looks like a leech...if it is, I never heard of them on chickens they can get on you in a pond if you been wading through some water. On second look with zoom it looks more like a maggot chickens can get maggots around there bum or in wounds its called "fly strike" I think, search it in the forum search. Sounds like you may have bought some infected birds. Mites are pretty easily controlled with premerthin...spelling? I use a liquid spray to keep flies off horses. Works very well but you need to spray coop and roosts thoroughly probably twice a few days to a week apart...it is safe to spray chickens also. There are directions for chickens on the label
