So i read this question to my husband who did vinyl siding for a living for years and had his own business.
he Says you must think like a mouse.
get down and look at the bottom flashing and see if it is capped off in all the lower areas.
he says if you are using vinyl or aluminum corner post , more often than not people do not cap the bottoms, so it leaves an open highway for rodents do travel all the way up to the soffit.
he would use aluminum stock and bend up a custom cap. to his knowledge they do not make a factory cap.
also there is this Great rodent and bug proof Foam that come in a pressurized can that you can spray fill gaps. its foam for insulating called GREAT STUFF, however i do not know that this should be used if it is exposed to your chickens , i am sure they will eat it if they can get to it, and it is toxic.

pictures of your coop would help, but with some investigative work you should be able to find the points of ingress, egress.
Good Luck!