**3. Migratory birds and flyway**

The geographical route that migratory birds move annually on the earth is called "flyway," and there are nine major flyways in the world. Japan is located on the East Asia-Australia flyway, and it is estimated that more than 50 million migratory birds, such as shorebirds, birds, and seabirds, travel over the flyway every year.

Summer migrants in Japan fly from the south mainly for breeding and spend the summer in Japan, and when the breeding season ends, they return south for overwintering. Winter migrants in Japan fly from the north mainly for overwintering and spend the winter in Japan, and then in the spring, they return north for breeding. Passage migrants breed in the country north of Japan and overwinter in the country south of Japan, and so they travel through Japan during the movement and are mainly observed in spring and autumn. As a whole, some millions of migratory birds are estimated to visit Japan annually.

The direct counting method by fluorescent staining revealed that the fecal matter of migratory birds contains ≥108 cells/g bacteria [16, 17]. Since disinfection treatments for bird feces is not performed like that for humans and livestock, there are possibilities that several live bacteria, along with 50 million migratory birds each year, travel the East Asia–Australia region flyway for a long distance. In order to clarify the dynamic of the microbiota, including pathogenic bacteria and antibiotic-resistant bacteria, as well as to verify their significance in public health and environmental microbiology, research has been performed to analyze the intestinal microbial community associated with migratory birds by NGS [16–22].
