Agree USAA is a great insurer. I also agree with those that are surprised by the move to cancel. I would call your insurance company and ask to speak to your agent. Not the secretary or some other home office person, but your actual agent that services your policy. Have an honest and frank conversation with them. If they are concerned about the chickens and/or coop specifically, they might be willing to write a waiver that exempts the chickens/coop, but they shouldn't cancel your homeowners altogether.
As an example of this type of negotiation, I will tell you my own story (with USAA). We live in a home north of Atlanta, GA, on just over an acre of land. Single family dwelling. We purchased 40 acres of undeveloped farmland in Tennessee this year. In order to get a better price on the land, which is currently an active plant nursery with over half the acreage planted in nursery stock, we agreed to let the nursery folks (and former owners of the property) continue to work the property, maintain it and maintain/remove their stock over the next 5 years.
There is an old, run down barn/shed structure up there (30x30 feet) and we restored it by adding siding, painting, re-hanging the doors, etc., and we bought a tractor to use up there. We wanted to store it in the barn (160 miles away from our primary residence) and were worried about theft of the tractor or intentional/accidental damage to the building we just restored. So I called USAA and told them about the property, the building and the tractor. I also told them about the arrangement with the former owners/nursery company, and that the nursery employees would be on the land periodically with tractors, bushhoging and digging up nursery stock.
USAA told me that our land was covered by an "extension" of our current primary residence, up to 40% of the value we have our primary residence covered for under the policy. They also told us the tractor and all other material items of ours were covered up to 10% of the primary residence value. And all this for no additional increase to our homeowners policy. They had a problem with the nursery workers, and had to "research" to decide what to do about that. After about a week, they came back to me and told me that the nursery workers, their equipment and any injuries related to them would be excluded from coverage.
I suspect at worst, the company might agree to exclude your chickens/coop, but I can't see why they'd actually cancel you just because you have chickens. If you don't get a satisfactory answer from them though, and you have access to USAA, then you have several people here who have recommended USAA plus an actual story from me on how USAA treats their customers. Good luck to you!