If the unusual behavior occurred after the obtainment of the chickens, then this certainly could be tied to the behavior. The chickens may have attracted predators such as coyotes and foxes to the area and the swans decided that it was time to get out with the cygnets instead of placing the family in danger. If the swans know that there are predators in the area, they may never return. The new swan may be a wayward juvenile who does not know about the territory but, was dispatched from another area and is looking for a suitable habitat. There are no guarantees that the original birds will or will not return. You will need to observe closely to ascertain if the swans are the same or a new group. Also, you will need to keep the chickens and the swans safe from any predators if you hope to keep either species. The Regal Swan