Hi Marco
We apologize for the delayed response, but we have been updating the website. There are several possibilities for the ring-like appearance around the neck. One reason could be that the swans are coming into contact with something under the water and are rubbing against the object causing an abrasion. The second possible cause is a parasite, protozoa or fungus in the water that is infecting the swans. The swans would need to be captured and examined by a licensed veterinarian to determine if an infectious agent is the cause.
However, more than likely, the cause of the lost plumage in this area is from swans pecking at each other during fights for mates and territories as springtime brings out defensive behavior. As long as there is no infection, the plumage should return very soon. Again, if this is an infection, a veterinarian should be consulted. The Regal Swan