My guess is that those are mites. You would be good to get some permethrin to treat the coop, nests, roosts, and the chickens after emptying the coop bedding. In the UK, it may take some investigating on where to find it. The birds need to be treated every 7 days until no mites are seen. Here are some links to read about mites and lice:
I find Netex total mite kill spray the best for keeping these blighters at a reasonable number. It contains ~ Contains: Permethrin 2.3%, Tetramethrin 0.23%, Benzalkonium chloride.
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I treat the coops every 3 days with it to try and break the cycle of them.
Unfortunately @Eggcessive i can’t find a permethrin spray etc to actually put on the birds over here in the UK. The mite kill above says do not apply to birds and I am thinking it’s because of the other ingredients that’s contained in it.