No they do not suddenly appear because you have chickens, they are already there you just have not seen them.
They will be attracted to your chicken coop just like they are attracted to garbage cans with no lids or not secured lids, dog food or cat food left out etc.
They will be attracted to your chicken coop just like they are attracted to garbage cans with no lids or not secured lids, dog food or cat food left out etc.