Not really, they carry ticks which can infect humans with Lymes disease but beyond that I've not heard about them being infectious to chickens. They will take a lot of your feed though. I'm trying to trap one at the moment that is sometimes on the top of my run but with no luck, bit too suspicious of my trap and I never have the air rifle handy when I see it, luckily next doors cat got two of them last month, we have nut trees and I would like to manage at least one hazelnut/walnut this autumn. You could do with your growers hurrying up and growing so you can get a real life cockerel in their to chase it off.