The older our little powder puffs get, the more my husband agrees with you that they are feather dusters, not chickens.
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Several of the chicks keep trying to eat my ring. And the one I wear around them is silicone, so now it has a small nick in it.

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With Indigo, the chicks were 1 day old when it “didn’t work”, and then 2-3 days old when she decided her babies were missing and tried to get back on to them.
With CoCo, they were already 3 days old when I didn’t feel it was working, and closer to 6-7 when I decided she was trying to adopt the bantams because she couldn’t find “her chicks”, and I out her back with them.
In both cases, I tried tucking the babies in around 10pm, and then checking about every hour (usually by trying to peek in quietly using a red flashlight). My concern both times was that they didn’t seem to be tucking them under, and would peck at them or push them out. I kept trying to tuck them underneath or under a wing. Since both did end up trying to find/raise their chicks after I separated them, I think it just took a little time for their maternal instinct to fully kick in, and I had been to afraid of a chick getting hurt to be patient.
I also had only moved the mom’s to the mini-coop nest box on the same day I gave them the chicks. If I had the opportunity to know I would be giving them chicks, or even suspected, I would have moved them out there several days before to settle in. And yes, I had them sitting on fake eggs, but did not put any empty egg shells. That is an interesting idea.
Coco has been my “extreme broody”. Starting from when she was only 4 or 5 months old, and doing it over and over each year. And every time she has seen or heard chicks. She has gone broody a few days later. Indigo had only been broody a few times.
So you definitely have Marek’s on the property? I hopefully don’t. My first 10 chickens were vaccinated, none of the rest have been. Sometimes you just have to take a chance and hope the vaccine kicks in early enough, but with Tassels I suspect you could wait until they are 5-6 days old. Especially if not too many of them. I actually had more trouble getting the chicks to accept Coco as mom, than I did getting her to accept the chicks.