There's a young killdeer where I live (hatched last year, already grown) who is trying to steal her new baby brothers from her mom and dad. She had lost her nest a few days before her parents hatched their own nest nearby. So, she hangs around the area where they are raising their chicks, watching them. If her parents aren't looking, she sneaks over and tries to interact with the babies. But, her mom chases her away as soon as she notices what's going on.
Also, we have at least one, possibly two ducklingnappers at our lake. These females hang around new, large broods of ducklings until some of them imprint on her. Then, she takes some of them away from the mom. And, there's also three sister ducks who all have ducklings and I've seen a few hatched out from one sister join the other sister's brood.