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I had no idea seeds were good for them in the MelonsHi and welcome to BYC! You got a lot of good advice about what treats to give from @New duck mommy 2021. There are a lot of websites that I have found which suggest all the treats and I'll add some here.
I don't take the seeds out of melons, they love them and it's the first thing they go for. (Some seeds are bad for them like apple seeds so you have to be careful.) I used to cut melons up nice and small and then take the skin off but now I just take a big knife, slice it in half, and put it on the ground for them. On the softer melons, they will eat the skin and all. They eat right down to the hard green skin on watermelons. They demolish melons in no time flat!
There are some things that are really bad for them that you need to know about too. These websites I've listed have information about them also.
https://kingofdoodles.blogspot.com/2011/06/plants-toxic-and-safe-to-pet-ducks-and.html
https://www.tyrantfarms.com/top-10-garden-plants-for-chickens-ducks/
https://www.backyardchickens.com/threads/the-ultimate-list-of-duck-treats-and-supplements.242460/