Eggshell microbiome as a potential microbial reservoir in a cavity nesting bird

Animals develop microbial associations (i.e., their microbiome) early in life that have been linked to changes in host development, physiology, and survival. However, microbial sources contributing to initial microbial colonization remain unclear, particularly in oviparous animals. We described a potential microbial source, the eggshell microbiome of Western Bluebirds ( Sialia mexicana ), a cavity nesting bird. We found bacteria commonly seen in the microbiomes of other birds; thus, eggshells could represent a source during initial microbial colonization. Eggshell microbiomes also represented an independent group from the gut microbiomes of similar species. Future work should explore the relative contribution of eggshells to initial microbial colonization of nestlings.