My older call ducklings (by 3 weeks) keep going after my younger ones. I had to seperate one of the younger ones because it was sick and not growing as fast and now they want to kill it too. They all (except for the littlest one that is caged with chicks) also go after chicks. Is this normal? Will it stop when they are all the same size? If I let any of them hatch next year will the others kill the ducklings?