hello; what about a “no-plumage ring-zone” in the top of the neck, just under the head?
a couple of captivity mute swan start from some weeks to loose plumage just under the head, the neck part that they usually use for preening the whole body.
I’ve never see something like this in a mute swan; it seems someone have cutted the plumage with a razor to get a ring shaped zone without plumage, around the neck and just above the head, about 5 to 10 cm high.
(how can I upload you an image to explain this?)
note: the place where they live, has been recently very cold (with snow and lake water totaly frozen) and rained a lot.
all the best,
Marco. (please reply also in my email)