Have you looked at the nostrils to see if there's food or debris stuck?
I don't see anything concerning in the video, no respiratory distress and with the sound all the way up, I don't hear any noticeable breathing sounds.
Shaking to ruffle feathers is normal. They are growing in new...
Photos of them and tucking of the feet?
Inspect the bottoms of the feet to be sure there's nothing stuck to the bottoms or any type of irritation.
Chickens can stand on one leg and tuck the other, but it's not that common.
Some chicks grow and feather out faster/slower than others. If...
This doesn't match...
I would stop cleaning/sanitizing the brooder.
A quick wash of water/feed stations when soiled is always a good idea.
Chicks need to be exposed to poop and each other, they will not be living in a sterile and sanitized environment outside. You want them to have exposure...
They look hot.
I'd provide a larger brooder with more space and raise them lamp.
It is not necessary to clean and sanitize a brooder daily. Chicks are not sterile.
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Can you please post photos of the chicks, their poop and the brooder?
What is the temperature on the warm side and cool side of the brooder?
What are you feeding?
You cleaned and sanitized the brooder - with what? Really all that needs to be done in a brooder is to keep...