We fed my sons day old ducklings with starter crumbles as there was no duck food available at tractor supplies. The addition of niacin -- we use brewer's yeast -- to duck diets is important but otherwise chick starter crumbles are fine.
It's not just that ducklings are noisy when on their own, a single duckling will need a lot -- hours each day -- of human attention if it doesn't have ducky friends. I have a rescued muscovy that I resuscitated and it somehow survived. It did well for 24 hours with me hourly hand feeding. But when I left it in the brooder to feed and drink itself, it lay face down in the corner and didn't move eat it drink. I recognized a depressed lonely duck and brought it out to be with me -- on the desk between me and my laptop. I did get a mirror but that worked for only a short time before my 4 day old duckling realized that there wasn't a duck there and only used the mirror to see me when he had his back to me.
The duckling thrived with fulltime human attention but I could not commit to 12 - 15 years as inseperable best friend to a duck. So at 8 weeks I contacted a wildlife rehabbed and she had ducklings of around the same age as mine. I took in another muscovy and a crested pekin and after a little adjustment they became tres amigos.
I suggest that you inquire of your local wildlife rehabbed and your branch of the American Society for the prevention of cruelty to Animals,(ASPCA) to see if they have any potential friends for your duckling
Its easiest to integrate ducklings that are the same size and that changes daily initially. It is thus easiest to find duckling friends when the ducks are 4 - 8 weeks. They will accept other ducklings a d are more equal in size if there are any pecking order tussles
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