21 In August 2018, hundreds of migratory birds died in the area of Dali Nouer Lake, Chifeng City, 22 China. We collected the remains of dead birds along with water and aquatic plants from the birds’ 23 environment. The bacterial communities of all samples were profiled by high-throughput 24 sequencing of the V3–V4 hypervariable region of the 16S rRNA amplicon. At the genus level, 25 Bacteroides, Clostridium, Plesiomonas, Vibrio, Fusobacterium, and Aeromonas were the 26 dominant genera in dead birds, the lake water, and aquatic plants in 2018. However, the relative 27 abundances of these bacterial genera were significantly reduced compared with the levels obtained 28 from healthy migratory bird feces, lake water, and aquatic plants from the same place and time 29 period in 2019. Combined with environmental factors such as the changes in salt content and pH, 30 the invasion and reproduction of those pathogens may have promoted the decline and death of the 31 birds. 32 33