Background <p>The gut microbiome is known from laboratory studies to be essential to host function and sociality, yet comparatively little is known about this association in wild animals. In wild birds, the gut microbiome seems to be broadly driven by environmental factors, and there is mixed evidence for a link with sociality. Here, we describe the gut microbiome composition of the Siberian jay (<i>Perisoreus infaustus</i>), a highly social group-living and food-caching corvid of the Eurasian boreal forest.</p> Results <p>We present evidence of potential environment-related variation in the gut microbiome of wild Siberian jays. Environmental acquisition of microbes may be an important process shaping their gut microbiome composition based on similarities to the local environmental microbial community, for which we propose an environment–oral–gut route as a potential underlying mechanism. We also identify an unexpected group-level convergence, wherein social horizontal transmission of gut microbes may be an incidental consequence of reciprocal cache pilfering among group members.</p> Conclusions <p>While the ecological significance of gut microbiome variation in Siberian jays is still unclear, our results paint a picture of passive microbiome assembly resulting from a combination of environmental acquisition and social transmission in a wild bird species.</p>