This was the scene four weeks ago: Mom sitting on the eggs, dad patrolling the pond, hissing and snarling at everything and anybody he deems dangerous for his family.
Yesterday the cygnets were already two weeks old and paddling around. Mom kept a watchful eye on them and dad patrolled the pond, hissing and snarling at everything and anybody he deemed dangerous for his family.
And what was happening behind the back of Cygne Père? Someone sneaked in and test-sat in the nest.
I assume that around bedtime there was some loud hissing and snarling to drive the pesky intruder away.