The Greggs family is into hunting and they hunt ducks often. Their neighbour's girl is very unhappy about this. She tried to stop them as she is friends with Philip and William Gregg but they wouldn't listen. The anger in her grew until a point where she used her magic finger to turn the Greggs into ducks.
When they woke up the next day, they were no longer humans. The ducks took over their house and shoot them later during the day. The family had to work hard to build nests up in the trees for shelter and live in fear as the ducks are not shooting them. Basically, the situation has been reversed. They learned their lesson and from that day on, they stopped shooting.
The girl (whose name wasn't mentioned in the book) is gifted with a magic finger. She had once used it on her schoolteacher for calling her stupid. After seeing the effect, she made a vow to herself to not use it ever again but the Greggs family needs a lesson and I believe they have learned!
Side note: Good read. :) Never forget to put yourself in others' shoes.