The Chinese crested tern (Thalasseus bersteini) is one of the most globally endangered species, listed as “Critically Endangered (CE)” on the IUCN Red List, with only approximately 30-49 individuals surviving in the wild. Chinese crested terns were discovered to breed in South Korea for the first time in 2016 while conducting a census on uninhabited islands. The Ministry of Environment has declared the breeding habitat of the Chinese crested terns as “Specified Island” to protect this CE species. However, brown rats (Rattus norvegicus) inhabiting the breeding grounds of the Chinese crested terns and Black-tailed gulls may potentially pose a threat to the breeding of these avian species. Therefore, we conducted a study on the feeding behavior of brown rats involving stable isotope analysis to determine their food sources. Fecal analysis showed that brown rats mainly fed on plants, whereas they scarcely fed on animals, such as insects. In addition, the stable isotope analysis showed that the δ13C values of brown rats, insects, and Indian goosegrasses were approximately –16 to –11‰, whereas the δ13C value of Chinese crested terns that obtained their food from the marine ecosystem was approximately –22 to –18‰. Hence, we conclude that the source of carbon for brown rats on this island is the terrestrial ecosystem. We ruled out the possibility of any direct prey– predator interaction between the brown rat and the Chinese crested tern or Black-tailed gull.