I had the very same problem, reddish 'fleas' mounded on the chicken's head and neck - terrible stuff.
we were to the point to consider burning down the 100' long coop to get rid of them. I had also read that was the only way to banish these parasites.
Finally read on this forum to spray the coops, nest boxes, ground, perches and all surfaces with the IG and Tenguard (permethrin) together, both came from
Amazon.
I also turned the feathers up on their backs and chests to give a good spray in those places; but I don't think I used the IG on the birds.
Then put clear Swat gel (horse product) all over their heads / necks, wherever I saw fleas, trying my best not to get in their eyes. but the fleas are stuck on so fiercely and very hard to remove in any way other than by suffocating / poisoning them. You could see the birds felt better almost immediately, maybe the fleas loosened their hold?? but you could see them physically relax after the Swat was on the fleas.
Maybe this wasn't an ideal treatment medical-wise, but we were infested with them for 2+ years and this was the ONLY treatment we used that was effective. Amazingly, they've stayed away as well. I thought for certain they would come back, but they haven't.
hope this helps.