Herons do eat ducklings, but I have never seen them eat more than one or two at a time. But, they could eat four in a day if there were no hiding places or the lake is small. At our lake, this year, gulls ate a lot of them, too. Bass will also eat them as do turtles, especially if the lake is small. Also, some mothers are better than others, but they can learn. Next time, she may be a little smarter about whatever took those ducklings and there will be a better chance they will survive.