I really don't know enough about plants and under what conditions they thrive to be of any help I'm sorry to write.
@DobieLover has a great chicken shelter bush in her garden. It's close to ideal but I don't know what it is. It may be a really slow grower. She's not too far from you as distances in the US go so maybe it would grow where you are.
Unfortunately the best options are not realized until it's too late by people building coops and runs and that is; leave everything you possibly can alone and fence around it.
I've seen coop projects where there were some great plants and shrubs, even trees and they just leveled the place and now it's a bare patch of mud.