Black-bellied Whistling Duck

Last year I had 13 then later 5 Black Bellied Whistling Ducks at my home near Hilton Head SC they left in early October then in april this year the 5 (i think) original returned but within a week or so the count went from 5 to 21 then to 41. They seem to be in 2 flocks of 21 or 20 and fly in v format. Has any one had same experience will they leave, will I have a hundred next year? They have fed under my bird feeders and this year I have fed them in a wooden ground feeder.