You can do one of two things - 1) let nature take its course, because weak ducklings usually grow up into weak ducks with lots of physical problems or 2) make sure there's enough food and water available that the weak duckling isn't having to fight for resources with the bigger stronger ducklings, or you could even separate the brood, depending on how many you have (you can't have a lone duckling, they get really stressed). For me personally, the only time I've ever culled a sick duckling or chick is when it was obviously struggling. I look at it as a quality of life issue - if it seems to be alert and happy, I will do whatever I can to help it thrive. But if it seems weak, lethargic, and just basically uninterested in living, then I will cull it because to me it's cruel to allow it to suffer, when in nature it would have died right away.