Simple Summary Intestinal microbes play an indispensable role in host physiology and their alteration can produce serious effects on vertebrates. In this study, we analyzed the characteristics of intestinal bacterial community of hooded crane and bean goose whose niches overlap at Shengjin Lake, China, and investigated how host internal factors and inter-species interactions affected the diversity and spread of intestinal bacteria of the two species over three wintering periods. We have found that direct or indirect contact with each other increased the diversity of host intestinal bacteria and caused bacteria to spread among species in the mixed-species flock. In addition, a total of 63 pathogens were identified, of which 38 (60.3%) were found in the gut of both species. These findings could help our understanding of the factors that influence gut bacteria in wild waterbirds, which are also major contributors to the spread of pathogens worldwide. Diversity of gut microbes is influenced by many aspects, including the host internal factors and even direct or indirect contact with other birds, which is particularly important for mixed-species wintering waterbird flocks. In this study, Illumina high-throughput sequencing was used to analyze the intestinal bacteria of the hooded crane and bean goose whose niches overlap at Shengjin Lake. We tested whether contact time enhances the trans-species spread of gut bacteria. Results indicate alpha-diversity and microbial composition displayed significant separation between the two hosts in every wintering period, although the number of bacteria types shared increased with increasing contact time. For the same species, with the lengthening of contact time, alpha-diversity and the number of operational taxonomic units (OTUs) in the host intestine augmented, and the common OTUs and structural similarity of microflora in the middle and late periods were more than in the early and middle periods. In addition, we found a very high proportion of shared pathogens. Our results indicate that, although intestinal microflora of different species were separated, direct or indirect contact in the mixed-species flock caused the spread of gut bacteria trans-species, indicating that more attention should be paid to intestinal pathogens in wild birds.
