Could be mites, some kind of ear infection, maybe something caught in throat that isn't passing down to the crop.
If it's throat or ear mites, I'd suggest a few drops of Ivermectin orally and a single drop in each ear. I use the injectable type, orally, topically and rarely injected for chickens. It's a powerful anti-parasitic that works fast and is reasonably safe. (It's also commonly used as a human dewormer in 3rd world countries, so is also relatively safe for humans.) When used topically very little is absorbed and goes systemic. Little if any would pass into eggs or meat. There is probably a withdrawal time on the packaging for meat animals.
I'd check down the throat of this chicken for anything that doesn't belong and examine the ears for any signs of parasites, swelling or irritation.
You may not see any mites in the ears or throat, but it really can't hurt to treat with Ivermectin anyway. If that stops the head shaking, you'll know it probably was mites. Would be good to treat the rest of the flock housed with this chicken to prevent an infestation.
If the head shaking is combined with neck stretching and hard swallowing efforts, there could be a foxtail seed stuck in its throat or other object that's hung up. I'd also check the feel of the crop contents gently from the outside, just below the neck. An impacted crop will feel hard, like one big lump of feed. That can cause some head shaking and attempts to relieve the discomfort.
A chicken with an impacted crop should be isolated and given no dry feed or seeds for about 24 hours, just water mixed with some apple cider vinegar. If the crop is still hard feeling, you can gently massage the mass from the outside to help break it up for digestion and continue on the water only for another 24 hours. You could give greens, maybe a little fruit, but no feed or seeds until the lump is digested. Adding more feed to an impacted crop makes it worse.
A normal feeling full crop should be mushy as feed is liquefied, dissolved and digested. In a flock environment, where chickens are roosting over night, not eating for about 8 hours, the crop should be empty in the morning. A slow or impacted crop can grow an unhealthy amount of yeast & bacteria, which can make a chicken sick.
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