Senoufou sent me an update.
I can barely credit my eyes.

The Mad Swans, after all these years,
have altered their ways.
They are still alive,
and their little what-d'yer-call-it of cygnets
is thriving.

And they stay AWAY from the route to that lake/pond/river
miles from their nest.
No more royal progress down the main thoroughfare.
But they returned to the same nest as before.

wonders never cease ...