I think what you’re doing is the best possible solution for this guy. If anything I’ve seen instances where people put them in cups, but I would not go that far. Personally I’d say to experiment with what you know. I just had an incubation batch screw up with one survivor in a similar predicament- not as severe
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All the eggs were malpositioned and some other issues. At the same time, another chick pipped but drowned because I didn’t make the hole bigger in time. I left it cause I thought it would keep doing it’s thing. So that’s why I think you’re doing alright by these ducks. If anything, let him suck up what he needs and sacrifice stress and time. I think he’ll make it with your help. These chicks might need an egg scalping to make it