Oh, I didn't want to imply that all spider mites are no problem. The regular type that get on my plants are he*l on earth to get rid of, and they are very tiny - even tinier than the red ones, and white or tan color, and they make little webs all over the plants, often the growing tips. Nasty, nasty things. I've had them numerous times on houseplants.
But, the little tiny red ones don't do that.
Beautiful bonsai collection - I just love it!
As for what mites can do to chickens, serious infestations can literally kill them. They can get a lot of scabs and blood loss and their immune system will be compromised as well, since they are having to deal with the bites and such. Mostly they are very irritating to the birds and egg production will also drop.