oceanodroma leucorhoa, the factor sex of individuals crucially impacts on the bacterial community variation . However, Taylor and colleagues , could not find a difference between oral samples from male and female birds of prey and there was no effect of sex detected in the skin microbiome of estrildid finches .
The composition of gut microbial communities in western cowbirds, a common North American brood parasite, is partially explained by the geographic origin of samples, which is even more pronounced than bird taxonomy or ecology . On the other hand, in Adelie penguins, genetically related individuals harbour very similar gut microbiomes, even if they inhabit distant geographical regions , which indicates a strong influence of the respective genetic background. To our knowledge, such examinations are lacking for the surface microbiota of birds such as skin, feathers or beak.
Besides spatial proximity, social interactions shape the microbiomes of socially living mammals like hyenas  and meerkats . In both species, bacterial communities from the anal pouches are transferred within the social environment during scent marking and therefore transferred to animals of the same social group [18, 19]. Studies like these underpin the importance of conspecifics and the individual’s family for the colonization of the skin with microbiota.
The nest environment, as well as the social environment, influences the bacterial community on the skin of various bird species (reviewed in ) and can therefore be considered to be drivers for diversity in the skin microbiomes of different families.
In order to understand the impact of social environment, we analysed samples from parents and their offspring from a natural population of zebra finches in the Australian desert. We expected that animals living in social groups and sharing the same environment harboured more similar skin microbiomes compared to those living in more distant environments. With the results that we present here, we aim to contribute to a better understanding of the physiological flora, the transmission and potential functions of skin bacteria in birds.