Complex and highly integrated functional dependencies have been established between animal hosts and their symbiotic microorganisms. In many cases, this dependence can evolve into systemic symbiosis, where closely related species exhibit greater similarity in the composition of their microbial communities, while distantly related species demonstrate more pronounced differences. However, evidence of systemic symbiosis has been lacking in studies investigating the avian gut microbiome. By utilizing the ABDOMEN model, we conducted an analysis on publicly available data pertaining to gut microbiota collected from passerine birds in New Guinea, revealing significant evidence of systemic symbiosis within the gut microbiota. Furthermore, our findings suggest that ecological factors such as diet and geographic location cannot fully account for this observed systemic symbiosis; thus implying that other evolutionarily conserved characteristics may play a pivotal role in shaping the composition of the microbiome. These results support considering vertebrate gut microbiota as a ” ecosystems on a leash” and emphasize its significance within studies encompassing ecology and evolutionary biology. This perspective provides novel insights into comprehending animal-microbial symbiosis and its ecological function.