A microbial take on bird life: Illuminating environmental context of bird microbiota dynamics and immune function

In a world dominated by invisible bacteria and other microorganisms, birds themselves are mobile ecosystems for microorganisms; every bird with its own microbiota. With a variety of functions, microbiota contribute to health and functioning of the bird in its environment. But should the microbiota be considered a trait of the individual bird or does the microbiota rather represent a reflection of the bird’s microbial environment? The answer to this question has fundamental merit to evolutionary ecology. Understanding the developmental process of microbiota and its symbiosis with the host, and understanding how the microbiota is maintained, insight can be gained into how symbiosis with microbiota affects health and disease and evolutionary success of failure. This thesis demonstrates that the effect of the microbial environment on the microbiota of larks in the Netherlands is substantial. Subsequent comparison with the microbiota of larks from the desert of Saudi Arabia and tropical grasslands of Kenia underscores the importance of the microbial environment to microbiota variation, also at a large geographical scale. By manipulating the microbial environment of zebra finches, not only the swift changes of bird microbiota upon a changing environment were apparent, also the antibody concentration in blood changed. The immune system can thus respond fast and flexibly to the microbial environment. The acquired experience can also be transferred to the next generation through maternal antibody deposition to the eggs. In conclusion, the microbial environment is of ecological relevance for a bird’s physiology and microbiota dynamics.