When I had 2 weeks old Runners and newly hatched Runners to keep from having separate brooders I made 2 brooders inside of one the one with the new ducklings was smaller of course but this way they could also share a heat lamp if that is what you are using. I used a piece of hardware cloth and wrapped the edges in gorilla tape so they wouldn't get poked this way they could see each other once the little ones were a couple days old I would put them in with the bigger ones and watch to make sure they wouldn't step on them or bite them They were a little rough at first but after a week of doing this they ended up all being together and they are still good friends. This was 2019.
My brooder was an X large dog crate that I put both top and bottom together with tie wraps to make one long brooder. I used a heat lamp when I had the 2 on one side and 2 on the other.
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