This is the territorial call of a male Limpkin, a large bird that specializes in eating apple snails and freshwater mussels. The males use this loud call to stake out their territory, telling other male Limpkins to stay away. Once one begins calling, it is not unusual for other males to start calling too. In areas where there are a lot of Limpkins, this can get VERY loud. This video was taken at Myakka State Park in Florida.